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, 1: panic if deadlock possible,
		2: try to avoid panic, 3: never panic or exit (for testing)
		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

  nosmp	Only use a single CPU

  maxcpus=NUMBER only use upto NUMBER CPUs

  cpumask=MASK   only use cpus with bits set in mask

  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=X   Fake X nodes and ignore NUMA setup of the actual machine.

  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

 iommu=[size][,noagp][,off][,force][,noforce][,leak][,memaper[=order]][,merge]
         [,forcesac][,fullflush][,nomerge][,noaperture]
   size  set size of iommu (in bytes)
   noagp don't initialize the AGP driver and use full aperture.
   off   don't use the IOMMU
   leak  turn on simple iommu leak tracing (only when CONFIG_IOMMU_LEAK is on)
   memaper[=order] allocate an own aperture over RAM with size 32MB^order.
   noforce don't force IOMMU usage. Default.
   force  Force IOMMU.
   merge  Do SG merging. Implies force (experimental)
   nomerge Don't do SG merging.
   forcesac For SAC mode for masks <40bits  (experimental)
   fullflush Flush IOMMU on each allocation (default)
   nofullflush Don't use IOMMU fullflush
   allowed  overwrite iommu off workarounds for specific chipsets.
   soft	 Use software bounce buffering (default for Intel machines)
   noaperture Don't touch the aperture for AGP.
   allowdac Allow DMA >4GB
	    When off all DMA over >4GB is forced through an IOMMU or bounce
	    buffering.
   nodac    Forbid DMA >4GB
   panic    Always panic when IOMMU overflows

  swiotlb=pages[,force]

  pages  Prereserve that many 128K pages for the software IO bounce buffering.
  force  Force all IO through the software TLB.

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

    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 that many 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)

  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)

Misc

  noreplacement  Don't replace instructions with more appropriate ones
		 for the CPU. This may be useful on asymmetric MP systems
		 where some CPU have less capabilities than the others.