# 1 Xen Hypervisor Command Line Options

This document covers the command line options which the Xen Hypervisor.

## 1.1 Types of parameter

Most parameters take the form `option=value`. Different options on the command line should be space delimited. All options are case sensitive, as are all values unless explicitly noted.

### 1.1.1 Boolean (`<boolean>`)

All boolean option may be explicitly enabled using a `value` of > `yes`, `on`, `true`, `enable` or `1`

They may be explicitly disabled using a `value` of > `no`, `off`, `false`, `disable` or `0`

In addition, a boolean option may be enabled by simply stating its name, and may be disabled by prefixing its name with `no-`.

####Examples

Enable noreboot mode > `noreboot=true`

Disable x2apic support (if present) > `x2apic=off`

Enable synchronous console mode > `sync_console`

Explicitly specifying any value other than those listed above is undefined, as is stacking a `no-` prefix with an explicit value.

### 1.1.2 Integer (`<integer>`)

An integer parameter will default to decimal and may be prefixed with a `-` for negative numbers. Alternatively, a hexadecimal number may be used by prefixing the number with `0x`, or an octal number may be used if a leading `0` is present.

Providing a string which does not validly convert to an integer is undefined.

### 1.1.3 Size (`<size>`)

A size parameter may be any integer, with a single size suffix

- `T` or `t`: TiB (2^40)
- `G` or `g`: GiB (2^30)
- `M` or `m`: MiB (2^20)
- `K` or `k`: KiB (2^10)
- `B` or `b`: Bytes

Without a size suffix, the default will be kilo. Providing a suffix other than those listed above is undefined.

### 1.1.4 String

Many parameters are more complicated and require more intricate configuration. The detailed description of each individual parameter specify which values are valid.

### 1.1.5 List

Some options take a comma separated list of values.

### 1.1.6 Combination

Some parameters act as combinations of the above, most commonly a mix of Boolean and String. These are noted in the relevant sections.

## 1.2 Parameter details

### 1.2.1 acpi

> `= force | ht | noirq | <boolean> | verbose`

String, or Boolean to disable.

By default, Xen will scan the DMI data and blacklist certain systems which are known to have broken ACPI setups. Providing `acpi=force` will cause Xen to ignore the blacklist and attempt to use all ACPI features.

Using `acpi=ht` causes Xen to parse the ACPI tables enough to enumerate all CPUs, but will not use other ACPI features. This is not common, and only has an effect if your system is blacklisted.

The `acpi=noirq` option causes Xen to not parse the ACPI MADT table looking for IO-APIC entries. This is also not common, and any system which requires this option to function should be blacklisted. Additionally, this will not prevent Xen from finding IO-APIC entries from the MP tables.

Further, any of the boolean false options can be used to disable ACPI usage entirely.

Because responsibility for ACPI processing is shared between Xen and the domain 0 kernel this option is automatically propagated to the domain 0 command line.

Finally, `acpi=verbose` will enable per-processor information logging which may otherwise be too noisy in particular on large systems.

### 1.2.2 acpi_apic_instance

> `= <integer>`

Specify which ACPI MADT table to parse for APIC information, if more than one is present.

### 1.2.3 acpi_pstate_strict (x86)

> `= <boolean>`

> Default: `false`

Enforce checking that P-state transitions by the ACPI cpufreq driver actually result in the nominated frequency to be established. A warning message will be logged if that isn’t the case.

### 1.2.4 acpi_skip_timer_override (x86)

> `= <boolean>`

Instruct Xen to ignore timer-interrupt override.

### 1.2.5 acpi_sleep (x86)

> `= s3_bios | s3_mode`

`s3_bios` instructs Xen to invoke video BIOS initialization during S3 resume.

`s3_mode` instructs Xen to set up the boot time (option `vga=`) video mode during S3 resume.

### 1.2.6 allow_unsafe (x86)

> `= <boolean>`

> Default: `false`

Force boot on potentially unsafe systems. By default Xen will refuse to boot on systems with the following errata:

- AMD Erratum 121. Processors with this erratum are subject to a guest triggerable Denial of Service. Override only if you trust all of your PV guests.

### 1.2.7 altp2m (Intel)

> `= <boolean>`

> Default: `false`

Permit multiple copies of host p2m.

### 1.2.8 apic (x86)

> `= bigsmp | default`

Override Xen’s logic for choosing the APIC driver. By default, if there are more than 8 CPUs, Xen will switch to `bigsmp` over `default`.

### 1.2.9 apicv (Intel)

> `= <boolean>`

> Default: `true`

Permit Xen to use APIC Virtualisation Extensions. This is an optimisation available as part of VT-x, and allows hardware to take care of the guests APIC handling, rather than requiring emulation in Xen.

### 1.2.10 apic_verbosity (x86)

> `= verbose | debug`

Increase the verbosity of the APIC code from the default value.

### 1.2.11 arat (x86)

> `= <boolean>`

> Default: `true`

Permit Xen to use “Always Running APIC Timer” support on compatible hardware in combination with cpuidle. This option is only expected to be useful for developers wishing Xen to fall back to older timing methods on newer hardware.

### 1.2.12 argo

    = List of [ <bool>, mac-permissive=<bool> ]

Controls for the Argo hypervisor-mediated interdomain communication service.

The functionality that this option controls is only available when Xen has been compiled with the build setting for Argo enabled in the build configuration.

Argo is a interdomain communication mechanism, where Xen acts as the central point of authority. Guests may register memory rings to recieve messages, query the status of other domains, and send messages by hypercall, all subject to appropriate auditing by Xen. Argo is disabled by default.

- The `mac-permissive` boolean controls whether wildcard receive rings may be registered (`mac-permissive=1`) or may not be registered (`mac-permissive=0`).

  This option is disabled by default, to protect domains from a DoS by a buggy or malicious other domain spamming the ring.

### 1.2.13 asid (x86)

> `= <boolean>`

> Default: `true`

Permit Xen to use Address Space Identifiers. This is an optimisation which tags the TLB entries with an ID per vcpu. This allows for guest TLB flushes to be performed without the overhead of a complete TLB flush.

### 1.2.14 async-show-all (x86)

> `= <boolean>`

> Default: `false`

Forces all CPUs’ full state to be logged upon certain fatal asynchronous exceptions (watchdog NMIs and unexpected MCEs).

### 1.2.15 ats (x86)

> `= <boolean>`

> Default: `false`

Permits Xen to set up and use PCI Address Translation Services. This is a performance optimisation for PCI Passthrough.

WARNING: Xen cannot currently safely use ATS because of its synchronous wait loops for Queued Invalidation completions.

### 1.2.16 availmem

> `= <size>`

> Default: `0` (no limit)

Specify a maximum amount of available memory, to which Xen will clamp the e820 table.

### 1.2.17 badpage

> `= List of [ <integer> | <integer>-<integer> ]`

Specify that certain pages, or certain ranges of pages contain bad bytes and should not be used. For example, if your memory tester says that byte `0x12345678` is bad, you would place `badpage=0x12345` on Xen’s command line.

### 1.2.18 bootscrub

> `= idle | <boolean>`

> Default: `idle`

Scrub free RAM during boot. This is a safety feature to prevent accidentally leaking sensitive VM data into other VMs if Xen crashes and reboots.

In `idle` mode, RAM is scrubbed in background on all CPUs during idle-loop with a guarantee that memory allocations always provide scrubbed pages. This option reduces boot time on machines with a large amount of RAM while still providing security benefits.

### 1.2.19 bootscrub_chunk

> `= <size>`

> Default: `128M`

Maximum RAM block size chunks to be scrubbed whilst holding the page heap lock and not running softirqs. Reduce this if softirqs are not being run frequently enough. Setting this to a high value may cause boot failure, particularly if the NMI watchdog is also enabled.

### 1.2.20 cet

    = List of [ shstk=<bool>, ibt=<bool> ]

    Applicability: x86

Controls for the use of Control-flow Enforcement Technology. CET is group a of hardware features designed to combat Return-oriented Programming (ROP, also call/jmp COP/JOP) attacks.

CET is incompatible with 32bit PV guests. If any CET sub-options are active, they will override the `pv=32` boolean to `false`. Backwards compatibility can be maintained with the pv-shim mechanism.

- The `shstk=` boolean controls whether Xen uses Shadow Stacks for its own protection.

  The option is available when `CONFIG_XEN_SHSTK` is compiled in, and defaults to `true` on hardware supporting CET-SS. Specifying `cet=no-shstk` will cause Xen not to use Shadow Stacks even when support is available in hardware.

- The `ibt=` boolean controls whether Xen uses Indirect Branch Tracking for its own protection.

  The option is available when `CONFIG_XEN_IBT` is compiled in, and defaults to `true` on hardware supporting CET-IBT. Specifying `cet=no-ibt` will cause Xen not to use Indirect Branch Tracking even when support is available in hardware.

### 1.2.21 clocksource (x86)

> `= pit | hpet | acpi | tsc`

If set, override Xen’s default choice for the platform timer. Having TSC as platform timer requires being explicitly set. This is because TSC can only be safely used if CPU hotplug isn’t performed on the system. On some platforms, the “maxcpus” option may need to be used to further adjust the number of allowed CPUs. When running on platforms that can guarantee a monotonic TSC across sockets you may want to adjust the “tsc” command line parameter to “stable:socket”.

### 1.2.22 cmci-threshold (Intel)

> `= <integer>`

> Default: `2`

Specify the event count threshold for raising Corrected Machine Check Interrupts. Specifying zero disables CMCI handling.

### 1.2.23 cmos-rtc-probe (x86)

> `= <boolean>`

> Default: `false`

Flag to indicate whether to probe for a CMOS Real Time Clock irrespective of ACPI indicating none to be there.

### 1.2.24 com1 (x86)

### 1.2.25 com2 (x86)

> `= <baud>[/<base-baud>][,[DPS][,[<io-base>|pci|amt][,[<irq>|msi][,[<port-bdf>][,[<bridge-bdf>]]]]]]`

Both option `com1` and `com2` follow the same format.

- `<baud>` may be either an integer baud rate, or the string `auto` if the bootloader or other earlier firmware has already set it up.
- Optionally, the base baud rate (usually the highest baud rate the device can communicate at) can be specified.
- `DPS` represents the number of data bits, the parity, and the number of stop bits.
  - `D` is an integer between 5 and 8 for the number of data bits.
  - `P` is a single character representing the type of parity:
    - `n` No
    - `o` Odd
    - `e` Even
    - `m` Mark
    - `s` Space
  - `S` is an integer 1 or 2 for the number of stop bits.
- `<io-base>` is an integer which specifies the IO base port for UART registers.
- `<irq>` is the IRQ number to use, or `0` to use the UART in poll mode only, or `msi` to set up a Message Signaled Interrupt.
- `<port-bdf>` is the PCI location of the UART, in `<bus>:<device>.<function>` notation.
- `<bridge-bdf>` is the PCI bridge behind which is the UART, in `<bus>:<device>.<function>` notation.
- `pci` indicates that Xen should scan the PCI bus for the UART, avoiding Intel AMT devices.
- `amt` indicated that Xen should scan the PCI bus for the UART, including Intel AMT devices if present.

A typical setup for most situations might be `com1=115200,8n1`

In addition to the above positional specification for UART parameters, name=value pair specfications are also supported. This is used to add flexibility for UART devices which require additional UART parameter configurations.

The comma separation still delineates positional parameters. Hence, unless the parameter is explicitly specified with name=value option, it will be considered a positional parameter.

The syntax consists of com1=(comma-separated positional parameters),(comma separated name-value pairs)

The accepted name keywords for name=value pairs are:

- `baud` - accepts integer baud rate (eg. 115200) or `auto`
- `bridge`- Similar to bridge-bdf in positional parameters. Used to determine the PCI bridge to access the UART device. Notation is xx:xx.x `<bus>:<device>.<function>`
- `clock-hz`- accepts large integers to setup UART clock frequencies. Do note - these values are multiplied by 16.
- `data-bits` - integer between 5 and 8
- `dev` - accepted values are `pci` OR `amt`. If this option is used to specify if the serial device is pci-based. The io_base cannot be specified when `dev=pci` or `dev=amt` is used.
- `io-base` - accepts integer which specified IO base port for UART registers
- `irq` - IRQ number to use
- `parity` - accepted values are same as positional parameters
- `port` - Used to specify which port the PCI serial device is located on Notation is xx:xx.x `<bus>:<device>.<function>`
- `reg-shift` - register shifts required to set UART registers
- `reg-width` - register width required to set UART registers (only accepts 1 and 4)
- `stop-bits` - only accepts 1 or 2 for the number of stop bits

The following are examples of correct specifications:

    com1=115200,8n1,0x3f8,4
    com1=115200,8n1,0x3f8,4,reg-width=4,reg-shift=2
    com1=baud=115200,parity=n,stop-bits=1,io-base=0x3f8,reg-width=4

### 1.2.26 conring_size

> `= <size>`

> Default: `conring_size=16k`

Specify the size of the console ring buffer.

### 1.2.27 console

> `= List of [ vga | com1[H,L] | com2[H,L] | pv | dbgp | none ]`

> Default: `console=com1,vga`

Specify which console(s) Xen should use.

`vga` indicates that Xen should try and use the vga graphics adapter.

`com1` and `com2` indicates that Xen should use serial ports 1 and 2 respectively. Optionally, these arguments may be followed by an `H` or `L`. `H` indicates that transmitted characters will have their MSB set, while received characters must have their MSB set. `L` indicates the converse; transmitted and received characters will have their MSB cleared. This allows a single port to be shared by two subsystems (e.g. console and debugger).

`pv` indicates that Xen should use Xen’s PV console. This option is only available when used together with `pv-in-pvh`.

`dbgp` indicates that Xen should use a USB debug port.

`none` indicates that Xen should not use a console. This option only makes sense on its own.

### 1.2.28 console_timestamps

> `= none | date | datems | boot | raw`

> Default: `none`

> Can be modified at runtime

Specify which timestamp format Xen should use for each console line.

- `none`: No timestamps
- `date`: Date and time information
  - `[YYYY-MM-DD HH:MM:SS]`
- `datems`: Date and time, with milliseconds
  - `[YYYY-MM-DD HH:MM:SS.mmm]`
- `boot`: Seconds and microseconds since boot
  - `[SSSSSS.uuuuuu]`
- `raw`: Raw platform ticks, architecture and implementation dependent
  - `[XXXXXXXXXXXXXXXX]`

For compatibility with the older boolean parameter, specifying `console_timestamps` alone will enable the `date` option.

### 1.2.29 console_to_ring

> `= <boolean>`

> Default: `false`

Flag to indicate whether all guest console output should be copied into the console ring buffer.

### 1.2.30 conswitch

> `= <switch char>[x]`

> Default: `conswitch=a`

> Can be modified at runtime

Specify which character should be used to switch serial input between Xen and dom0. The required sequence is CTRL-<switch char> three times.

The optional trailing `x` indicates that Xen should not automatically switch the console input to dom0 during boot. Any other value, including omission, causes Xen to automatically switch to the dom0 console during dom0 boot. Use `conswitch=ax` to keep the default switch character, but for xen to keep the console.

### 1.2.31 core_parking

> `= power | performance`

> Default: `power`

### 1.2.32 cpu_type (x86)

> `= arch_perfmon`

If set, force use of the performance counters for oprofile, rather than detecting available support.

### 1.2.33 cpufreq

> `= none | {{ <boolean> | xen } [:[powersave|performance|ondemand|userspace][,<maxfreq>][,[<minfreq>][,[verbose]]]]} | dom0-kernel`

> Default: `xen`

Indicate where the responsibility for driving power states lies. Note that the choice of `dom0-kernel` is deprecated and not supported by all Dom0 kernels.

- Default governor policy is ondemand.
- `<maxfreq>` and `<minfreq>` are integers which represent max and min processor frequencies respectively.
- `verbose` option can be included as a string or also as `verbose=<integer>`

### 1.2.34 cpuid (x86)

> `= List of comma separated booleans`

This option allows for fine tuning of the facilities Xen will use, after accounting for hardware capabilities as enumerated via CPUID.

Unless otherwise noted, options only have any effect in their negative form, to hide the named feature(s). Ignoring a feature using this mechanism will cause Xen not to use the feature, nor offer them as usable to guests.

Currently accepted:

The Speculation Control hardware features `srbds-ctrl`, `md-clear`, `ibrsb`, `stibp`, `ibpb`, `l1d-flush` and `ssbd` are used by default if available and applicable. They can all be ignored.

`rdrand` and `rdseed` have multiple interactions.

- For Special Register Buffer Data Sampling (SRBDS, XSA-320, CVE-2020-0543), RDRAND and RDSEED can be ignored.

  Due to the absence of microcode to address SRBDS on IvyBridge client hardware, the RDRAND feature is hidden by default for guests, unless `rdrand` is used in its positive form. Irrespective of the setting here, VMs can use RDRAND if explicitly enabled in guest config file, and VMs already using RDRAND can migrate in.

- The RDRAND feature is disabled by default on AMD Fam15/16 systems, due to possible malfunctions after ACPI S3 suspend/resume. `rdrand` may be used in its positive form to override Xen’s default behaviour on these systems, and make the feature fully usable.

### 1.2.35 cpuid_mask_cpu

> `= fam_0f_rev_[cdefg] | fam_10_rev_[bc] | fam_11_rev_b`

> Applicability: AMD

If none of the other cpuid_mask_* options are given, Xen has a set of pre-configured masks to make the current processor appear to be family/revision specified.

See below for general information on masking.

Warning: This option is not fully effective on Family 15h processors or later.

### 1.2.36 cpuid_mask_ecx

### 1.2.37 cpuid_mask_edx

### 1.2.38 cpuid_mask_ext_ecx

### 1.2.39 cpuid_mask_ext_edx

### 1.2.40 cpuid_mask_l7s0_eax

### 1.2.41 cpuid_mask_l7s0_ebx

### 1.2.42 cpuid_mask_thermal_ecx

### 1.2.43 cpuid_mask_xsave_eax

> `= <integer>`

> Applicability: x86. Default: `~0` (all bits set)

The availability of these options are model specific. Some processors don’t support any of them, and no processor supports all of them. Xen will ignore options on processors which are lacking support.

These options can be used to alter the features visible via the `CPUID` instruction. Settings applied here take effect globally, including for Xen and all guests.

Note: Since Xen 4.7, it is no longer necessary to mask a host to create migration safety in heterogeneous scenarios. All necessary CPUID settings should be provided in the VM configuration file. Furthermore, it is recommended not to use this option, as doing so causes an unnecessary reduction of features at Xen’s disposal to manage guests.

### 1.2.44 cpuidle (x86)

> `= <boolean>`

### 1.2.45 cpuinfo (x86)

> `= <boolean>`

### 1.2.46 crash-debug-debugkey

### 1.2.47 crash-debug-hwdom

### 1.2.48 crash-debug-kexeccmd

### 1.2.49 crash-debug-panic

### 1.2.50 crash-debug-watchdog

> `= <string>`

> Can be modified at runtime

Specify debug-key actions in cases of crashes. Each of the parameters applies to a different crash reason. The `<string>` is a sequence of debug key characters, with `+` having the special meaning of a 10 millisecond pause.

`crash-debug-debugkey` will be used for crashes induced by the `C` debug key (i.e. manually induced crash).

`crash-debug-hwdom` denotes a crash of dom0.

`crash-debug-kexeccmd` is an explicit request of dom0 to continue with the kdump kernel via kexec. Only available on hypervisors built with CONFIG_KEXEC.

`crash-debug-panic` is a crash of the hypervisor.

`crash-debug-watchdog` is a crash due to the watchdog timer expiring.

It should be noted that dumping diagnosis data to the console can fail in multiple ways (missing data, hanging system, …) depending on the reason of the crash, which might have left the hypervisor in a bad state. In case a debug-key action leads to another crash recursion will be avoided, so no additional debug-key actions will be performed in this case. A crash in the early boot phase will not result in any debug-key action, as the system might not yet be in a state where the handlers can work.

So e.g. `crash-debug-watchdog=0+0r` would dump dom0 state twice with 10 milliseconds between the two state dumps, followed by the run queues of the hypervisor, if the system crashes due to a watchdog timeout.

Depending on the reason of the system crash it might happen that triggering some debug key action will result in a hang instead of dumping data and then doing a reboot or crash dump.

### 1.2.51 crashinfo_maxaddr

> `= <size>`

> Default: `4G`

Specify the maximum address to allocate certain structures, if used in combination with the low_crashinfo command line option.

### 1.2.52 crashkernel

> `= <ramsize-range>:<size>[,...][{@,<}<offset>]` `= <size>[{@,<}<offset>]` `= <size>,below=offset`

Specify sizes and optionally placement of the crash kernel reservation area. The `<ramsize-range>:<size>` pairs indicate how much memory to set aside for a crash kernel (`<size>`) for a given range of installed RAM (`<ramsize-range>`). Each `<ramsize-range>` is of the form `<start>-[<end>]`.

A trailing `@<offset>` specifies the exact address this area should be placed at, whereas `<` in place of `@` just specifies an upper bound of the address range the area should fall into.

< and below are synonyomous, the latter being useful for grub2 systems which would otherwise require escaping of the < option

### 1.2.53 credit2_balance_over

> `= <integer>`

### 1.2.54 credit2_balance_under

> `= <integer>`

### 1.2.55 credit2_cap_period_ms

> `= <integer>`

> Default: `10`

Domains subject to a cap receive a replenishment of their runtime budget once every cap period interval. Default is 10 ms. The amount of budget they receive depends on their cap. For instance, a domain with a 50% cap will receive 50% of 10 ms, so 5 ms.

### 1.2.56 credit2_load_precision_shift

> `= <integer>`

> Default: `18`

Specify the number of bits to use for the fractional part of the values involved in Credit2 load tracking and load balancing math.

### 1.2.57 credit2_load_window_shift

> `= <integer>`

> Default: `30`

Specify the number of bits to use to represent the length of the window (in nanoseconds) we use for load tracking inside Credit2. This means that, with the default value (30), we use 2^30 nsec ~= 1 sec long window.

Load tracking is done by means of a variation of exponentially weighted moving average (EWMA). The window length defined here is what tells for how long we give value to previous history of the load itself. In fact, after a full window has passed, what happens is that we discard all previous history entirely.

A short window will make the load balancer quick at reacting to load changes, but also short-sighted about previous history (and hence, e.g., long term load trends). A long window will make the load balancer thoughtful of previous history (and hence capable of capturing, e.g., long term load trends), but also slow in responding to load changes.

The default value of `1 sec` is rather long.

### 1.2.58 credit2_runqueue

> `= cpu | core | socket | node | all`

> Default: `socket`

Specify how host CPUs are arranged in runqueues. Runqueues are kept balanced with respect to the load generated by the vCPUs running on them. Smaller runqueues (as in with `core`) means more accurate load balancing (for instance, it will deal better with hyperthreading), but also more overhead.

Available alternatives, with their meaning, are: * `cpu`: one runqueue per each logical pCPUs of the host; * `core`: one runqueue per each physical core of the host; * `socket`: one runqueue per each physical socket (which often, but not always, matches a NUMA node) of the host; * `node`: one runqueue per each NUMA node of the host; * `all`: just one runqueue shared by all the logical pCPUs of the host

### 1.2.59 dbgp

> `= ehci[ <integer> | @pci<bus>:<slot>.<func> ]`

Specify the USB controller to use, either by instance number (when going over the PCI busses sequentially) or by PCI device (must be on segment 0).

### 1.2.60 debug_stack_lines

> `= <integer>`

> Default: `20`

Limits the number lines printed in Xen stack traces.

### 1.2.61 debugtrace

> `= [cpu:]<size>`

> Default: `128`

Specify the size of the console debug trace buffer. By specifying `cpu:` additionally a trace buffer of the specified size is allocated per cpu. The debug trace feature is only enabled in debugging builds of Xen.

### 1.2.62 dma_bits

> `= <integer>`

Specify the bit width of the DMA heap.

### 1.2.63 dom0

    = List of [ pv | pvh, shadow=<bool>, verbose=<bool>,
                cpuid-faulting=<bool>, msr-relaxed=<bool> ]

    Applicability: x86

Controls for how dom0 is constructed on x86 systems.

- The `pv` and `pvh` options select the virtualisation mode of dom0.

  The `pv` option is only available when `CONFIG_PV` is compiled in. The `pvh` option is only available when `CONFIG_HVM` is compiled in. When both options are compiled in, the default is PV.

  In addition, the following requirements must be met:

  - The dom0 kernel selected by the boot loader must be capable of the selected mode.
  - For a PVH dom0, the hardware must have VT-x/SVM extensions available.

- The `shadow` boolean allows dom0 to be explicitly constructed using shadow paging. This option is unavailable when `CONFIG_SHADOW_PAGING` is disabled.

  For PVH, dom0 defaults to using HAP on capable hardware, and falls back to shadow paging otherwise. A PVH dom0 cannot be used if Xen is compiled without shadow paging support, and the hardware lacks HAP support.

  For PV, the use of dom0 shadow mode is only for development purposes. PV guests do no require any paging support by default.

- The `verbose` boolean is intended for diagnostics, and prints out extra information during the dom0 build. It defaults to the compile time choice of `CONFIG_VERBOSE_DEBUG`.

- The `cpuid-faulting` boolean is an interim option, is only applicable to PV dom0, and defaults to true.

  Before Xen 4.13, the domain builder logic for guest construction depended on seeing host CPUID values to function correctly. As a result, CPUID Faulting was never activated for PV dom0’s, even on capable hardware.

  In Xen 4.13, the domain builder logic has been fixed, and no longer has this dependency. As a consequence, CPUID Faulting is activated by default even for PV dom0’s.

  However, as PV dom0’s have always seen host CPUID data in the past, there is a chance that further dependencies exist. This boolean can be used to restore the pre-4.13 behaviour. If specifying `no-cpuid-faulting` fixes an issue in dom0, please report a bug.

- The `msr-relaxed` boolean is an interim option, and defaults to false.

  In Xen 4.15, the default behaviour for unhandled MSRs has been changed, to avoid leaking host data into guests, and to avoid breaking guest logic which uses #GP probing to identify the availability of MSRs.

  However, this new stricter behaviour has the possibility to break guests, and a more 4.14-like behaviour can be selected by specifying `dom0=msr-relaxed`.

  If using this option is necessary to fix an issue, please report a bug.

### 1.2.64 dom0-iommu

    = List of [ passthrough=<bool>, strict=<bool>, map-inclusive=<bool>,
                map-reserved=<bool>, none ]

Controls for the dom0 IOMMU setup.

- The `passthrough` boolean controls whether IOMMU translation functionality is disabled for devices in dom0 (`passthrough=1`) or whether the IOMMU is used to ensure that dom0 can only DMA to its permitted areas of RAM (`passthrough=0`).

  This option is only applicable to x86 PV dom0’s, and defaults to false.

  Some older Intel VT-d hardware isn’t capable of disabling translation functionality on a per-device basis, and will cause this option to be ignored and assumed to be 0. Similar behaviour on such systems is only available by fully disabling all IOMMUs.

  This option is hardwired to false for x86 PVH dom0’s (where a non-identity transform is required for dom0 to function), and is ignored for ARM.

- The `strict` boolean is applicable to x86 PV dom0’s only and defaults to false. It controls whether dom0 can have IOMMU mappings for all domain RAM in the system, or only for its allocated RAM (and grant mappings etc.)

  This option is hardwired to true for x86 PVH dom0’s (as RAM belonging to other domains in the system don’t live in a compatible address space), and is ignored for ARM.

- The `map-inclusive` boolean is applicable to x86 PV dom0’s, and sets up identity IOMMU mappings for all non-RAM regions below 4GB except for unusable ranges, and ranges belonging to Xen.

  Typically, some devices in a system use bits of RAM for communication, and these areas should be listed as reserved in the E820 table and identified via RMRR or IVMD entries in the ACPI tables, so Xen can ensure that they are identity-mapped in the IOMMU. However, some firmware makes mistakes, and this option is a coarse-grain workaround for those errors.

  Where possible, finer grain corrections should be made with the `rmrr=`, `ivmd=`, `ivrs_hpet[]=`, or `ivrs_ioapic[]=` command line options.

  This option is disabled by default, and deprecated and intended for removal in future versions of Xen. If specifying `map-inclusive` is the only way to make your system boot, please report a bug.

- The `map-reserved` functionality is very similar to `map-inclusive`.

  The differences from `map-inclusive` are that `map-reserved` is applicable to both x86 PV and PVH dom0’s, is enabled by default, and represents a subset of the correction by only mapping reserved memory regions rather than all non-RAM regions.

- The `none` option is intended for development purposes only, and skips certain safety checks pertaining to the correct IOMMU configuration for dom0 to boot.

  Incorrect use of this option may result in a malfunctioning system.

### 1.2.65 dom0_ioports_disable (x86)

> `= List of <hex>-<hex>`

Specify a list of IO ports to be excluded from dom0 access.

### 1.2.66 dom0_max_vcpus

Either:

> `= <integer>`.

The number of VCPUs to give to dom0. This number of VCPUs can be more than the number of PCPUs on the host. The default is the number of PCPUs.

Or:

> `= <min>-<max>` where `<min>` and `<max>` are integers.

Gives dom0 a number of VCPUs equal to the number of PCPUs, but always at least `<min>` and no more than `<max>`. Using `<min>` may give more VCPUs than PCPUs. `<min>` or `<max>` may be omitted and the defaults of 1 and unlimited respectively are used instead.

For example, with `dom0_max_vcpus=4-8`:

>        Number of
>     PCPUs | Dom0 VCPUs
>      2    |  4
>      4    |  4
>      6    |  6
>      8    |  8
>     10    |  8

### 1.2.67 dom0_mem (ARM)

> `= <size>`

Set the amount of memory for the initial domain (dom0). It must be greater than zero. This parameter is required.

### 1.2.68 dom0_mem (x86)

> `= List of ( min:<sz> | max:<sz> | <sz> )`

Set the amount of memory for the initial domain (dom0). If a size is positive, it represents an absolute value. If a size is negative, it is subtracted from the total available memory.

- `<sz>` specifies the exact amount of memory.
- `min:<sz>` specifies the minimum amount of memory.
- `max:<sz>` specifies the maximum amount of memory.

If `<sz>` is not specified, the default is all the available memory minus some reserve. The reserve is 1/16 of the available memory or 128 MB (whichever is smaller).

The amount of memory will be at least the minimum but never more than the maximum (i.e., `max` overrides the `min` option). If there isn’t enough memory then as much as possible is allocated.

`max:<sz>` also sets the maximum reservation (the maximum amount of memory dom0 can balloon up to). If this is omitted then the maximum reservation is unlimited.

For example, to set dom0’s initial memory allocation to 512MB but allow it to balloon up as far as 1GB use `dom0_mem=512M,max:1G`

> `<sz>` is: `<size> | [<size>+]<frac>%` `<frac>` is an integer < 100

- `<frac>` specifies a fraction of host memory size in percent.

So `<sz>` being `1G+25%` on a 256 GB host would result in 65 GB.

If you use this option then it is highly recommended that you disable any dom0 autoballooning feature present in your toolstack. See the xl.conf(5) man page or Xen Best Practices.

This option doesn’t have effect if pv-shim mode is enabled.

### 1.2.69 dom0_nodes (x86)

> `= List of [ <integer> | relaxed | strict ]`

> Default: `strict`

Specify the NUMA nodes to place Dom0 on. Defaults for vCPU-s created and memory assigned to Dom0 will be adjusted to match the node restrictions set up here. Note that the values to be specified here are ACPI PXM ones, not Xen internal node numbers. `relaxed` sets up vCPU affinities to prefer but be not limited to the specified node(s).

### 1.2.70 dom0_vcpus_pin

> `= <boolean>`

> Default: `false`

Pin dom0 vcpus to their respective pcpus

### 1.2.71 dtuart (ARM)

> `= path [:options]`

> Default: `""`

Specify the full path in the device tree for the UART. If the path doesn’t start with `/`, it is assumed to be an alias. The options are device specific.

### 1.2.72 e820-mtrr-clip (x86)

> `= <boolean>`

Flag that specifies if RAM should be clipped to the highest cacheable MTRR.

> Default: `true` on Intel CPUs, otherwise `false`

### 1.2.73 e820-verbose (x86)

> `= <boolean>`

> Default: `false`

Flag that enables verbose output when processing e820 information and applying clipping.

### 1.2.74 edd (x86)

> `= off | on | skipmbr`

Control retrieval of Extended Disc Data (EDD) from the BIOS during boot.

### 1.2.75 edid (x86)

> `= no | force`

Either force retrieval of monitor EDID information via VESA DDC, or disable it (edid=no). This option should not normally be required except for debugging purposes.

### 1.2.76 efi

    = List of [ rs=<bool>, attr=no|uc ]

Controls for interacting with the system Extended Firmware Interface.

- The `rs` boolean controls whether Runtime Services are used. By default, Xen uses Runtime Services itself, and proxies certain calls on behalf of dom0. Selecting `rs=0` prohibits all use of Runtime Services.

- The `attr=` string exists to specify what to do with memory regions of unknown/unrecognised cacheability. `attr=no` is the default and will leave the memory regions unmapped, while `attr=uc` will map them as fully uncacheable.

### 1.2.77 ept

> `= List of [ ad=<bool>, pml=<bool>, exec-sp=<bool> ]`

> Applicability: Intel

Extended Page Tables are a feature of Intel’s VT-x technology, whereby hardware manages the virtualisation of HVM guest pagetables. EPT was introduced with the Nehalem architecture.

- The `ad` boolean controls hardware tracking of Access and Dirty bits in the EPT pagetables, and was first introduced in Broadwell Server.

  By default, Xen will use A/D tracking when available in hardware, except on Avoton processors affected by erratum AVR41. Explicitly choosing `ad=0` will disable the use of A/D tracking on capable hardware, whereas choosing `ad=1` will cause tracking to be used even on AVR41-affected hardware.

- The `pml` boolean controls the use of Page Modification Logging, which is also introduced in Broadwell Server.

  PML is a feature whereby the processor generates a list of pages which have been dirtied. This is necessary information for operations such as live migration, and having the processor maintain the list of dirtied pages is more efficient than traditional software implementations where all guest writes trap into Xen so the dirty bitmap can be maintained.

  By default, Xen will use PML when it is available in hardware. PML functionally depends on A/D tracking, so choosing `ad=0` will implicitly disable PML. `pml=0` can be used to prevent the use of PML on otherwise capable hardware.

- The `exec-sp` boolean controls whether EPT superpages with execute permissions are permitted. In general this is good for performance.

  However, on processors vulnerable CVE-2018-12207, HVM guest kernels can use executable superpages to crash the host. By default, executable superpages are disabled on affected hardware.

  If HVM guest kernels are trusted not to mount a DoS against the system, this option can enabled to regain performance.

  This boolean may be modified at runtime using `xl set-parameters ept=[no-]exec-sp` to switch between fast and secure.

  - When switching from secure to fast, preexisting HVM domains will run at their current performance until they are rebooted; new domains will run without any overhead.

  - When switching from fast to secure, all HVM domains will immediately suffer a performance penalty.

  Warning: No guarantee is made that this runtime option will be retained indefinitely, or that it will retain this exact behaviour. It is intended as an emergency option for people who first chose fast, then change their minds to secure, and wish not to reboot.

### 1.2.78 extra_guest_irqs

> `= [<domU number>][,<dom0 number>]`

> Default: `32,<variable>`

Change the number of PIRQs available for guests. The optional first number is common for all domUs, while the optional second number (preceded by a comma) is for dom0. Changing the setting for domU has no impact on dom0 and vice versa. For example to change dom0 without changing domU, use `extra_guest_irqs=,512`. The default value for Dom0 and an eventual separate hardware domain is architecture dependent. Note that specifying zero as domU value means zero, while for dom0 it means to use the default.

### 1.2.79 ext_regions (Arm)

> `= <boolean>`

> Default : `true`

Flag to enable or disable support for extended regions for Dom0.

Extended regions are ranges of unused address space exposed to Dom0 as “safe to use” for special memory mappings. Disable if your board device tree is incomplete.

### 1.2.80 flask

> `= permissive | enforcing | late | disabled`

> Default: `enforcing`

Specify how the FLASK security server should be configured. This option is only available if the hypervisor was compiled with FLASK support. This can be enabled by running either: - make -C xen config and enabling XSM and FLASK. - make -C xen menuconfig and enabling ‘FLux Advanced Security Kernel support’ and ‘Xen Security Modules support’

- `permissive`: This is intended for development and is not suitable for use with untrusted guests. If a policy is provided by the bootloader, it will be loaded; errors will be reported to the ring buffer but will not prevent booting. The policy can be changed to enforcing mode using “xl setenforce”.
- `enforcing`: This will cause the security server to enter enforcing mode prior to the creation of domain 0. If an valid policy is not provided by the bootloader and no built-in policy is present, the hypervisor will not continue booting.
- `late`: This disables loading of the built-in security policy or the policy provided by the bootloader. FLASK will be enabled but will not enforce access controls until a policy is loaded by a domain using “xl loadpolicy”. Once a policy is loaded, FLASK will run in enforcing mode unless “xl setenforce” has changed that setting.
- `disabled`: This causes the XSM framework to revert to the dummy module. The dummy module provides the same security policy as is used when compiling the hypervisor without support for XSM. The xsm_op hypercall can also be used to switch to this mode after boot, but there is no way to re-enable FLASK once the dummy module is loaded.

### 1.2.81 font

> `= <height>` where height is `8x8 | 8x14 | 8x16`

Specify the font size when using the VESA console driver.

### 1.2.82 force-ept (Intel)

> `= <boolean>`

> Default: `false`

Allow EPT to be enabled when VMX feature `VM_ENTRY_LOAD_GUEST_PAT` is not present.

Warning: Due to CVE-2013-2212, VMX feature `VM_ENTRY_LOAD_GUEST_PAT` is by default required as a prerequisite for using EPT. If you are not using PCI Passthrough, or trust the guest administrator who would be using passthrough, then the requirement can be relaxed. This option is particularly useful for nested virtualization, to allow the L1 hypervisor to use EPT even if the L0 hypervisor does not provide `VM_ENTRY_LOAD_GUEST_PAT`.

### 1.2.83 gdb

> `= com1[H,L] | com2[H,L] | dbgp`

> Default: ``

Specify which console gdbstub should use. See console.

### 1.2.84 gnttab

> `= List of [ max-ver:<integer>, transitive=<bool> ]`

> Default: `gnttab=max-ver:2,transitive`

Control various aspects of the grant table behaviour available to guests.

- `max-ver` Select the maximum grant table version to offer to guests. Valid version are 1 and 2.
- `transitive` Permit or disallow the use of transitive grants. Note that the use of grant table v2 without transitive grants is an ABI breakage from the guests point of view.

The usage of gnttab v2 is not security supported on ARM platforms.

### 1.2.85 gnttab_max_frames

> `= <integer>`

> Default: `64`

> Can be modified at runtime

Specify the maximum number of frames which any domain may use as part of its grant table. This value is an upper boundary of the per-domain value settable via Xen tools.

Dom0 is using this value for sizing its grant table.

### 1.2.86 gnttab_max_maptrack_frames

> `= <integer>`

> Default: `1024`

> Can be modified at runtime

Specify the maximum number of frames to use as part of a domains maptrack array. This value is an upper boundary of the per-domain value settable via Xen tools.

Dom0 is using this value for sizing its maptrack table.

### 1.2.87 global-pages

    = <boolean>

    Applicability: x86
    Default: true unless running virtualized on AMD or Hygon hardware

Control whether to use global pages for PV guests, and thus the need to perform TLB flushes by writing to CR4. This is a performance trade-off.

AMD SVM does not support selective trapping of CR4 writes, which means that a global TLB flush (two CR4 writes) takes two VMExits, and massively outweigh the benefit of using global pages to begin with. This case is easy for Xen to spot, and is accounted for in the default setting.

Other cases where this option might be a benefit is on VT-x hardware when selective CR4 writes are not supported/enabled by the hypervisor, or in any virtualised case using shadow paging. These are not easy for Xen to spot, so are not accounted for in the default setting.

### 1.2.88 guest_loglvl

> `= <level>[/<rate-limited level>]` where level is `none | error | warning | info | debug | all`

> Default: `guest_loglvl=none/warning`

> Can be modified at runtime

Set the logging level for Xen guests. Any log message with equal more more importance will be printed.

The optional `<rate-limited level>` option instructs which severities should be rate limited.

### 1.2.89 hap (x86)

> `= <boolean>`

> Default: `true`

Flag to globally enable or disable support for Hardware Assisted Paging (HAP)

### 1.2.90 hap_1gb (x86)

> `= <boolean>`

> Default: `true`

Flag to enable 1 GB host page table support for Hardware Assisted Paging (HAP).

### 1.2.91 hap_2mb (x86)

> `= <boolean>`

> Default: `true`

Flag to enable 2 MB host page table support for Hardware Assisted Paging (HAP).

### 1.2.92 hardware_dom

> `= <domid>`

> Default: `0`

Enable late hardware domain creation using the specified domain ID. This is intended to be used when domain 0 is a stub domain which builds a disaggregated system including a hardware domain with the specified domain ID. This option is supported only when compiled with XSM on x86.

### 1.2.93 hest_disable

> `= <boolean>`

> Default: `false`

Control Xens use of the APEI Hardware Error Source Table, should one be found.

### 1.2.94 highmem-start (x86)

> `= <size>`

Specify the memory boundary past which memory will be treated as highmem (x86 debug hypervisor only).

### 1.2.95 hmp-unsafe (arm)

> `= <boolean>`

> Default : `false`

Say yes at your own risk if you want to enable heterogenous computing (such as big.LITTLE). This may result to an unstable and insecure platform, unless you manually specify the cpu affinity of all domains so that all vcpus are scheduled on the same class of pcpus (big or LITTLE but not both). vcpu migration between big cores and LITTLE cores is not supported. See docs/misc/arm/big.LITTLE.txt for more information.

When the hmp-unsafe option is disabled (default), CPUs that are not identical to the boot CPU will be parked and not used by Xen.

### 1.2.96 hpet

    = List of [ <bool> | broadcast=<bool> | legacy-replacement=<bool> ]

    Applicability: x86

Controls Xen’s use of the system’s High Precision Event Timer. By default, Xen will use an HPET when available and not subject to errata. Use of the HPET can be disabled by specifying `hpet=0`.

- The `broadcast` boolean is disabled by default, but forces Xen to keep using the broadcast for CPUs in deep C-states even when an RTC interrupt is enabled. This then also affects raising of the RTC interrupt.

- The `legacy-replacement` boolean allows for control over whether Legacy Replacement mode is enabled.

  Legacy Replacement mode is intended for hardware which does not have an 8254 PIT, and allows the HPET to be configured into a compatible mode. Intel chipsets from Skylake/ApolloLake onwards can turn the PIT off for power saving reasons, and there is no platform-agnostic mechanism for discovering this.

  By default, Xen will not change hardware configuration, unless the PIT appears to be absent, at which point Xen will try to enable Legacy Replacement mode before falling back to pre-IO-APIC interrupt routing options.

  This behaviour can be inhibited by specifying `legacy-replacement=0`. Alternatively, this mode can be enabled unconditionally (if available) by specifying `legacy-replacement=1`.

### 1.2.97 hpetbroadcast (x86)

> `= <boolean>`

Deprecated alternative of `hpet=broadcast`.

### 1.2.98 hvm_debug (x86)

> `= <integer>`

The specified value is a bit mask with the individual bits having the following meaning:

>     Bit  0 - debug level 0 (unused at present)
>     Bit  1 - debug level 1 (Control Register logging)
>     Bit  2 - debug level 2 (VMX logging of MSR restores when context switching)
>     Bit  3 - debug level 3 (unused at present)
>     Bit  4 - I/O operation logging
>     Bit  5 - vMMU logging
>     Bit  6 - vLAPIC general logging
>     Bit  7 - vLAPIC timer logging
>     Bit  8 - vLAPIC interrupt logging
>     Bit  9 - vIOAPIC logging
>     Bit 10 - hypercall logging
>     Bit 11 - MSR operation logging

Recognized in debug builds of the hypervisor only.

### 1.2.99 hvm_fep (x86)

> `= <boolean>`

> Default: `false`

Allow use of the Forced Emulation Prefix in HVM guests, to allow emulation of arbitrary instructions.

This option is intended for development and testing purposes.

Warning As this feature opens up the instruction emulator to arbitrary instruction from an HVM guest, don’t use this in production system. No security support is provided when this flag is set.

### 1.2.100 hvm_port80 (x86)

> `= <boolean>`

> Default: `true`

Specify whether guests are to be given access to physical port 80 (often used for debugging purposes), to override the DMI based detection of systems known to misbehave upon accesses to that port.

### 1.2.101 idle_latency_factor (x86)

> `= <integer>`

### 1.2.102 ioapic_ack (x86)

> `= old | new`

> Default: `new` unless directed-EOI is supported

### 1.2.103 iommu

    = List of [ <bool>, verbose, debug, force, required, quarantine[=scratch-page],
                sharept, intremap, intpost, crash-disable,
                snoop, qinval, igfx, amd-iommu-perdev-intremap,
                dom0-{passthrough,strict} ]

    All sub-options are boolean in nature.

I/O Memory Memory Units perform a function similar to the CPU MMU (hence the name), but typically exist as a discrete device, integrated as part of a PCI Root Complex. The most common configuration is to have one IOMMU per package (for on-die PCIe devices and directly attached PCIe lanes), and one IOMMU covering the remaining I/O in the system.

The functionality in an IOMMU commonly falls into two orthogonal categories:

1. DMA remapping which uses a pagetable-like hierarchical structure and maps I/O Virtual Addresses (DFNs - Device Frame Numbers in Xen’s terminology) to System Physical Addresses (MFNs - Machine Frame Numbers in Xen’s terminology).

2. Interrupt Remapping, which controls incoming Message Signalled Interrupt requests, including their routing to specific CPUs.

IOMMU functionality can be used to provide a translation which the hardware device driver isn’t aware of (e.g. PCI Passthrough and a native driver inside the guest) and/or to enforce fine-grained control over the memory and interrupts which a device is attempting to access.

By default, IOMMUs are configured for use if they are available. An overall boolean (e.g. `iommu=no`) can override this and leave the IOMMUs disabled.

- The `verbose` and `debug` booleans can be used to print additional diagnostic information. Neither are active by default.

- The `force` and `required` booleans are synonymous and, when requested, will prevent Xen from booting if IOMMUs aren’t discovered and enabled successfully.

- The `quarantine` option can be used to control Xen’s behavior when de-assigning devices from guests.

  When a PCI device is assigned to an untrusted domain, it is possible for that domain to program the device to DMA to an arbitrary address. The IOMMU is used to protect the host from malicious DMA by making sure that the device addresses can only target memory assigned to the guest. However, when the guest domain is torn down, assigning the device back to the hardware domain would allow any in-flight DMA to potentially target critical host data. To avoid this, quarantining should be enabled. Quarantining can be done in two ways: In its basic form, all in-flight DMA will simply be forced to encounter IOMMU faults. Since there are systems where doing so can cause host lockup, an alternative form is available where writes to memory will be made fault, but reads will be directed to a scratch page. The implication here is that such reads will go unnoticed, i.e. an admin may not become aware of the underlying problem.

  Therefore, if this option is set to true (the default), Xen always quarantines such devices; they must be explicitly assigned back to Dom0 before they can be used there again. If set to “scratch-page”, still active DMA reads will additionally be directed to a “scratch” page. If set to false, Xen will only quarantine devices the toolstack has arranged for getting quarantined, and only in the “basic” form.

  This option is only valid on builds supporting PCI.

- The `sharept` boolean controls whether the IOMMU pagetables are shared with the CPU-side HAP pagetables, or allocated separately. Sharing reduces the memory overhead, but doesn’t work in combination with CPU-side pagefault-based features, e.g. dirty VRAM tracking when a PCI device is assigned.

  Due to implementation choices, sharing pagetables doesn’t work on AMD hardware, and this option is ignored. It is enabled by default on Intel systems.

  This option is ignored on ARM, and the pagetables are always shared.

- The `intremap` boolean controls the Interrupt Remapping sub-feature, and is active by default on compatible hardware. On x86 systems, the first generation of IOMMUs only supported DMA remapping, and Interrupt Remapping appeared in the second generation.

  This option is only valid on x86.

- The `intpost` boolean controls the Posted Interrupt sub-feature. In combination with APIC acceleration (VT-x APICV, SVM AVIC), the IOMMU can be configured to deliver interrupts from assigned PCI devices directly into the guest, without trapping out into hypervisor context.

  This option depends on `intremap`, and is disabled by default due to some corner cases in the implementation which have yet to be resolved.

  This option is only valid on x86, and only builds of Xen with HVM support.

- The `crash-disable` boolean controls disabling IOMMU functionality (DMAR/IR/QI) before switching to a crash kernel. This option is inactive by default and is for compatibility with older kdump kernels only. Modern kernels copy all the necessary tables from the previous one following kexec which makes the transition transparent for them with IOMMU functions still on.

The following options are specific to Intel VT-d hardware:

- The `snoop` boolean controls the Snoop Control sub-feature, and is active by default on compatible hardware.

  An incoming DMA request may specify Snooped (query the CPU caches for the appropriate lines) or Non-Snooped (don’t query the CPU caches). Non-Snooped accesses incur less latency, but behind-the-scenes hypervisor activity can invalidate the expectations of the device driver, and Snoop Control allows the hypervisor to force DMA requests to be Snooped when they would otherwise not be.

- The `qinval` boolean controls the Queued Invalidation sub-feature, and is active by default on compatible hardware. Queued Invalidation is a feature in second-generation IOMMUs and is a functional prerequisite for Interrupt Remapping. Note that Xen disregards this setting for Intel VT-d version 6 and greater as Registered-Based Invalidation isn’t supported by them.

- The `igfx` boolean is active by default, and controls whether IOMMUs in front of solely graphics devices get enabled or not.

  It is intended as a debugging mechanism for graphics issues, and to be similar to Linux’s `intel_iommu=igfx_off` option. If specifying `no-igfx` fixes anything, please report the problem.

The following options are specific to AMD-Vi hardware:

- The `amd-iommu-perdev-intremap` boolean controls whether the interrupt remapping table is per device (the default), or a single global table for the entire system.

  Using a global table is not security supported as it allows all devices to impersonate each other as far as interrupts as concerned (see XSA-36), but it is a workaround for SP5100 Erratum 28.

WARNING: The `dom0-passthrough` and `dom0-strict` booleans are both deprecated, and superseded by dom0-iommu={passthrough,strict} respectively - using both the old and new command line options in combination is undefined.

### 1.2.104 iommu_dev_iotlb_timeout

> `= <integer>`

> Default: `1000`

Specify the timeout of the device IOTLB invalidation in milliseconds. By default, the timeout is 1000 ms. When you see error ‘Queue invalidate wait descriptor timed out’, try increasing this value.

### 1.2.105 iommu_inclusive_mapping

> `= <boolean>`

WARNING: This command line option is deprecated, and superseded by dom0-iommu=map-inclusive - using both options in combination is undefined.

### 1.2.106 irq_ratelimit (x86)

> `= <integer>`

### 1.2.107 irq_vector_map (x86)

### 1.2.108 ivmd (x86)

> `= <start>[-<end>][=<bdf1>[-<bdf1'>][,<bdf2>[-<bdf2'>][,...]]][;<start>...]`

Define IVMD-like ranges that are missing from ACPI tables along with the device(s) they belong to, and use them for 1:1 mapping. End addresses can be omitted when exactly one page is meant. The ranges are inclusive when start and end are specified. Note that only PCI segment 0 is supported at this time, but it is fine to specify it explicitly.

‘start’ and ‘end’ values are page numbers (not full physical addresses), in hexadecimal format (can optionally be preceded by “0x”).

Omitting the optional (range of) BDF spcifiers signals that the range is to be applied to all devices.

Usage example: If device 0:0:1d.0 requires one page (0xd5d45) to be reserved, and devices 0:0:1a.0…0:0:1a.3 collectively require three pages (0xd5d46 thru 0xd5d48) to be reserved, one usage would be:

ivmd=d5d45=0:1d.0;0xd5d46-0xd5d48=0:1a.0-0:1a.3

Note: grub2 requires to escape or quote special characters, like ‘;’ when multiple ranges are specified - refer to the grub2 documentation.

### 1.2.109 ivrs_hpet[`<hpet>`] (AMD)

> `=[<seg>:]<bus>:<device>.<func>`

Force the use of `[<seg>:]<bus>:<device>.<func>` as device ID of HPET `<hpet>` instead of the one specified by the IVHD sub-tables of the IVRS ACPI table.

### 1.2.110 ivrs_ioapic[`<ioapic>`] (AMD)

> `=[<seg>:]<bus>:<device>.<func>`

Force the use of `[<seg>:]<bus>:<device>.<func>` as device ID of IO-APIC `<ioapic>` instead of the one specified by the IVHD sub-tables of the IVRS ACPI table.

### 1.2.111 lapic (x86)

> `= <boolean>`

Force the use of use of the local APIC on a uniprocessor system, even if left disabled by the BIOS.

### 1.2.112 lapic_timer_c2_ok (x86)

> `= <boolean>`

### 1.2.113 ler (x86)

> `= <boolean>`

> Default: false

This option is intended for debugging purposes only. Enable MSR_DEBUGCTL.LBR in hypervisor context to be able to dump the Last Interrupt/Exception To/From record with other registers.

### 1.2.114 loglvl

> `= <level>[/<rate-limited level>]` where level is `none | error | warning | info | debug | all`

> Default: `loglvl=warning`

> Can be modified at runtime

Set the logging level for Xen. Any log message with equal more more importance will be printed.

The optional `<rate-limited level>` option instructs which severities should be rate limited.

### 1.2.115 low_crashinfo

> `= none | min | all`

> Default: `none` if not specified at all, or to `min` if low_crashinfo is present without qualification.

This option is only useful for hosts with a 32bit dom0 kernel, wishing to use kexec functionality in the case of a crash. It represents which data structures should be deliberately allocated in low memory, so the crash kernel may find find them. Should be used in combination with crashinfo_maxaddr.

### 1.2.116 low_mem_virq_limit

> `= <size>`

> Default: `64M`

Specify the threshold below which Xen will inform dom0 that the quantity of free memory is getting low. Specifying `0` will disable this notification.

### 1.2.117 maxcpus (x86)

> `= <integer>`

Specify the maximum number of CPUs that should be brought up.

This option is ignored in pv-shim mode.

### 1.2.118 max_cstate (x86)

> `= <integer>[,<integer>]`

Specify the deepest C-state CPUs are permitted to be placed in, and optionally the maximum sub C-state to be used used. The latter only applies to the highest permitted C-state.

### 1.2.119 max_gsi_irqs (x86)

> `= <integer>`

Specifies the number of interrupts to be use for pin (IO-APIC or legacy PIC) based interrupts. Any higher IRQs will be available for use via PCI MSI.

### 1.2.120 max_lpi_bits (arm)

> `= <integer>`

Specifies the number of ARM GICv3 LPI interrupts to allocate on the host, presented as the number of bits needed to encode it. This must be at least 14 and not exceed 32, and each LPI requires one byte (configuration) and one pending bit to be allocated. Defaults to 20 bits (to cover at most 1048576 interrupts).

### 1.2.121 mce (x86)

> `= <integer>`

### 1.2.122 mce_fb (Intel)

> `= <integer>`

### 1.2.123 mce_verbosity (x86)

> `= verbose`

Specify verbose machine check output.

### 1.2.124 mem (x86)

> `= <size>`

Specify the maximum address of physical RAM. Any RAM beyond this limit is ignored by Xen.

### 1.2.125 memop-max-order

> `= [<domU>][,[<ctldom>][,[<hwdom>][,<ptdom>]]]`

> x86 default: `9,18,12,12` ARM default: `9,18,10,10`

Change the maximum order permitted for allocation (or allocation-like) requests issued by the various kinds of domains (in this order: ordinary DomU, control domain, hardware domain, and - when supported by the platform - DomU with pass-through device assigned).

### 1.2.126 mmcfg (x86)

> `= <boolean>[,amd-fam10]`

> Default: `1`

Specify if the MMConfig space should be enabled.

### 1.2.127 mmio-relax (x86)

> `= <boolean> | all`

> Default: `false`

By default, domains may not create cached mappings to MMIO regions. This option relaxes the check for Domain 0 (or when using `all`, all PV domains), to permit the use of cacheable MMIO mappings.

### 1.2.128 msi (x86)

> `= <boolean>`

> Default: `true`

Force Xen to (not) use PCI-MSI, even if ACPI FADT says otherwise.

### 1.2.129 mtrr.show (x86)

> `= <boolean>`

> Default: `false`

Print boot time MTRR state.

### 1.2.130 mwait-idle (x86)

> `= <boolean>`

> Default: `true`

Use the MWAIT idle driver (with model specific C-state knowledge) instead of the ACPI based one.

### 1.2.131 nmi (x86)

> `= ignore | dom0 | fatal`

> Default: `fatal` for a debug build, or `dom0` for a non-debug build

Specify what Xen should do in the event of an NMI parity or I/O error. `ignore` discards the error; `dom0` causes Xen to report the error to dom0, while ‘fatal’ causes Xen to print diagnostics and then hang.

### 1.2.132 noapic (x86)

Instruct Xen to ignore any IOAPICs that are present in the system, and instead continue to use the legacy PIC. This is not recommended with pvops type kernels.

Because responsibility for APIC setup is shared between Xen and the domain 0 kernel this option is automatically propagated to the domain 0 command line.

### 1.2.133 invpcid (x86)

> `= <boolean>`

> Default: `true`

By default, Xen will use the INVPCID instruction for TLB management if it is available. This option can be used to cause Xen to fall back to older mechanisms, which are generally slower.

### 1.2.134 noirqbalance (x86)

> `= <boolean>`

Disable software IRQ balancing and affinity. This can be used on systems such as Dell 1850/2850 that have workarounds in hardware for IRQ routing issues.

### 1.2.135 nolapic (x86)

> `= <boolean>`

> Default: `false`

Ignore the local APIC on a uniprocessor system, even if enabled by the BIOS.

### 1.2.136 no-real-mode (x86)

> `= <boolean>`

Do not execute real-mode bootstrap code when booting Xen. This option should not be used except for debugging. It will effectively disable the vga option, which relies on real mode to set the video mode.

### 1.2.137 noreboot

> `= <boolean>`

Do not automatically reboot after an error. This is useful for catching debug output. Defaults to automatically reboot after 5 seconds.

### 1.2.138 nosmp (x86)

> `= <boolean>`

Disable SMP support. No secondary processors will be booted. Defaults to booting secondary processors.

This option is ignored in pv-shim mode.

### 1.2.139 nr_irqs (x86)

> `= <integer>`

### 1.2.140 irq-max-guests (x86)

> `= <integer>`

> Default: `32`

Maximum number of guests any individual IRQ could be shared between, i.e. a limit on the number of guests it is possible to start each having assigned a device sharing a common interrupt line. Accepts values between 1 and 255.

### 1.2.141 numa (x86)

> `= on | off | fake=<integer> | noacpi`

> Default: `on`

### 1.2.142 pci

    = List of [ serr=<bool>, perr=<bool> ]

    Default: Signaling left as set by firmware.

Override the firmware settings, and explicitly enable or disable the signalling of PCI System and Parity errors.

### 1.2.143 pci-phantom

> `=[<seg>:]<bus>:<device>,<stride>`

Mark a group of PCI devices as using phantom functions without actually advertising so, so the IOMMU can create translation contexts for them.

All numbers specified must be hexadecimal ones.

This option can be specified more than once (up to 8 times at present).

### 1.2.144 pci-passthrough (arm)

> `= <boolean>`

> Default: `false`

Flag to enable or disable support for PCI passthrough

### 1.2.145 pcid (x86)

> `= <boolean> | xpti=<bool>`

> Default: `xpti`

> Can be modified at runtime (change takes effect only for domains created afterwards)

If available, control usage of the PCID feature of the processor for 64-bit pv-domains. PCID can be used either for no domain at all (`false`), for all of them (`true`), only for those subject to XPTI (`xpti`) or for those not subject to XPTI (`no-xpti`). The feature is used only in case INVPCID is supported and not disabled via `invpcid=false`.

### 1.2.146 pku (x86)

> `= <boolean>`

> Default: `true`

Flag to enable Memory Protection Keys.

The protection-key feature provides an additional mechanism by which IA-32e paging controls access to usermode addresses.

### 1.2.147 ple_gap

> `= <integer>`

### 1.2.148 ple_window (Intel)

> `= <integer>`

### 1.2.149 psr (Intel)

> `= List of ( cmt:<boolean> | rmid_max:<integer> | cat:<boolean> | cos_max:<integer> | cdp:<boolean> )`

> Default: `psr=cmt:0,rmid_max:255,cat:0,cos_max:255,cdp:0`

Platform Shared Resource(PSR) Services. Intel Haswell and later server platforms offer information about the sharing of resources.

To use the PSR monitoring service for a certain domain, a Resource Monitoring ID(RMID) is used to bind the domain to corresponding shared resource. RMID is a hardware-provided layer of abstraction between software and logical processors.

To use the PSR cache allocation service for a certain domain, a capacity bitmasks(CBM) is used to bind the domain to corresponding shared resource. CBM represents cache capacity and indicates the degree of overlap and isolation between domains. In hypervisor a Class of Service(COS) ID is allocated for each unique CBM.

The following resources are available:

- Cache Monitoring Technology (Haswell and later). Information regarding the L3 cache occupancy.

  - `cmt` instructs Xen to enable/disable Cache Monitoring Technology.
  - `rmid_max` indicates the max value for rmid.

- Memory Bandwidth Monitoring (Broadwell and later). Information regarding the total/local memory bandwidth. Follow the same options with Cache Monitoring Technology.

- Cache Allocation Technology (Broadwell and later). Information regarding the cache allocation.

  - `cat` instructs Xen to enable/disable Cache Allocation Technology.
  - `cos_max` indicates the max value for COS ID.

- Code and Data Prioritization Technology (Broadwell and later). Information regarding the code cache and the data cache allocation. CDP is based on CAT.

  - `cdp` instructs Xen to enable/disable Code and Data Prioritization. Note that `cos_max` of CDP is a little different from `cos_max` of CAT. With CDP, one COS will corespond two CBMs other than one with CAT, due to the sum of CBMs is fixed, that means actual `cos_max` in use will automatically reduce to half when CDP is enabled.

### 1.2.150 pv

    = List of [ 32=<bool> ]

    Applicability: x86

Controls for aspects of PV guest support.

- The `32` boolean controls whether 32bit PV guests can be created. It defaults to `true`, and is ignored when `CONFIG_PV32` is compiled out.

  32bit PV guests are incompatible with CET Shadow Stacks. If Xen is using shadow stacks, this option will be overridden to `false`. Backwards compatibility can be maintained with the `pv-shim` mechanism.

### 1.2.151 pv-linear-pt (x86)

> `= <boolean>`

> Default: `true`

Only available if Xen is compiled with `CONFIG_PV_LINEAR_PT` support enabled.

Allow PV guests to have pagetable entries pointing to other pagetables of the same level (i.e., allowing L2 PTEs to point to other L2 pages). This technique is often called “linear pagetables”, and is sometimes used to allow operating systems a simple way to consistently map the current process’s pagetables into its own virtual address space.

Linux and MiniOS don’t use this technique. NetBSD and Novell Netware do; there may be other custom operating systems which do. If you’re certain you don’t plan on having PV guests which use this feature, turning it off can reduce the attack surface.

### 1.2.152 pv-l1tf (x86)

> `= List of [ <bool>, dom0=<bool>, domu=<bool> ]`

> Default: `false` on believed-unaffected hardware, or in pv-shim mode. `domu` on believed-affected hardware.

Mitigations for L1TF / XSA-273 / CVE-2018-3620 for PV guests.

For backwards compatibility, we may not alter an architecturally-legitimate pagetable entry a PV guest chooses to write. We can however force such a guest into shadow mode so that Xen controls the PTEs which are reachable by the CPU pagewalk.

Shadowing is performed at the point where a PV guest first tries to write an L1TF-vulnerable PTE. Therefore, a PV guest kernel which has been updated with its own L1TF mitigations will not trigger shadow mode if it is well behaved.

If `CONFIG_SHADOW_PAGING` is not compiled in, this mitigation instead crashes the guest when an L1TF-vulnerable PTE is written, which still allows updated, well-behaved PV guests to run, despite Shadow being compiled out.

In the pv-shim case, Shadow is expected to be compiled out, and a malicious guest kernel can only leak data from the shim Xen, rather than the host Xen.

### 1.2.153 pv-shim (x86)

> `= <boolean>`

> Default: `false`

This option is intended for use by a toolstack, when choosing to run a PV guest compatibly inside an HVM container.

In this mode, the kernel and initrd passed as modules to the hypervisor are constructed into a plain unprivileged PV domain.

### 1.2.154 rcu-idle-timer-period-ms

> `= <integer>`

> Default: `10`

How frequently a CPU which has gone idle, but with pending RCU callbacks, should be woken up to check if the grace period has completed, and the callbacks are safe to be executed. Expressed in milliseconds; maximum is 100, and it can’t be 0.

### 1.2.155 reboot (x86)

> `= t[riple] | k[bd] | a[cpi] | p[ci] | P[ower] | e[fi] | n[o] [, [w]arm | [c]old]`

> Default: `0`

Specify the host reboot method.

`warm` instructs Xen to not set the cold reboot flag.

`cold` instructs Xen to set the cold reboot flag.

`no` instructs Xen to not automatically reboot after panics or crashes.

`triple` instructs Xen to reboot the host by causing a triple fault.

`kbd` instructs Xen to reboot the host via the keyboard controller.

`acpi` instructs Xen to reboot the host using RESET_REG in the ACPI FADT.

`pci` instructs Xen to reboot the host using PCI reset register (port CF9).

`Power` instructs Xen to power-cycle the host using PCI reset register (port CF9).

‘efi’ instructs Xen to reboot using the EFI reboot call (in EFI mode by default it will use that method first).

`xen` instructs Xen to reboot using Xen’s SCHEDOP hypercall (this is the default when running nested Xen)

### 1.2.156 rmrr

> `= start<-end>=[s1]bdf1[,[s1]bdf2[,...]];start<-end>=[s2]bdf1[,[s2]bdf2[,...]]`

Define RMRR units that are missing from ACPI table along with device they belong to and use them for 1:1 mapping. End addresses can be omitted and one page will be mapped. The ranges are inclusive when start and end are specified. If segment of the first device is not specified, segment zero will be used. If other segments are not specified, first device segment will be used. If a segment is specified for other than the first device and it does not match the one specified for the first one, an error will be reported.

‘start’ and ‘end’ values are page numbers (not full physical addresses), in hexadecimal format (can optionally be preceded by “0x”).

Usage example: If device 0:0:1d.0 requires one page (0xd5d45) to be reserved, and device 0:0:1a.0 requires three pages (0xd5d46 thru 0xd5d48) to be reserved, one usage would be:

rmrr=d5d45=0:0:1d.0;0xd5d46-0xd5d48=0:0:1a.0

Note: grub2 requires to escape or use quotations if special characters are used, namely ‘;’, refer to the grub2 documentation if multiple ranges are specified.

### 1.2.157 ro-hpet (x86)

> `= <boolean>`

> Default: `true`

Map the HPET page as read only in Dom0. If disabled the page will be mapped with read and write permissions.

### 1.2.158 sched

> `= credit | credit2 | arinc653 | rtds | null`

> Default: `sched=credit2`

Choose the default scheduler. Note the default scheduler is selectable via Kconfig and depends on enabled schedulers. Check `CONFIG_SCHED_DEFAULT` to see which scheduler is the default.

### 1.2.159 sched_credit2_max_cpus_runqueue

> `= <integer>`

> Default: `16`

Defines how many CPUs will be put, at most, in each Credit2 runqueue.

Runqueues are still arranged according to the host topology (and following what indicated by the ‘credit2_runqueue’ parameter). But we also have a cap to the number of CPUs that share each runqueues.

A value that is a submultiple of the number of online CPUs is recommended, as that would likely produce a perfectly balanced runqueue configuration.

### 1.2.160 sched_credit2_migrate_resist

> `= <integer>`

### 1.2.161 sched_credit_tslice_ms

> `= <integer>`

Set the timeslice of the credit1 scheduler, in milliseconds. The default is 30ms. Reasonable values may include 10, 5, or even 1 for very latency-sensitive workloads.

### 1.2.162 sched-gran (x86)

> `= cpu | core | socket`

> Default: `sched-gran=cpu`

Set the scheduling granularity. In case the granularity is larger than 1 (e.g. `core`on a SMT-enabled system, or `socket`) multiple vcpus are assigned statically to a “scheduling unit” which will then be subject to scheduling. This assignment of vcpus to scheduling units is fixed.

`cpu`: Vcpus will be scheduled individually on single cpus (e.g. a hyperthread using x86/Intel terminology)

`core`: As many vcpus as there are cpus on a physical core are scheduled together on a physical core.

`socket`: As many vcpus as there are cpus on a physical sockets are scheduled together on a physical socket.

Note: a value other than `cpu` will result in rejecting a runtime modification attempt of the “smt” setting.

Note: for AMD x86 processors before Fam17 the terminology in the official data sheets is different: a cpu is named “core” and multiple “cores” are running in the same “compute unit”. As from Fam17 on AMD is using the same names as Intel (“thread” and “core”) the topology levels are named “cpu”, “core” and “socket” even on older AMD processors.

### 1.2.163 sched_ratelimit_us

> `= <integer>`

In order to limit the rate of context switching, set the minimum amount of time that a vcpu can be scheduled for before preempting it, in microseconds. The default is 1000us (1ms). Setting this to 0 disables it altogether.

### 1.2.164 sched_smt_power_savings

> `= <boolean>`

Normally Xen will try to maximize performance and cache utilization by spreading out vcpus across as many different divisions as possible (i.e, numa nodes, sockets, cores threads, &c). This often maximizes throughput, but also maximizes energy usage, since it reduces the depth to which a processor can sleep.

This option inverts the logic, so that the scheduler in effect tries to keep the vcpus on the smallest amount of silicon possible; i.e., first fill up sibling threads, then sibling cores, then sibling sockets, &c. This will reduce performance somewhat, particularly on systems with hyperthreading enabled, but should reduce power by enabling more sockets and cores to go into deeper sleep states.

### 1.2.165 scrub-domheap

> `= <boolean>`

> Default: `false`

Scrub domains’ freed pages. This is a safety net against a (buggy) domain accidentally leaking secrets by releasing pages without proper sanitization.

### 1.2.166 serial_tx_buffer

> `= <size>`

> Default: `16kB`

Set the serial transmit buffer size.

### 1.2.167 serrors (ARM)

> `= diverse | panic`

> Default: `diverse`

This parameter is provided to administrators to determine how the hypervisor handles SErrors.

- `diverse`: The hypervisor will distinguish guest SErrors from hypervisor SErrors:
  - The guest generated SErrors will be forwarded to the currently running guest.
  - The hypervisor generated SErrors will cause the whole system to crash
- `panic`: All SErrors will cause the whole system to crash. This option should only be used if you trust all your guests and/or they don’t have a gadget (e.g. device) to generate SErrors in normal run.

### 1.2.168 shim_mem (x86)

> `= List of ( min:<size> | max:<size> | <size> )`

Set the amount of memory that xen-shim uses. Only has effect if pv-shim mode is enabled. Note that this value accounts for the memory used by the shim itself plus the free memory slack given to the shim for runtime allocations.

- `min:<size>` specifies the minimum amount of memory. Ignored if greater than max.
- `max:<size>` specifies the maximum amount of memory.
- `<size>` specifies the exact amount of memory. Overrides both min and max.

By default, the amount of free memory slack given to the shim for runtime usage is 1MB.

### 1.2.169 smap (x86)

> `= <boolean> | hvm`

> Default: `true` unless running in pv-shim mode on AMD or Hygon hardware

Flag to enable Supervisor Mode Access Prevention Use `smap=hvm` to allow SMAP use by HVM guests only.

In PV shim mode on AMD or Hygon hardware due to significant performance impact in some cases and generally lower security risk the option defaults to false.

### 1.2.170 smep (x86)

> `= <boolean> | hvm`

> Default: `true` unless running in pv-shim mode on AMD or Hygon hardware

Flag to enable Supervisor Mode Execution Protection Use `smep=hvm` to allow SMEP use by HVM guests only.

In PV shim mode on AMD or Hygon hardware due to significant performance impact in some cases and generally lower security risk the option defaults to false.

### 1.2.171 smt (x86)

> `= <boolean>`

Default: `true`

Control bring up of multiple hyper-threads per CPU core.

### 1.2.172 snb_igd_quirk

> `= <boolean> | cap | <integer>`

A true boolean value enables legacy behavior (1s timeout), while `cap` enforces the maximum theoretically necessary timeout of 670ms. Any number is being interpreted as a custom timeout in milliseconds. Zero or boolean false disable the quirk workaround, which is also the default.

### 1.2.173 spec-ctrl (Arm)

> `= List of [ ssbd=force-disable|runtime|force-enable ]`

Controls for speculative execution sidechannel mitigations.

The option `ssbd=` is used to control the state of Speculative Store Bypass Disable (SSBD) mitigation.

- `ssbd=force-disable` will keep the mitigation permanently off. The guest will not be able to control the state of the mitigation.
- `ssbd=runtime` will always turn on the mitigation when running in the hypervisor context. The guest will be to turn on/off the mitigation for itself by using the firmware interface `ARCH_WORKAROUND_2`.
- `ssbd=force-enable` will keep the mitigation permanently on. The guest will not be able to control the state of the mitigation.

By default SSBD will be mitigated at runtime (i.e `ssbd=runtime`).

### 1.2.174 spec-ctrl (x86)

> `= List of [ <bool>, xen=<bool>, {pv,hvm,msr-sc,rsb,md-clear}=<bool>, bti-thunk=retpoline|lfence|jmp, {ibrs,ibpb,ssbd,eager-fpu, l1d-flush,branch-harden,srb-lock}=<bool> ]`

Controls for speculative execution sidechannel mitigations. By default, Xen will pick the most appropriate mitigations based on compiled in support, loaded microcode, and hardware details, and will virtualise appropriate mitigations for guests to use.

WARNING: Any use of this option may interfere with heuristics. Use with extreme care.

An overall boolean value, `spec-ctrl=no`, can be specified to turn off all mitigations, including pieces of infrastructure used to virtualise certain mitigation features for guests. This also includes settings which `xpti`, `smt`, `pv-l1tf`, `tsx` control, unless the respective option(s) have been specified earlier on the command line.

Alternatively, a slightly more restricted `spec-ctrl=no-xen` can be used to turn off all of Xen’s mitigations, while leaving the virtualisation support in place for guests to use.

Use of a positive boolean value for either of these options is invalid.

The booleans `pv=`, `hvm=`, `msr-sc=`, `rsb=` and `md-clear=` offer fine grained control over the alternative blocks used by Xen. These impact Xen’s ability to protect itself, and Xen’s ability to virtualise support for guests to use.

- `pv=` and `hvm=` offer control over all suboptions for PV and HVM guests respectively.
- `msr-sc=` offers control over Xen’s support for manipulating `MSR_SPEC_CTRL` on entry and exit. These blocks are necessary to virtualise support for guests and if disabled, guests will be unable to use IBRS/STIBP/SSBD/etc.
- `rsb=` offers control over whether to overwrite the Return Stack Buffer / Return Address Stack on entry to Xen.
- `md-clear=` offers control over whether to use VERW to flush microarchitectural buffers on idle and exit from Xen. Note: For compatibility with development versions of this fix, `mds=` is also accepted on Xen 4.12 and earlier as an alias. Consult vendor documentation in preference to here.

If Xen was compiled with INDIRECT_THUNK support, `bti-thunk=` can be used to select which of the thunks gets patched into the `__x86_indirect_thunk_%reg` locations. The default thunk is `retpoline` (generally preferred), with the alternatives being `jmp` (a `jmp *%reg` gadget, minimal overhead), and `lfence` (an `lfence; jmp *%reg` gadget).

On hardware supporting IBRS (Indirect Branch Restricted Speculation), the `ibrs=` option can be used to force or prevent Xen using the feature itself. If Xen is not using IBRS itself, functionality is still set up so IBRS can be virtualised for guests.

On hardware supporting IBPB (Indirect Branch Prediction Barrier), the `ibpb=` option can be used to force (the default) or prevent Xen from issuing branch prediction barriers on vcpu context switches.

On hardware supporting SSBD (Speculative Store Bypass Disable), the `ssbd=` option can be used to force or prevent Xen using the feature itself. On AMD hardware, this is a global option applied at boot, and not virtualised for guest use. On Intel hardware, the feature is virtualised for guests, independently of Xen’s choice of setting.

On all hardware, the `eager-fpu=` option can be used to force or prevent Xen from using fully eager FPU context switches. This is currently implemented as a global control. By default, Xen will choose to use fully eager context switches on hardware believed to speculate past #NM exceptions.

On hardware supporting L1D_FLUSH, the `l1d-flush=` option can be used to force or prevent Xen from issuing an L1 data cache flush on each VMEntry. Irrespective of Xen’s setting, the feature is virtualised for HVM guests to use. By default, Xen will enable this mitigation on hardware believed to be vulnerable to L1TF.

If Xen is compiled with `CONFIG_SPECULATIVE_HARDEN_BRANCH`, the `branch-harden=` boolean can be used to force or prevent Xen from using speculation barriers to protect selected conditional branches. By default, Xen will enable this mitigation.

On hardware supporting SRBDS_CTRL, the `srb-lock=` option can be used to force or prevent Xen from protect the Special Register Buffer from leaking stale data. By default, Xen will enable this mitigation, except on parts where MDS is fixed and TAA is fixed/mitigated (in which case, there is believed to be no way for an attacker to obtain the stale data).

### 1.2.175 sync_console

> `= <boolean>`

> Default: `false`

Flag to force synchronous console output. Useful for debugging, but not suitable for production environments due to incurred overhead.

### 1.2.176 tboot (x86)

> `= 0x<phys_addr>`

Specify the physical address of the trusted boot shared page.

### 1.2.177 tbuf_size

> `= <integer>`

Specify the per-cpu trace buffer size in pages.

### 1.2.178 tdt (x86)

> `= <boolean>`

> Default: `true`

Flag to enable TSC deadline as the APIC timer mode.

### 1.2.179 tevt_mask

> `= <integer>`

Specify a mask for Xen event tracing. This allows Xen tracing to be enabled at boot. Refer to the xentrace(8) documentation for a list of valid event mask values. In order to enable tracing, a buffer size (in pages) must also be specified via the tbuf_size parameter.

### 1.2.180 tickle_one_idle_cpu

> `= <boolean>`

### 1.2.181 timer_slop

> `= <integer>`

### 1.2.182 tsc (x86)

> `= unstable | skewed | stable:socket`

### 1.2.183 tsx

    = <bool>

    Applicability: x86
    Default: false on parts vulnerable to TAA, true otherwise

Controls for the use of Transactional Synchronization eXtensions.

Several microcode updates are relevant:

- March 2019, fixing the TSX memory ordering errata on all TSX-enabled CPUs to date. Introduced MSR_TSX_FORCE_ABORT on SKL/SKX/KBL/WHL/CFL parts. The errata workaround uses Performance Counter 3, so the user can select between working TSX and working perfcounters.

- November 2019, fixing the TSX Async Abort speculative vulnerability. Introduced MSR_TSX_CTRL on all TSX-enabled MDS_NO parts to date, CLX/WHL-R/CFL-R, with the controls becoming architectural moving forward and formally retiring HLE from the architecture. The user can disable TSX to mitigate TAA, and elect to hide the HLE/RTM CPUID bits. Also causes VERW to once-again flush the microarchiectural buffers in case a TAA mitigation is wanted along with TSX being enabled.

- June 2021, removing the workaround for March 2019 on client CPUs and formally de-featured TSX on SKL/KBL/WHL/CFL (Note: SKX still retains the March 2019 fix). Introduced the ability to hide the HLE/RTM CPUID bits. PCR3 works fine, and TSX is disabled by default, but the user can re-enable TSX at their own risk, accepting that the memory order erratum is unfixed.

- February 2022, removing the VERW flushing workaround from November 2019 on client CPUs and formally de-featuring TSX on WHL-R/CFL-R (Note: CLX still retains the VERW flushing workaround). TSX defaults to disabled, and is locked off when SGX is enabled in the BIOS. When SGX is not enabled, TSX can be re-enabled at the users own risk, as it reintroduces the TSX Async Abort speculative vulnerability.

On systems with the ability to configure TSX, this boolean offers system wide control of whether TSX is enabled or disabled.

When TSX is disabled, transactions unconditionally abort. This is compatible with the TSX spec, which requires software to have a non-transactional path as a fallback. The RTM and HLE CPUID bits are hidden from VMs by default, but can be re-enabled if required. This allows VMs which previously saw RTM/HLE to be migrated in, although any TSX-enabled software will run with reduced performance.

- When TSX is locked off by firmware, `tsx=` is ignored and treated as `false`.

- An explicit `tsx=` choice is honoured, even if it is `true` and would result in a vulnerable system.

- When no explicit `tsx=` choice is given, parts vulnerable to TAA will be mitigated by disabling TSX, as this is the lowest overhead option.

- When no explicit `tsx=` option is given, parts susceptible to the memory ordering errata default to `true` to enable working TSX. Alternatively, selecting `tsx=0` will disable TSX and restore PCR3 to a working state.

  SKX and SKL/KBL/WHL/CFL on pre-June 2021 microcode default to `true`. Alternatively, selecting `tsx=0` will disable TSX and restore PCR3 to a working state.

  SKL/KBL/WHL/CFL on the June 2021 microcode or later default to `false`. Alternatively, selecting `tsx=1` will re-enable TSX at the users own risk.

### 1.2.184 ucode

> `= List of [ <integer> | scan=<bool>, nmi=<bool>, allow-same=<bool> ]`

    Applicability: x86
    Default: `nmi`

Controls for CPU microcode loading. For early loading, this parameter can specify how and where to find the microcode update blob. For late loading, this parameter specifies if the update happens within a NMI handler.

‘integer’ specifies the CPU microcode update blob module index. When positive, this specifies the n-th module (in the GrUB entry, zero based) to be used for updating CPU micrcode. When negative, counting starts at the end of the modules in the GrUB entry (so with the blob commonly being last, one could specify `ucode=-1`). Note that the value of zero is not valid here (entry zero, i.e. the first module, is always the Dom0 kernel image). Note further that use of this option has an unspecified effect when used with xen.efi (there the concept of modules doesn’t exist, and the blob gets specified via the `ucode=<filename>` config file/section entry; see EFI configuration file description).

‘scan’ instructs the hypervisor to scan the multiboot images for an cpio image that contains microcode. Depending on the platform the blob with the microcode in the cpio name space must be: - on Intel: kernel/x86/microcode/GenuineIntel.bin - on AMD : kernel/x86/microcode/AuthenticAMD.bin When using xen.efi, the `ucode=<filename>` config file setting takes precedence over `scan`.

‘nmi’ determines late loading is performed in NMI handler or just in stop_machine context. In NMI handler, even NMIs are blocked, which is considered safer. The default value is `true`.

‘allow-same’ alters the default acceptance policy for new microcode to permit trying to reload the same version. Many CPUs will actually reload microcode of the same version, and this allows for easy testing of the late microcode loading path.

### 1.2.185 unrestricted_guest (Intel)

> `= <boolean>`

### 1.2.186 vcpu_migration_delay

> `= <integer>`

> Default: `0`

Specify a delay, in microseconds, between migrations of a VCPU between PCPUs when using the credit1 scheduler. This prevents rapid fluttering of a VCPU between CPUs, and reduces the implicit overheads such as cache-warming. 1ms (1000) has been measured as a good value.

### 1.2.187 vesa-map

> `= <integer>`

### 1.2.188 vesa-mtrr

> `= <integer>`

### 1.2.189 vesa-ram

> `= <integer>`

### 1.2.190 vga

> `= ( ask | current | text-80x<rows> | gfx-<width>x<height>x<depth> | mode-<mode> )[,keep]`

`ask` causes Xen to display a menu of available modes and request the user to choose one of them.

`current` causes Xen to use the graphics adapter in its current state, without further setup.

`text-80x<rows>` instructs Xen to set up text mode. Valid values for `<rows>` are `25, 28, 30, 34, 43, 50, 80`

`gfx-<width>x<height>x<depth>` instructs Xen to set up graphics mode with the specified width, height and depth.

`mode-<mode>` instructs Xen to use a specific mode, as shown with the `ask` option. (N.B menu modes are displayed in hex, so `<mode>` should be a hexadecimal number)

The optional `keep` parameter causes Xen to continue using the vga console even after dom0 has been started. The default behaviour is to relinquish control to dom0.

### 1.2.191 viridian-spinlock-retry-count (x86)

> `= <integer>`

> Default: `2047`

Specify the maximum number of retries before an enlightened Windows guest will notify Xen that it has failed to acquire a spinlock.

### 1.2.192 viridian-version (x86)

> `= [<major>],[<minor>],[<build>]`

> Default: `6,0,0x1772`

, and must be integers. The values will be encoded in guest CPUID 0x40000002 if viridian enlightenments are enabled.

### 1.2.193 vpid (Intel)

> `= <boolean>`

> Default: `true`

Use Virtual Processor ID support if available. This prevents the need for TLB flushes on VM entry and exit, increasing performance.

### 1.2.194 vpmu (x86)

    = List of [ <bool>, bts, ipc, arch, rtm-abort=<bool> ]

    Applicability: x86.  Default: false

Controls for Performance Monitoring Unit virtualisation.

Performance monitoring facilities tend to be very hardware specific, and provide access to a wealth of low level processor information.

- An overall boolean can be used to enable or disable vPMU support. vPMU is disabled by default.

  When enabled, guests have full access to all performance counter settings, including model specific functionality. This is a superset of the functionality offered by `ipc` and/or `arch`, but a subset of the functionality offered by `bts`.

  Xen’s watchdog functionality is implemented using performance counters. As a result, use of the watchdog option will override and disable vPMU.

- The `bts` option enables performance monitoring, and permits additional access to the Branch Trace Store controls. BTS is an Intel feature where the processor can write data into a buffer whenever a branch occurs. However, as this feature isn’t virtualised, a misconfiguration by the guest can lock the entire system up.

- The `ipc` option allows access to the most minimal set of counters possible: instructions, cycles, and reference cycles. These can be used to calculate instructions per cycle (IPC).

- The `arch` option allows access to the pre-defined architectural events.

- The `rtm-abort` boolean has been superseded. Use `tsx=0` instead.

Warning: As the virtualisation is not 100% safe, don’t use the vpmu flag on production systems (see https://xenbits.xen.org/xsa/advisory-163.html)!

### 1.2.195 vwfi (arm)

> `= trap | native`

> Default: `trap`

WFI is the ARM instruction to “wait for interrupt”. WFE is similar and means “wait for event”. This option, which is ARM specific, changes the way guest WFI and WFE are implemented in Xen. By default, Xen traps both instructions. In the case of WFI, Xen blocks the guest vcpu; in the case of WFE, Xen yield the guest vcpu. When setting vwfi to `native`, Xen doesn’t trap either instruction, running them in guest context. Setting vwfi to `native` reduces irq latency significantly. It can also lead to suboptimal scheduling decisions, but only when the system is oversubscribed (i.e., in total there are more vCPUs than pCPUs).

### 1.2.196 watchdog (x86)

> `= force | <boolean>`

> Default: `false`

Run an NMI watchdog on each processor. If a processor is stuck for longer than the watchdog_timeout, a panic occurs. When `force` is specified, in addition to running an NMI watchdog on each processor, unknown NMIs will still be processed.

### 1.2.197 watchdog_timeout (x86)

> `= <integer>`

> Default: `5`

Set the NMI watchdog timeout in seconds. Specifying `0` will turn off the watchdog.

### 1.2.198 x2apic (x86)

> `= <boolean>`

> Default: `true`

Permit use of x2apic setup for SMP environments.

### 1.2.199 x2apic_phys (x86)

> `= <boolean>`

> Default: `true` if FADT mandates physical mode or if interrupt remapping is not available, `false` otherwise.

In the case that x2apic is in use, this option switches between physical and clustered mode. The default, given no hint from the FADT, is cluster mode.

### 1.2.200 xenheap_megabytes (arm32)

> `= <size>`

> Default: `0` (1/32 of RAM)

Amount of RAM to set aside for the Xenheap. Must be an integer multiple of 32.

By default will use 1/32 of the RAM up to a maximum of 1GB and with a minimum of 32M, subject to a suitably aligned and sized contiguous region of memory being available.

### 1.2.201 xpti (x86)

> `= List of [ default | <boolean> | dom0=<bool> | domu=<bool> ]`

> Default: `false` on hardware known not to be vulnerable to Meltdown (e.g. AMD) Default: `true` everywhere else

Override default selection of whether to isolate 64-bit PV guest page tables.

`true` activates page table isolation even on hardware not vulnerable by Meltdown for all domains.

`false` deactivates page table isolation on all systems for all domains.

`default` sets the default behaviour.

With `dom0` and `domu` it is possible to control page table isolation for dom0 or guest domains only.

### 1.2.202 xsave (x86)

> `= <boolean>`

> Default: `true`

Permit use of the `xsave/xrstor` instructions.

### 1.2.203 xsm

> `= dummy | flask | silo`

> Default: selectable via Kconfig. Depends on enabled XSM modules.

Specify which XSM module should be enabled. This option is only available if the hypervisor was compiled with `CONFIG_XSM` enabled.

- `dummy`: this is the default choice. Basic restriction for common deployment (the dummy module) will be applied. It’s also used when XSM is compiled out.
- `flask`: this is the policy based access control. To choose this, the separated option in kconfig must also be enabled.
- `silo`: this will deny any unmediated communication channels between unprivileged VMs. To choose this, the separated option in kconfig must also be enabled.