xref: /kernel/linux/linux-6.6/Documentation/arch/x86/x86_64/boot-options.rst

.. SPDX-License-Identifier: GPL-2.0

===========================
AMD64 Specific Boot Options
===========================

There are many others (usually documented in driver documentation), but
only the AMD64 specific ones are listed here.

Machine check
=============
Please see Documentation/arch/x86/x86_64/machinecheck.rst for sysfs runtime tunables.

   mce=off
		Disable machine check
   mce=no_cmci
		Disable CMCI(Corrected Machine Check Interrupt) that
		Intel processor supports.  Usually this disablement is
		not recommended, but it might be handy if your hardware
		is misbehaving.
		Note that you'll get more problems without CMCI than with
		due to the shared banks, i.e. you might get duplicated
		error logs.
   mce=dont_log_ce
		Don't make logs for corrected errors.  All events reported
		as corrected are silently cleared by OS.
		This option will be useful if you have no interest in any
		of corrected errors.
   mce=ignore_ce
		Disable features for corrected errors, e.g. polling timer
		and CMCI.  All events reported as corrected are not cleared
		by OS and remained in its error banks.
		Usually this disablement is not recommended, however if
		there is an agent checking/clearing corrected errors
		(e.g. BIOS or hardware monitoring applications), conflicting
		with OS's error handling, and you cannot deactivate the agent,
		then this option will be a help.
   mce=no_lmce
		Do not opt-in to Local MCE delivery. Use legacy method
		to broadcast MCEs.
   mce=bootlog
		Enable logging of machine checks left over from booting.
		Disabled by default on AMD Fam10h and older because some BIOS
		leave bogus ones.
		If your BIOS doesn't do that it's a good idea to enable though
		to make sure you log even machine check events that result
		in a reboot. On Intel systems it is enabled by default.
   mce=nobootlog
		Disable boot machine check logging.
   mce=monarchtimeout (number)
		monarchtimeout:
		Sets the time in us to wait for other CPUs on machine checks. 0
		to disable.
   mce=bios_cmci_threshold
		Don't overwrite the bios-set CMCI threshold. This boot option
		prevents Linux from overwriting the CMCI threshold set by the
		bios. Without this option, Linux always sets the CMCI
		threshold to 1. Enabling this may make memory predictive failure
		analysis less effective if the bios sets thresholds for memory
		errors since we will not see details for all errors.
   mce=recovery
		Force-enable recoverable machine check code paths

   nomce (for compatibility with i386)
		same as mce=off

   Everything else is in sysfs now.

APICs
=====

   apic
	Use IO-APIC. Default

   noapic
	Don't use the IO-APIC.

   disableapic
	Don't use the local APIC

   nolapic
     Don't use the local APIC (alias for i386 compatibility)

   pirq=...
	See Documentation/arch/x86/i386/IO-APIC.rst

   noapictimer
	Don't set up the APIC timer

   no_timer_check
	Don't check the IO-APIC timer. This can work around
	problems with incorrect timer initialization on some boards.

   apicpmtimer
	Do APIC timer calibration using the pmtimer. Implies
	apicmaintimer. Useful when your PIT timer is totally broken.

Timing
======

  notsc
    Deprecated, use tsc=unstable instead.

  nohpet
    Don't use the HPET timer.

Idle loop
=========

  idle=poll
    Don't do power saving in the idle loop using HLT, but poll for rescheduling
    event. This will make the CPUs eat a lot more power, but may be useful
    to get slightly better performance in multiprocessor benchmarks. It also
    makes some profiling using performance counters more accurate.
    Please note that on systems with MONITOR/MWAIT support (like Intel EM64T
    CPUs) this option has no performance advantage over the normal idle loop.
    It may also interact badly with hyperthreading.

Rebooting
=========

   reboot=b[ios] | t[riple] | k[bd] | a[cpi] | e[fi] | p[ci] [, [w]arm | [c]old]
      bios
        Use the CPU reboot vector for warm reset
      warm
        Don't set the cold reboot flag
      cold
        Set the cold reboot flag
      triple
        Force a triple fault (init)
      kbd
        Use the keyboard controller. cold reset (default)
      acpi
        Use the ACPI RESET_REG in the FADT. If ACPI is not configured or
        the ACPI reset does not work, the reboot path attempts the reset
        using the keyboard controller.
      efi
        Use efi reset_system runtime service. If EFI is not configured or
        the EFI reset does not work, the reboot path attempts the reset using
        the keyboard controller.
      pci
        Use a write to the PCI config space register 0xcf9 to trigger reboot.

   Using warm reset will be much faster especially on big memory
   systems because the BIOS will not go through the memory check.
   Disadvantage is that not all hardware will be completely reinitialized
   on reboot so there may be boot problems on some systems.

   reboot=force
     Don't stop other CPUs on reboot. This can make reboot more reliable
     in some cases.

   reboot=default
     There are some built-in platform specific "quirks" - you may see:
     "reboot: <name> series board detected. Selecting <type> for reboots."
     In the case where you think the quirk is in error (e.g. you have
     newer BIOS, or newer board) using this option will ignore the built-in
     quirk table, and use the generic default reboot actions.

NUMA
====

  numa=off
    Only set up a single NUMA node spanning all memory.

  numa=noacpi
    Don't parse the SRAT table for NUMA setup

  numa=nohmat
    Don't parse the HMAT table for NUMA setup, or soft-reserved memory
    partitioning.

  numa=fake=<size>[MG]
    If given as a memory unit, fills all system RAM with nodes of
    size interleaved over physical nodes.

  numa=fake=<N>
    If given as an integer, fills all system RAM with N fake nodes
    interleaved over physical nodes.

  numa=fake=<N>U
    If given as an integer followed by 'U', it will divide each
    physical node into N emulated nodes.

ACPI
====

  acpi=off
    Don't enable ACPI
  acpi=ht
    Use ACPI boot table parsing, but don't enable ACPI interpreter
  acpi=force
    Force ACPI on (currently not needed)
  acpi=strict
    Disable out of spec ACPI workarounds.
  acpi_sci={edge,level,high,low}
    Set up ACPI SCI interrupt.
  acpi=noirq
    Don't route interrupts
  acpi=nocmcff
    Disable firmware first mode for corrected errors. This
    disables parsing the HEST CMC error source to check if
    firmware has set the FF flag. This may result in
    duplicate corrected error reports.

PCI
===

  pci=off
    Don't use PCI
  pci=conf1
    Use conf1 access.
  pci=conf2
    Use conf2 access.
  pci=rom
    Assign ROMs.
  pci=assign-busses
    Assign busses
  pci=irqmask=MASK
    Set PCI interrupt mask to MASK
  pci=lastbus=NUMBER
    Scan up to NUMBER busses, no matter what the mptable says.
  pci=noacpi
    Don't use ACPI to set up PCI interrupt routing.

IOMMU (input/output memory management unit)
===========================================
Multiple x86-64 PCI-DMA mapping implementations exist, for example:

   1. <kernel/dma/direct.c>: use no hardware/software IOMMU at all
      (e.g. because you have < 3 GB memory).
      Kernel boot message: "PCI-DMA: Disabling IOMMU"

   2. <arch/x86/kernel/amd_gart_64.c>: AMD GART based hardware IOMMU.
      Kernel boot message: "PCI-DMA: using GART IOMMU"

   3. <arch/x86_64/kernel/pci-swiotlb.c> : Software IOMMU implementation. Used
      e.g. if there is no hardware IOMMU in the system and it is need because
      you have >3GB memory or told the kernel to us it (iommu=soft))
      Kernel boot message: "PCI-DMA: Using software bounce buffering
      for IO (SWIOTLB)"

::

  iommu=[<size>][,noagp][,off][,force][,noforce]
  [,memaper[=<order>]][,merge][,fullflush][,nomerge]
  [,noaperture]

General iommu options:

    off
      Don't initialize and use any kind of IOMMU.
    noforce
      Don't force hardware IOMMU usage when it is not needed. (default).
    force
      Force the use of the hardware IOMMU even when it is
      not actually needed (e.g. because < 3 GB memory).
    soft
      Use software bounce buffering (SWIOTLB) (default for
      Intel machines). This can be used to prevent the usage
      of an available hardware IOMMU.

iommu options only relevant to the AMD GART hardware IOMMU:

    <size>
      Set the size of the remapping area in bytes.
    allowed
      Overwrite iommu off workarounds for specific chipsets.
    fullflush
      Flush IOMMU on each allocation (default).
    nofullflush
      Don't use IOMMU fullflush.
    memaper[=<order>]
      Allocate an own aperture over RAM with size 32MB<<order.
      (default: order=1, i.e. 64MB)
    merge
      Do scatter-gather (SG) merging. Implies "force" (experimental).
    nomerge
      Don't do scatter-gather (SG) merging.
    noaperture
      Ask the IOMMU not to touch the aperture for AGP.
    noagp
      Don't initialize the AGP driver and use full aperture.
    panic
      Always panic when IOMMU overflows.

iommu options only relevant to the software bounce buffering (SWIOTLB) IOMMU
implementation:

    swiotlb=<slots>[,force,noforce]
      <slots>
        Prereserve that many 2K slots for the software IO bounce buffering.
      force
        Force all IO through the software TLB.
      noforce
        Do not initialize the software TLB.


Miscellaneous
=============

  nogbpages
    Do not use GB pages for kernel direct mappings.
  gbpages
    Use GB pages for kernel direct mappings.


AMD SEV (Secure Encrypted Virtualization)
=========================================
Options relating to AMD SEV, specified via the following format:

::

   sev=option1[,option2]

The available options are:

   debug
     Enable debug messages.