1KVM CPUID bits 2Glauber Costa <glommer@redhat.com>, Red Hat Inc, 2010 3===================================================== 4 5A guest running on a kvm host, can check some of its features using 6cpuid. This is not always guaranteed to work, since userspace can 7mask-out some, or even all KVM-related cpuid features before launching 8a guest. 9 10KVM cpuid functions are: 11 12function: KVM_CPUID_SIGNATURE (0x40000000) 13returns : eax = 0x40000001, 14 ebx = 0x4b4d564b, 15 ecx = 0x564b4d56, 16 edx = 0x4d. 17Note that this value in ebx, ecx and edx corresponds to the string "KVMKVMKVM". 18The value in eax corresponds to the maximum cpuid function present in this leaf, 19and will be updated if more functions are added in the future. 20Note also that old hosts set eax value to 0x0. This should 21be interpreted as if the value was 0x40000001. 22This function queries the presence of KVM cpuid leafs. 23 24 25function: define KVM_CPUID_FEATURES (0x40000001) 26returns : ebx, ecx 27 eax = an OR'ed group of (1 << flag), where each flags is: 28 29 30flag || value || meaning 31============================================================================= 32KVM_FEATURE_CLOCKSOURCE || 0 || kvmclock available at msrs 33 || || 0x11 and 0x12. 34------------------------------------------------------------------------------ 35KVM_FEATURE_NOP_IO_DELAY || 1 || not necessary to perform delays 36 || || on PIO operations. 37------------------------------------------------------------------------------ 38KVM_FEATURE_MMU_OP || 2 || deprecated. 39------------------------------------------------------------------------------ 40KVM_FEATURE_CLOCKSOURCE2 || 3 || kvmclock available at msrs 41 || || 0x4b564d00 and 0x4b564d01 42------------------------------------------------------------------------------ 43KVM_FEATURE_ASYNC_PF || 4 || async pf can be enabled by 44 || || writing to msr 0x4b564d02 45------------------------------------------------------------------------------ 46KVM_FEATURE_STEAL_TIME || 5 || steal time can be enabled by 47 || || writing to msr 0x4b564d03. 48------------------------------------------------------------------------------ 49KVM_FEATURE_PV_EOI || 6 || paravirtualized end of interrupt 50 || || handler can be enabled by writing 51 || || to msr 0x4b564d04. 52------------------------------------------------------------------------------ 53KVM_FEATURE_PV_UNHALT || 7 || guest checks this feature bit 54 || || before enabling paravirtualized 55 || || spinlock support. 56------------------------------------------------------------------------------ 57KVM_FEATURE_PV_TLB_FLUSH || 9 || guest checks this feature bit 58 || || before enabling paravirtualized 59 || || tlb flush. 60------------------------------------------------------------------------------ 61KVM_FEATURE_ASYNC_PF_VMEXIT || 10 || paravirtualized async PF VM exit 62 || || can be enabled by setting bit 2 63 || || when writing to msr 0x4b564d02 64------------------------------------------------------------------------------ 65KVM_FEATURE_PV_SEND_IPI || 11 || guest checks this feature bit 66 || || before using paravirtualized 67 || || send IPIs. 68------------------------------------------------------------------------------ 69KVM_FEATURE_CLOCKSOURCE_STABLE_BIT || 24 || host will warn if no guest-side 70 || || per-cpu warps are expected in 71 || || kvmclock. 72------------------------------------------------------------------------------ 73 74 edx = an OR'ed group of (1 << flag), where each flags is: 75 76 77flag || value || meaning 78================================================================================== 79KVM_HINTS_REALTIME || 0 || guest checks this feature bit to 80 || || determine that vCPUs are never 81 || || preempted for an unlimited time, 82 || || allowing optimizations 83---------------------------------------------------------------------------------- 84
