Xen Hypervisor Command Line Options

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

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.

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-.


Enable noreboot mode


Disable x2apic support (if present)


Enable synchronous console mode


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

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.

Size (<size>)

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

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


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


Some options take a comma separated list of values.


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

Parameter details


= force | ht | noirq | <boolean>

String, or Boolean to disable.

The acpi option is used to control a set of four related boolean flags; acpi_force, acpi_ht, acpi_noirq and acpi_disabled.

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.

Finally, 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


= <integer>

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


= <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.


= <boolean>

Instruct Xen to ignore timer-interrupt override.


= 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.


= <boolean>

Default: true

Permit Xen to use superpages when performing memory management.

altp2m (Intel)

= <boolean>

Default: false

Permit multiple copies of host p2m.


= 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.


= <boolean>

Default: false

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


= <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.


= verbose | debug

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


= <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.


= <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.


= <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.


= <size>

Default: 0 (no limit)

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


= 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.


= <boolean>

Default: true

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.


= <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.

bti (x86)

= List of [ thunk=retpoline|lfence|jmp, ibrs=<bool>, ibpb=<bool>, rsb_{vmexit,native}=<bool> ]

WARNING: This command line option is deprecated, and superseded by spec-ctrl= - using both options in combination is undefined.

Branch Target Injection controls. By default, Xen will pick the most appropriate BTI mitigations based on compiled in support, loaded microcode, and hardware details.

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

If Xen was compiled with INDIRECT_THUNK support, 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 for Intel hardware), with the alternatives being jmp (a jmp *%reg gadget, minimal overhead), and lfence (an lfence; jmp *%reg gadget, preferred for AMD).

On hardware supporting IBRS, 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, the ibpb= option can be used to prevent Xen from issuing Branch Prediction Barriers on vcpu context switches.

The rsb_vmexit= and rsb_native= options can be used to fine tune when the RSB gets overwritten. There are individual controls for an entry from HVM context, and an entry from a native (PV or Xen) context.

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.


= pit | hpet | acpi

If set, override Xen's default choice for the platform timer.


= <integer>

Default: 2

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


= <boolean>

Default: false

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


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

Both option com1 and com2 follow the same format.

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


= <size>

Default: conring_size=16k

Specify the size of the console ring buffer.


= List of [ vga | com1[H,L] | com2[H,L] | 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).

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.


= none | date | datems | boot

Default: none

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

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


= <boolean>

Default: false

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


= <switch char>[x]

Default: conswitch=a

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.


= power | performance

Default: power


= arch_perfmon

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


= dom0-kernel | none | xen[,[powersave|performance|ondemand|userspace][,<maxfreq>][,[<minfreq>][,[verbose]]]]

Default: xen

Indicate where the responsibility for driving power states lies.

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.

Currently accepted:

The Speculation Control hardware features ibrsb, stibp, ibpb, l1d-flush and ssbd are used by default if available and applicable. They can be ignored, e.g. no-ibrsb, at which point Xen won't use them itself, and won't offer them to guests.

cpuid_mask_cpu (AMD only)

= fam_0f_rev_c | fam_0f_rev_d | fam_0f_rev_e | fam_0f_rev_f | fam_0f_rev_g | fam_10_rev_b | fam_10_rev_c | fam_11_rev_b

If the other cpuid_mask_{,ext_,thermal_,l7s0_}e{a,b,c,d}x options are fully set (unspecified on the command line), specify a pre-canned cpuid mask to mask the current processor down to appear as the specified processor. It is important to ensure that all hosts in a pool appear the same to guests to allow successful live migration.


= <integer>

Default: ~0 (all bits set)

These four command line parameters are used to specify cpuid masks to help with cpuid levelling across a pool of hosts. Setting a bit in the mask indicates that the feature should be enabled, while clearing a bit in the mask indicates that the feature should be disabled. It is important to ensure that all hosts in a pool appear the same to guests to allow successful live migration.

cpuid_mask_xsave_eax (Intel only)

= <integer>

Default: ~0 (all bits set)

This command line parameter is also used to specify a cpuid mask to help with cpuid levelling across a pool of hosts. See the description of the other respective options above.

cpuid_mask_{l7s0_{eax,ebx},thermal_ecx} (AMD only)

= <integer>

Default: ~0 (all bits set)

These three command line parameters are also used to specify cpuid masks to help with cpuid levelling across a pool of hosts. See the description of the other respective options above.


= <boolean>


= <boolean>


= <size>

Default: 4G

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


= <ramsize-range>:<size>[,...][@<offset>]


= <integer>


= <integer>


= <integer>


= 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).


= <integer>

Default: 20

Limits the number lines printed in Xen stack traces.


= <integer>

Default: 128

Specify the size of the console debug trace buffer in KiB. The debug trace feature is only enabled in debugging builds of Xen.


= <integer>

Specify the bit width of the DMA heap.


= List of <hex>-<hex>

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



= <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.


= <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


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

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.

If <size> 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:<size> 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

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.


= 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).


= <boolean>


= <boolean>

Default: false

Pin dom0 vcpus to their respective pcpus


= <boolean>

Default: false

Flag that makes a 64bit dom0 boot in PVH mode. No 32bit support at present.

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.


= <boolean>

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

Default: true on Intel CPUs, otherwise false


= <boolean>

Default: false

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

edd (x86)

= off | on | skipmbr

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

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.


= List of [ rs | attr ]

All options are of boolean kind and can be prefixed with no- to effect the inverse meaning.


Default: true

Force or disable use of EFI runtime services.


Default: off

Allows mapping of RuntimeServices which have no cachability attribute set as UC.


= [<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.


= permissive | enforcing | late | disabled

Default: permissive

Specify how the FLASK security server should be configured. This option is only available if the hypervisor was compiled with XSM support (which can be enabled by setting XSM_ENABLE = y in .config).


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

Specify the font size when using the VESA console driver.

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.

ept (Intel)

= List of ( {no-}pml | {no-}ad )

Controls EPT related features.



Default: false

PML is a new hardware feature in Intel's Broadwell Server and further platforms which reduces hypervisor overhead of log-dirty mechanism by automatically recording GPAs (guest physical addresses) when guest memory gets dirty, and therefore significantly reducing number of EPT violation caused by write protection of guest memory, which is a necessity to implement log-dirty mechanism before PML.


Default: Hardware dependent

Have hardware keep accessed/dirty (A/D) bits updated.


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

Specify the serial parameters for the GDB stub.


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

Default: gnttab=max-ver:2,transitive

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

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


= <integer>

Default: 32

Specify the maximum number of frames which any domain may use as part of its grant table.


= <integer>

Default: 8 * gnttab_max_frames

Specify the maximum number of frames to use as part of a domains maptrack array.


= <integer>

Deprecated Use gnttab_max_frames and gnttab_max_maptrack_frames instead.

Specify the maximum number of frames per grant table operation and the maximum number of maptrack frames domain.


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

Default: guest_loglvl=none/warning

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.


= <boolean>

Default: true

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


= <boolean>

Default: true

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


= <boolean>

Default: true

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


= <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_ENABLE=y on x86.


= <boolean>

Default: false

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


= <boolean>


= <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.


= <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 purposes, and is only available in debug builds of the hypervisor.


= <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.


= <size>

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


= <integer>


= old | new

Default: new unless directed-EOI is supported


= List of [ <boolean> | force | required | intremap | qinval | snoop | sharept | dom0-passthrough | dom0-strict | amd-iommu-perdev-intremap | workaround_bios_bug | igfx | verbose | debug ]



Default: on

Control the use of IOMMU(s) in the system.

All other sub-options are of boolean kind and can be prefixed with no- to effect the inverse meaning.

force or required

Default: false

Don't continue booting unless IOMMU support is found and can be initialized successfully.


Default: true

Control the use of interrupt remapping (DMA remapping will always be enabled if IOMMU functionality is enabled).

qinval (VT-d)

Default: true

Control the use of Queued Invalidation.

snoop (Intel)

Default: true

Control the use of Snoop Control.


Default: true

Control whether CPU and IOMMU page tables should be shared.


Default: false

Control whether to disable DMA remapping for Dom0.


Default: false

Control whether to set up DMA remapping only for the memory Dom0 actually got assigned. Implies no-dom0-passthrough.


Default: true

Control whether to set up interrupt remapping data structures per device rather that once for the entire system. Turning this off is making PCI device pass-through insecure and hence unsupported.

workaround_bios_bug (VT-d)

Default: false

Causes DRHD entries without any PCI discoverable devices under them to be ignored (normally IOMMU setup fails if any of the devices listed by a DRHD entry aren't PCI discoverable).

igfx (VT-d)

Default: true

Enable IOMMU for Intel graphics devices. The intended usage of this option is no-igfx, which is similar to Linux intel_iommu=igfx_off option used to workaround graphics issues. If adding no-igfx fixes anything, you should file a bug reporting the problem.


Default: false

Increase IOMMU code's verbosity.


Default: false

Enable IOMMU debugging code (implies verbose).

iommu_inclusive_mapping (VT-d)

= <boolean>

Default: false

Use this to work around firmware issues providing correct RMRR entries. Rather than only mapping RAM pages for IOMMU accesses for Dom0, with this option all pages not marked as unusable in the E820 table will get a mapping established.


= <integer>




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.



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.


= <boolean>

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


= <boolean>


= <boolean>


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

Default: loglvl=warning

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.


= 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.


= <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.


= [<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).


= <integer>


= <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.


= <integer>


= <integer>


= <integer>


= verbose

Specify verbose machine check output.


= <size>

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


= <boolean>[,amd-fam10]

Default: 1

Specify if the MMConfig space should be enabled.


= <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.


= <boolean>

Default: true

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


= <boolean>

Default: false

Print boot time MTRR state (x86 only).


= <boolean>

Default: true

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


= 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.


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.

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.


= <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.


= <boolean>

Default: false

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

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.


= <boolean>

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


= <boolean>

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


= <integer>


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

Default: on


= {no-}serr | {no-}perr

Default: Signaling left as set by firmware.

Disable signaling of SERR (system errors) and/or PERR (parity errors) on all PCI devices.



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).


= <integer>


= <integer>

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.

psr (Intel)

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

Default: psr=cmt:0,rmid_max:255,cat:0,cos_max:255

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:


= <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.

pv-l1tf (x86)

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

Default: false on believed-unaffected hardware. 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.


= 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).


= <boolean>

Default: true

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


= credit | credit2 | arinc653

Default: sched=credit

Choose the default scheduler.


= <integer>


= <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.


= <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.


= <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.


= <size>

Default: 16kB

Set the serial transmit buffer size.


= <boolean>

Default: true

Flag to enable Supervisor Mode Execution Protection


= <boolean>

Default: true

Flag to enable Supervisor Mode Access Prevention

smt (x86)

= <boolean>

Default: true

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


= <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.

spec-ctrl (x86)

= List of [ <bool>, xen=<bool>, {pv,hvm,msr-sc,rsb}=<bool>, bti-thunk=retpoline|lfence|jmp, {ibrs,ibpb,ssbd,eager-fpu, l1d-flush}=<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 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= and rsb= 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.

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 for Intel hardware), with the alternatives being jmp (a jmp *%reg gadget, minimal overhead), and lfence (an lfence; jmp *%reg gadget, preferred for AMD).

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.


= <boolean>

Default: false

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


= 0x<phys_addr>

Specify the physical address of the trusted boot shared page.


= <integer>

Specify the per-cpu trace buffer size in pages.


= <boolean>

Default: true

Flag to enable TSC deadline as the APIC timer mode.


= <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.


= <boolean>


= <integer>


= <boolean>


= <boolean>


= <boolean>


= <boolean>


= <integer>


= unstable | skewed


= [<integer> | scan]

Specify how and where to find CPU microcode update blob.

'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


= <boolean>


= <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.


= <integer>


= <integer>


= <integer>


= ( 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.

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.


= ( bts )

Default: off

Switch on the virtualized performance monitoring unit for HVM guests.

If the current cpu isn't supported a message like 'VPMU: Initialization failed. ...' is printed on the hypervisor serial log.

For some Intel Nehalem processors a quirk handling exist for an unknown wrong behaviour (see handle_pmc_quirk()).

If 'vpmu=bts' is specified the virtualisation of the Branch Trace Store (BTS) feature is switched on on Intel processors supporting this feature.

Note that if watchdog option is also specified vpmu will be turned off.

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


`= 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).


= 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.


= <integer>

Default: 5

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


= <boolean>

Default: true

Permit use of x2apic setup for SMP environments.


= <boolean>

Default: true if FADT mandates physical mode, 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.


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

Default: false on hardware 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.

* WARNING: Not yet a complete isolation implementation, but better than nothing. *


= <boolean>

Default: true

Permit use of the xsave/xrstor instructions.