linux/Documentation/virtual/kvm/cpuid.txt
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   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, edx = 0
  27          eax = and 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_CLOCKSOURCE_STABLE_BIT ||    24 || host will warn if no guest-side
  47                                   ||       || per-cpu warps are expected in
  48                                   ||       || kvmclock.
  49------------------------------------------------------------------------------
  50
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