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-.

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.

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 size suffix

• 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.

String

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

List

Some options take a comma separated list of values.

Combination

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

acpi

= 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

acpi_apic_instance

= <integer>

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

acpi_pstate_strict

= <integer>

acpi_skip_timer_override

= <boolean>

Instruct Xen to ignore timer-interrupt override.

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

acpi_sleep

= s3_bios | s3_mode

allowsuperpage

= <boolean>

Default: true

Permit Xen to use superpages when performing memory management.

apic

= summit | 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.

allow_unsafe

= <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.

apic_verbosity

= verbose | debug

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

ats

= <boolean>

Default: true

Permits Xen to set up and use PCI Address Translation Services, which is required for PCI Passthrough.

availmem

= <size>

Default: 0 (no limit)

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

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.

bootscrub

= <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.

cachesize

= <size>

If set, override Xen's calculation of the level 2 cache line size.

clocksource

= pit | hpet | cyclone | acpi

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

com1,com2

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

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, a clock speed measured in hz 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.
• <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

conring_size

= <size>

Default: conring_size=16k

Specify the size of the console ring buffer.

console

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

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

console_timestamps

= <boolean>

Default: false

Flag to indicate whether include a timestamp with each console line.

console_to_ring

= <boolean>

Default: false

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

conswitch

= <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.

cpu_type

= arch_perfmon

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

cpufreq

= dom0-kernel | none | xen

Default: xen

Indicate where the responsibility for driving power states lies.

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_}e{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.

cpuid_mask_ ecx,edx,ext_ecx,ext_edx,xsave_eax

= <integer>

Default: ~0 (all bits set)

These five 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.

cpuidle

= <boolean>

cpuinfo

= <boolean>

crashinfo_maxaddr

= <size>

Default: 4G

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

crashkernel

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

credit2_balance_over

= <integer>

credit2_balance_under

= <integer>

credit2_load_window_shift

= <integer>

debug_stack_lines

= <integer>

Default: 20

Limits the number lines printed in Xen stack traces.

debugtrace

= <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.

dma_bits

= <integer>

Specify the bit width of the DMA heap.

dom0_ioports_disable

= List of <hex>-<hex>

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

dom0_max_vcpus

= <integer>

Specify the maximum number of vcpus to give to dom0. This defaults to the number of pcpus on the host.

dom0_mem

= 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.

• <size> specifies the exact amount of memory.
• min:<size> specifies the minimum amount of memory.
• max:<size> specifies the maximum amount of 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.

dom0_shadow

= <boolean>

dom0_vcpus_pin

= <boolean>

Default: false

Pin dom0 vcpus to their respective pcpus

e820-mtrr-clip

= <boolean>

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

Default: true on Intel CPUs, otherwise false

e820-verbose

= <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.

extra_guest_irqs

= <number>

Increase the number of PIRQs available for the guest. The default is 32.

flask_enabled

= <integer>

flask_enforcing

= <integer>

font

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

Specify the font size when using the VESA console driver.

gdb

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

Specify the serial parameters for the GDB stub.

gnttab_max_nr_frames

= <integer>

Specify the maximum number of frames per grant table operation.

guest_loglvl

= <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.

hap_1gb

= <boolean>

Default: true

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

hap_2mb

= <boolean>

Default: true

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

hpetbroadcast

= <boolean>

hvm_debug

= <integer>

hvm_port80

= <boolean>

idle_latency_factor

= <integer>

ioapic_ack

iommu

iommu_inclusive_mapping

= <boolean>

irq_ratelimit

= <integer>

irq_vector_map

lapic

Force the use of use of the local APIC on a uniprocessor system, even if left disabled by the BIOS. This option will accept any value at all.

lapic_timer_c2_ok

= <boolean>

ler

= <boolean>

loglvl

= <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.

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.

max_cstate

= <integer>

max_gsi_irqs

= <integer>

maxcpus

= <integer>

mce

= <integer>

mce_fb

= <integer>

mce_verbosity

= verbose

Specify verbose machine check output.

mem

= <size>

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

mmcfg

= <boolean>[,amd-fam10]

Default: 1

Specify if the MMConfig space should be enabled.

msi

= <boolean>

Default: true

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

nmi

= ignore | dom0 | fatal

Default: nmi=fatal

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.

noapic

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.

nofxsr

= <boolean>

noirqbalance

= <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.

nolapic

= <boolean>

Default: false

Ignore the local APIC on a uniprocessor system, even if enabled by the BIOS. This option will accept value.

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.

noreboot

= <boolean>

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

noserialnumber

= <boolean>

Disable CPU serial number reporting.

nosmp

= <boolean>

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

nr_irqs

= <integer>

numa

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

Default: on

pci

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

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

Default: Signaling left as set by firmware.

ple_gap

= <integer>

ple_window

= <integer>

reboot

= b[ios] | t[riple] | k[bd] | 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.

bios instructs Xen to reboot the host by jumping to BIOS. This is only available on 32-bit x86 platforms.

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.

sched

= credit | credit2 | sedf | arinc653

Default: sched=credit

Choose the default scheduler.

sched_credit2_migrate_resist

= <integer>

sched_credit_default_yield

= <boolean>

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.

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.

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.

serial_tx_buffer

= <size>

Default: 16kB

Set the serial transmit buffer size.

smep

= <boolean>

Default: true

Flag to enable Supervisor Mode Execution Protection

snb_igd_quirk

= <boolean>

sync_console

= <boolean>

Default: false

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

tboot

= 0x<phys_addr>

Specify the physical address of the trusted boot shared page.

tbuf_size

= <integer>

Specify the per-cpu trace buffer size in pages.

tdt

= <boolean>

Default: true

Flag to enable TSC deadline as the APIC timer mode.

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.

tickle_one_idle_cpu

= <boolean>

timer_slop

= <integer>

tmem

= <boolean>

tmem_compress

= <boolean>

tmem_dedup

= <boolean>

tmem_lock

= <integer>

tmem_shared_auth

= <boolean>

tmem_tze

= <integer>

tsc

= unstable | skewed

ucode

= <integer>

Specify 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).

unrestricted_guest

= <boolean>

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.

vesa-map

= <integer>

vesa-mtrr

= <integer>

vesa-ram

= <integer>

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.

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.

vpmu

= ( 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.

Warning: As the BTS virtualisation is not 100% safe and because of the nehalem quirk don't use the vpmu flag on production systems with Intel cpus!

watchdog

= <boolean>

Default: false

Run an NMI watchdog on each processor. If a processor is stuck for longer than the watchdog_timeout, a panic occurs.

watchdog_timeout

= <integer>

Default: 5

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

x2apic

= <boolean>

Default: true

Permit use of x2apic setup for SMP environments.

x2apic_phys

= <boolean>

Default: true

Use the x2apic physical apic driver. The alternative is the x2apic cluster driver.

xsave

= <boolean>

Default: true

Permit use of the xsave/xrstor instructions.