Kernel-4.18.0-80.el8_boot-options

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/x86/x86_64/machinecheck 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=tolerancelevel[,monarchtimeout] (number,number)
tolerance levels:
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.
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/x86/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.
apicpmtimer
Do APIC timer calibration using the pmtimer. Implies
apicmaintimer. Useful when your PIT timer is totally
broken.

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.

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] [, [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.

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

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

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

numa=fake=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. <lib/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)”

  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=[][,noagp][,off][,force][,noforce][,leak[=]
    [,memaper[=]][,merge][,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:
    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[=] 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.
    calgary Use the Calgary IOMMU if it is available

    iommu options only relevant to the software bounce buffering (SWIOTLB) IOMMU
    implementation:
    swiotlb=[,force]
    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=]
    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= - 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.

Miscellaneous

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