boot-options.txt

AMD64 specific boot options

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

Machine check

   mce=off disable machine check
   mce=bootlog Enable logging of machine checks left over from booting.
               Disabled by default on AMD 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=tolerancelevel (number)
		0: always panic on uncorrected errors, log corrected errors
		1: panic or SIGBUS on uncorrected errors, log corrected errors
		2: SIGBUS or log uncorrected errors, log corrected errors
		3: never panic or SIGBUS, log all errors (for testing only)
		Default is 1
		Can be also set using sysfs which is preferable.

   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/i386/IO-APIC.txt

   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.

   apicmaintimer Run time keeping from the local APIC timer instead
                 of using the PIT/HPET interrupt for this. This is useful
                 when the PIT/HPET interrupts are unreliable.

   noapicmaintimer  Don't do time keeping using the APIC timer.
		 Useful when this option was auto selected, but doesn't work.

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

   disable_8254_timer / enable_8254_timer
		 Enable interrupt 0 timer routing over the 8254 in addition to over
	         the IO-APIC. The kernel tries to set a sensible default.

Early Console

   syntax: earlyprintk=vga
           earlyprintk=serial[,ttySn[,baudrate]]

   The early console is useful when the kernel crashes before the
   normal console is initialized. It is not enabled by
   default because it has some cosmetic problems.
   Append ,keep to not disable it when the real console takes over.
   Only vga or serial at a time, not both.
   Currently only ttyS0 and ttyS1 are supported.
   Interaction with the standard serial driver is not very good.
   The VGA output is eventually overwritten by the real console.

Timing

  notsc
  Don't use the CPU time stamp counter to read the wall time.
  This can be used to work around timing problems on multiprocessor systems
  with not properly synchronized CPUs.

  report_lost_ticks
  Report when timer interrupts are lost because some code turned off
  interrupts for too long.

  nmi_watchdog=NUMBER[,panic]
  NUMBER can be:
  0 don't use an NMI watchdog
  1 use the IO-APIC timer for the NMI watchdog
  2 use the local APIC for the NMI watchdog using a performance counter. Note
  This will use one performance counter and the local APIC's performance
  vector.
  When panic is specified panic when an NMI watchdog timeout occurs.
  This is useful when you use a panic=... timeout and need the box
  quickly up again.

  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] [, [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)

   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.

Non Executable Mappings

  noexec=on|off

  on      Enable(default)
  off     Disable

SMP

  additional_cpus=NUM Allow NUM more CPUs for hotplug
		 (defaults are specified by the BIOS, see Documentation/x86_64/cpu-hotplug-spec)

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=fake=CMDLINE
		If a number, fakes CMDLINE nodes and ignores NUMA setup of the
		actual machine.  Otherwise, system memory is configured
		depending on the sizes and coefficients listed.  For example:
			numa=fake=2*512,1024,4*256,*128
		gives two 512M nodes, a 1024M node, four 256M nodes, and the
		rest split into 128M chunks.  If the last character of CMDLINE
		is a *, the remaining memory is divided up equally among its
		coefficient:
			numa=fake=2*512,2*
		gives two 512M nodes and the rest split into two nodes.
		Otherwise, the remaining system RAM is allocated to an
		additional node.

  numa=hotadd=percent
		Only allow hotadd memory to preallocate page structures upto
		percent of already available memory.
		numa=hotadd=0 will disable hotadd memory.

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

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 upto 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)

 Currently four x86-64 PCI-DMA mapping implementations exist:

   1. <arch/x86_64/kernel/pci-nommu.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_64/kernel/pci-gart.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)"

   4. <arch/x86_64/pci-calgary.c> : IBM Calgary hardware IOMMU. Used in IBM
      pSeries and xSeries servers. This hardware IOMMU supports DMA address
      mapping with memory protection, etc.
      Kernel boot message: "PCI-DMA: Using Calgary IOMMU"

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

  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.
    leak               Turn on simple iommu leak tracing (only when
                       CONFIG_IOMMU_LEAK is on). Default number of leak pages
                       is 20.
    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.
    forcesac           Force single-address cycle (SAC) mode for masks <40bits
                       (experimental).
    noagp              Don't initialize the AGP driver and use full aperture.
    allowdac           Allow double-address cycle (DAC) mode, i.e. DMA >4GB.
                       DAC is used with 32-bit PCI to push a 64-bit address in
                       two cycles. When off all DMA over >4GB is forced through
                       an IOMMU or software bounce buffering.
    nodac              Forbid DAC mode, i.e. DMA >4GB.
    panic              Always panic when IOMMU overflows.
    calgary            Use the Calgary IOMMU if it is available

  iommu options only relevant to the software bounce buffering (SWIOTLB) IOMMU
  implementation:
    swiotlb=<pages>[,force]
    <pages>            Prereserve that many 128K pages for the software IO
                       bounce buffering.
    force              Force all IO through the software TLB.

  Settings for the IBM Calgary hardware IOMMU currently found in IBM
  pSeries and xSeries machines:

    calgary=[64k,128k,256k,512k,1M,2M,4M,8M]
    calgary=[translate_empty_slots]
    calgary=[disable=<PCI bus number>]
    panic              Always panic when IOMMU overflows

    64k,...,8M - Set the size of each PCI slot's translation table
    when using the Calgary IOMMU. This is the size of the translation
    table itself in main memory. The smallest table, 64k, covers an IO
    space of 32MB; the largest, 8MB table, can cover an IO space of
    4GB. Normally the kernel will make the right choice by itself.

    translate_empty_slots - Enable translation even on slots that have
    no devices attached to them, in case a device will be hotplugged
    in the future.

    disable=<PCI bus number> - Disable translation on a given PHB. For
    example, the built-in graphics adapter resides on the first bridge
    (PCI bus number 0); if translation (isolation) is enabled on this
    bridge, X servers that access the hardware directly from user
    space might stop working. Use this option if you have devices that
    are accessed from userspace directly on some PCI host bridge.

Debugging

  oops=panic	Always panic on oopses. Default is to just kill the process,
		but there is a small probability of deadlocking the machine.
		This will also cause panics on machine check exceptions.
		Useful together with panic=30 to trigger a reboot.

  kstack=N	Print N words from the kernel stack in oops dumps.

  pagefaulttrace  Dump all page faults. Only useful for extreme debugging
		and will create a lot of output.

  call_trace=[old|both|newfallback|new]
		old: use old inexact backtracer
		new: use new exact dwarf2 unwinder
 		both: print entries from both
		newfallback: use new unwinder but fall back to old if it gets
			stuck (default)

Miscellaneous