# 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 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.

### 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

> `= <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.

### acpi_skip_timer_override

> `= <boolean>`

Instruct Xen to ignore timer-interrupt override.

### acpi_sleep

> `= 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.

### allowsuperpage

> `= <boolean>`

> Default: `true`

Permit Xen to use superpages when performing memory management.

### altp2m (Intel)

> `= <boolean>`

> Default: `false`

Permit multiple copies of host p2m.

### apic

> `= 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.

### apicv

> `= <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.

### apic_verbosity

> `= verbose | debug`

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

### arat

> `= <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.

### asid

> `= <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.

### async-show-all

> `= <boolean>`

> Default: `false`

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

### ats

> `= <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.

### 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.

### 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.

### 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.

### clocksource

> `= 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".

### cmci-threshold

> `= <integer>`

> Default: `2`

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

### cmos-rtc-probe

> `= <boolean>`

> Default: `false`

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

### com1,com2

> `= <baud>[/<base-baud>][,[DPS][,[<io-base>|pci|amt][,[<irq>][,[<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.
- `<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] | 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.

### console_timestamps

> `= none | date | datems | boot`

> Default: `none`

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]`

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

### 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. Use `conswitch=ax` to keep the default switch character, but for xen to keep the console.

### core_parking

> `= power | performance`

> Default: `power`

### cpu_type

> `= arch_perfmon`

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

### 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>`

### 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.

### cpuid_mask_{{,ext_}ecx,edx}

> `= <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.

### 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>]` `= <size>[{@,<}<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.

### credit2_balance_over

> `= <integer>`

### credit2_balance_under

> `= <integer>`

### 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.

### 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.

### credit2_runqueue

> `= 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: `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

### 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).

### 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

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

### dom0_mem (ARM)

> `= <size>`

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

### dom0_mem (x86)

> `= 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_nodes

> `= 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).

### dom0_vcpus_pin

> `= <boolean>`

> Default: `false`

Pin dom0 vcpus to their respective pcpus

### dom0

> `= List of [ pvh | shadow ]`

> Sub-options:

> `pvh`

> Default: `false`

Flag that makes a dom0 boot in PVHv2 mode.

> `shadow`

> Default: `false`

Flag that makes a dom0 use shadow paging. Only works when "pvh" is enabled.

### 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.

### 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.

### efi

> `= List of [ rs | attr ]`

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

> `rs`

> Default: `true`

> > Force or disable use of EFI runtime services.

> `attr=uc`

> Default: `off`

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

### 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.

### 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.

### font

> `= <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.

> Sub-options:

> `pml`

> Default: `true`

> > 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.

> `ad`

> Default: Hardware dependent

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

### gdb

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

> Default: ``

Specify which console gdbstub should use. See console.

### gnttab_max_frames

> `= <integer>`

> Default: `32`

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

### gnttab_max_maptrack_frames

> `= <integer>`

> Default: `8 * gnttab_max_frames`

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

### gnttab_max_nr_frames

> `= <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.

### 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

> `= <boolean>`

> Default: `true`

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

### 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 2 MB host page table support for Hardware Assisted Paging (HAP).

### 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.

### hest_disable

> `= <boolean>`

> Default: `false`

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

### hpetbroadcast

> `= <boolean>`

### hvm_debug

> `= <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.

### hvm_fep

> `= <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.

### hvm_port80

> `= <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.

### highmem-start

> `= <size>`

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

### idle_latency_factor

> `= <integer>`

### ioapic_ack

> `= old | new`

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

### iommu

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

> Sub-options:

> `<boolean>`

> 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.

> `intremap`

> Default: `true`

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

> `intpost`

> Default: `false`

> > Control the use of interrupt posting, which depends on the availability of interrupt remapping.

> `qinval` (VT-d)

> Default: `true`

> > Control the use of Queued Invalidation.

> `snoop` (Intel)

> Default: `true`

> > Control the use of Snoop Control.

> `sharept`

> Default: `true`

> > Control whether CPU and IOMMU page tables should be shared.

> `dom0-passthrough`

> Default: `false`

> > Control whether to disable DMA remapping for Dom0.

> `dom0-strict`

> Default: `false`

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

> `amd-iommu-perdev-intremap`

> 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.

> `verbose`

> Default: `false`

> > Increase IOMMU code's verbosity.

> `debug`

> Default: `false`

> > Enable IOMMU debugging code (implies `verbose`).

### 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.

### iommu_inclusive_mapping (VT-d)

> `= <boolean>`

> Default: `true`

Use this to work around firmware issues providing incorrect 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.

### irq_ratelimit

> `= <integer>`

### irq_vector_map

### ivrs_hpet[`<hpet>`]

> `=[<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.

### ivrs_ioapic[`<ioapic>`]

> `=[<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.

### lapic

> `= <boolean>`

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

### 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.

### 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.

### 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).

### max_cstate

> `= <integer>`

### max_gsi_irqs

> `= <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.

### maxcpus

> `= <integer>`

### max_lpi_bits

> `= <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).

### 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.

### mmio-relax

> `= <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.

### msi

> `= <boolean>`

> Default: `true`

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

### mtrr.show

> `= <boolean>`

> Default: `false`

Print boot time MTRR state (x86 only).

### mwait-idle

> `= <boolean>`

> Default: `true`

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

### nmi

> `= 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.

### 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.

### 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.

### 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.

### 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`

> Default: Signaling left as set by firmware.

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

### 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).

### ple_gap

> `= <integer>`

### ple_window

> `= <integer>`

### pku

> `= <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.

### 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.

### pv-linear-pt

> `= <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.

### reboot

> `= 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).

### 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.

### ro-hpet

> `= <boolean>`

> Default: `true`

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

### sched

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

> Default: `sched=credit`

Choose the default scheduler.

### sched_credit2_migrate_resist

> `= <integer>`

### 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.

### serrors (ARM)

> `= diverse | forward | panic`

> Default: `diverse`

This parameter is provided to administrators to determine how the hypervisors handle SErrors.

In order to distinguish guest-generated SErrors from hypervisor-generated SErrors we have to place SError checking code in every EL1 <-> EL2 paths. That will cause overhead on entries and exits due to dsb/isb. However, not all platforms need to categorize SErrors. For example, a host that is running with trusted guests. The administrator can confirm that all guests that are running on the host will not trigger such SErrors. In this case, the administrator can use this parameter to skip categorizing SErrors and reduce the overhead of dsb/isb.

We provided the following 3 options to administrators to determine how the hypervisors handle SErrors:

- `diverse`: The hypervisor will distinguish guest SErrors from hypervisor SErrors. The guest generated SErrors will be forwarded to guests, the hypervisor generated SErrors will cause the whole system to crash. It requires:
- dsb/isb on all EL1 -> EL2 trap entries to categorize SErrors correctly.
- dsb/isb on EL2 -> EL1 return paths to prevent slipping hypervisor SErrors to guests.

- dsb/isb in context switch to isolate SErrors between 2 vCPUs.

- `forward`: The hypervisor will not distinguish guest SErrors from hypervisor SErrors. All SErrors will be forwarded to guests, except the SErrors generated when the idle vCPU is running. The idle domain doesn't have the ability to handle SErrors, so we have to crash the whole system when we get SErros with the idle vCPU. This option will avoid most overhead of the dsb/isb, except the dsb/isb in context switch which is used to isolate the SErrors between 2 vCPUs.

- `panic`: The hypervisor will not distinguish guest SErrors from hypervisor SErrors. All SErrors will crash the whole system. This option will avoid all overhead of the dsb/isb pairs.

### smap

> `= <boolean> | hvm`

> Default: `true`

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

### smep

> `= <boolean> | hvm`

> Default: `true`

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

### 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.

### 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>`

### tsc

> `= unstable | skewed | stable:socket`

### ucode

> `= [<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

### 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.

### viridian-version

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

> Default: `6,0,0x1772`

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

### viridian-spinlock-retry-count

> `= <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.

### 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

> `= ( <boolean> | { bts | ipc | arch [, ...] } )`

> 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.

vpmu=ipc enables performance monitoring, but restricts the counters to the most minimum set possible: instructions, cycles, and reference cycles. These can be used to calculate instructions per cycle (IPC).

vpmu=arch enables performance monitoring, but restricts the counters to the pre-defined architectural events only. These are exposed by cpuid, and listed in the Pre-Defined Architectural Performance Events table from the Intel 64 and IA-32 Architectures Software Developer's Manual, Volume 3B, System Programming Guide, Part 2.

If a boolean is not used, combinations of flags are allowed, comma separated. For example, vpmu=arch,bts.

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)!

### vwfi

> `= 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).

### watchdog

> `= 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.

### 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` 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.

### xsave

> `= <boolean>`

> Default: `true`

Permit use of the `xsave/xrstor` instructions.