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