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=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 gives two 512M nodes, a 1024M node, and four 256M nodes. The remaining system RAM is allocated to an additional node. If the last character of CMDLINE is a *, the remaining system RAM is instead divided up equally among its coefficient. 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. : use no hardware/software IOMMU at all (e.g. because you have < 3 GB memory). Kernel boot message: "PCI-DMA: Disabling IOMMU" 2. : AMD GART based hardware IOMMU. Kernel boot message: "PCI-DMA: using GART IOMMU" 3. : 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. : 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][,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: 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<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=[,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. 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