KVM CPUID bitsΒΆ

Author:Glauber Costa <glommer@gmail.com>

A guest running on a kvm host, can check some of its features using cpuid. This is not always guaranteed to work, since userspace can mask-out some, or even all KVM-related cpuid features before launching a guest.

KVM cpuid functions are:

function: KVM_CPUID_SIGNATURE (0x40000000)

returns:

eax = 0x40000001
ebx = 0x4b4d564b
ecx = 0x564b4d56
edx = 0x4d

Note that this value in ebx, ecx and edx corresponds to the string "KVMKVMKVM". The value in eax corresponds to the maximum cpuid function present in this leaf, and will be updated if more functions are added in the future. Note also that old hosts set eax value to 0x0. This should be interpreted as if the value was 0x40000001. This function queries the presence of KVM cpuid leafs.

function: define KVM_CPUID_FEATURES (0x40000001)

returns:

ebx, ecx
eax = an OR'ed group of (1 << flag)

where flag is defined as below:

flag value meaning
KVM_FEATURE_CLOCKSOURCE 0 kvmclock available at msrs 0x11 and 0x12
KVM_FEATURE_NOP_IO_DELAY 1 not necessary to perform delays on PIO operations
KVM_FEATURE_MMU_OP 2 deprecated
KVM_FEATURE_CLOCKSOURCE2 3

kvmclock available at msrs

0x4b564d00 and 0x4b564d01

KVM_FEATURE_ASYNC_PF 4 async pf can be enabled by writing to msr 0x4b564d02
KVM_FEATURE_STEAL_TIME 5 steal time can be enabled by writing to msr 0x4b564d03
KVM_FEATURE_PV_EOI 6 paravirtualized end of interrupt handler can be enabled by writing to msr 0x4b564d04
KVM_FEATURE_PV_UNHAULT 7 guest checks this feature bit before enabling paravirtualized spinlock support
KVM_FEATURE_PV_TLB_FLUSH 9 guest checks this feature bit before enabling paravirtualized tlb flush
KVM_FEATURE_ASYNC_PF_VMEXIT 10 paravirtualized async PF VM EXIT can be enabled by setting bit 2 when writing to msr 0x4b564d02
KVM_FEATURE_PV_SEND_IPI 11 guest checks this feature bit before enabling paravirtualized sebd IPIs
KVM_FEATURE_PV_POLL_CONTROL 12 host-side polling on HLT can be disabled by writing to msr 0x4b564d05.
KVM_FEATURE_PV_SCHED_YIELD 13 guest checks this feature bit before using paravirtualized sched yield.
KVM_FEATURE_CLOCSOURCE_STABLE_BIT 24 host will warn if no guest-side per-cpu warps are expeced in kvmclock
edx = an OR'ed group of (1 << flag)

Where flag here is defined as below:

flag value meaning
KVM_HINTS_REALTIME 0 guest checks this feature bit to determine that vCPUs are never preempted for an unlimited time allowing optimizations