Documentation/virtual/kvm/cpuid.txt

   1 KVM CPUID bits
   2 Glauber Costa <glommer@redhat.com>, Red Hat Inc, 2010
   3 =====================================================
   4
   5 A guest running on a kvm host, can check some of its features using
   6 cpuid. This is not always guaranteed to work, since userspace can
   7 mask-out some, or even all KVM-related cpuid features before launching
   8 a guest.
   9
  10 KVM cpuid functions are:
  11
  12 function: KVM_CPUID_SIGNATURE (0x40000000)
  13 returns : eax = 0x40000001,
  14           ebx = 0x4b4d564b,
  15           ecx = 0x564b4d56,
  16           edx = 0x4d.
  17 Note that this value in ebx, ecx and edx corresponds to the string "KVMKVMKVM".
  18 The value in eax corresponds to the maximum cpuid function present in this leaf,
  19 and will be updated if more functions are added in the future.
  20 Note also that old hosts set eax value to 0x0. This should
  21 be interpreted as if the value was 0x40000001.
  22 This function queries the presence of KVM cpuid leafs.
  23
  24
  25 function: define KVM_CPUID_FEATURES (0x40000001)
  26 returns : ebx, ecx, edx = 0
  27           eax = and OR'ed group of (1 << flag), where each flags is:
  28
  29
  30 flag                               || value || meaning
  31 =============================================================================
  32 KVM_FEATURE_CLOCKSOURCE            ||     0 || kvmclock available at msrs
  33                                    ||       || 0x11 and 0x12.
  34 ------------------------------------------------------------------------------
  35 KVM_FEATURE_NOP_IO_DELAY           ||     1 || not necessary to perform delays
  36                                    ||       || on PIO operations.
  37 ------------------------------------------------------------------------------
  38 KVM_FEATURE_MMU_OP                 ||     2 || deprecated.
  39 ------------------------------------------------------------------------------
  40 KVM_FEATURE_CLOCKSOURCE2           ||     3 || kvmclock available at msrs
  41                                    ||       || 0x4b564d00 and 0x4b564d01
  42 ------------------------------------------------------------------------------
  43 KVM_FEATURE_ASYNC_PF               ||     4 || async pf can be enabled by
  44                                    ||       || writing to msr 0x4b564d02
  45 ------------------------------------------------------------------------------
  46 KVM_FEATURE_STEAL_TIME             ||     5 || steal time can be enabled by
  47                                    ||       || writing to msr 0x4b564d03.
  48 ------------------------------------------------------------------------------
  49 KVM_FEATURE_PV_EOI                 ||     6 || paravirtualized end of interrupt
  50                                    ||       || handler can be enabled by writing
  51                                    ||       || to msr 0x4b564d04.
  52 ------------------------------------------------------------------------------
  53 KVM_FEATURE_PV_UNHALT              ||     7 || guest checks this feature bit
  54                                    ||       || before enabling paravirtualized
  55                                    ||       || spinlock support.
  56 ------------------------------------------------------------------------------
  57 KVM_FEATURE_PV_TLB_FLUSH           ||     9 || guest checks this feature bit
  58                                    ||       || before enabling paravirtualized
  59                                    ||       || tlb flush.
  60 ------------------------------------------------------------------------------
  61 KVM_FEATURE_CLOCKSOURCE_STABLE_BIT ||    24 || host will warn if no guest-side
  62                                    ||       || per-cpu warps are expected in
  63                                    ||       || kvmclock.
  64 ------------------------------------------------------------------------------