Documentation/virtual/kvm/cpuid.rst

   1 .. SPDX-License-Identifier: GPL-2.0
   2
   3 ==============
   4 KVM CPUID bits
   5 ==============
   6
   7 :Author: Glauber Costa <glommer@gmail.com>
   8
   9 A guest running on a kvm host, can check some of its features using
  10 cpuid. This is not always guaranteed to work, since userspace can
  11 mask-out some, or even all KVM-related cpuid features before launching
  12 a guest.
  13
  14 KVM cpuid functions are:
  15
  16 function: KVM_CPUID_SIGNATURE (0x40000000)
  17
  18 returns::
  19
  20    eax = 0x40000001
  21    ebx = 0x4b4d564b
  22    ecx = 0x564b4d56
  23    edx = 0x4d
  24
  25 Note that this value in ebx, ecx and edx corresponds to the string "KVMKVMKVM".
  26 The value in eax corresponds to the maximum cpuid function present in this leaf,
  27 and will be updated if more functions are added in the future.
  28 Note also that old hosts set eax value to 0x0. This should
  29 be interpreted as if the value was 0x40000001.
  30 This function queries the presence of KVM cpuid leafs.
  31
  32 function: define KVM_CPUID_FEATURES (0x40000001)
  33
  34 returns::
  35
  36           ebx, ecx
  37           eax = an OR'ed group of (1 << flag)
  38
  39 where ``flag`` is defined as below:
  40
  41 ================================= =========== ================================
  42 flag                              value       meaning
  43 ================================= =========== ================================
  44 KVM_FEATURE_CLOCKSOURCE           0           kvmclock available at msrs
  45                                               0x11 and 0x12
  46
  47 KVM_FEATURE_NOP_IO_DELAY          1           not necessary to perform delays
  48                                               on PIO operations
  49
  50 KVM_FEATURE_MMU_OP                2           deprecated
  51
  52 KVM_FEATURE_CLOCKSOURCE2          3           kvmclock available at msrs
  53
  54                                               0x4b564d00 and 0x4b564d01
  55 KVM_FEATURE_ASYNC_PF              4           async pf can be enabled by
  56                                               writing to msr 0x4b564d02
  57
  58 KVM_FEATURE_STEAL_TIME            5           steal time can be enabled by
  59                                               writing to msr 0x4b564d03
  60
  61 KVM_FEATURE_PV_EOI                6           paravirtualized end of interrupt
  62                                               handler can be enabled by
  63                                               writing to msr 0x4b564d04
  64
  65 KVM_FEATURE_PV_UNHAULT            7           guest checks this feature bit
  66                                               before enabling paravirtualized
  67                                               spinlock support
  68
  69 KVM_FEATURE_PV_TLB_FLUSH          9           guest checks this feature bit
  70                                               before enabling paravirtualized
  71                                               tlb flush
  72
  73 KVM_FEATURE_ASYNC_PF_VMEXIT       10          paravirtualized async PF VM EXIT
  74                                               can be enabled by setting bit 2
  75                                               when writing to msr 0x4b564d02
  76
  77 KVM_FEATURE_PV_SEND_IPI           11          guest checks this feature bit
  78                                               before enabling paravirtualized
  79                                               sebd IPIs
  80
  81 KVM_FEATURE_PV_POLL_CONTROL       12          host-side polling on HLT can
  82                                               be disabled by writing
  83                                               to msr 0x4b564d05.
  84
  85 KVM_FEATURE_PV_SCHED_YIELD        13          guest checks this feature bit
  86                                               before using paravirtualized
  87                                               sched yield.
  88
  89 KVM_FEATURE_CLOCSOURCE_STABLE_BIT 24          host will warn if no guest-side
  90                                               per-cpu warps are expeced in
  91                                               kvmclock
  92 ================================= =========== ================================
  93
  94 ::
  95
  96       edx = an OR'ed group of (1 << flag)
  97
  98 Where ``flag`` here is defined as below:
  99
 100 ================== ============ =================================
 101 flag               value        meaning
 102 ================== ============ =================================
 103 KVM_HINTS_REALTIME 0            guest checks this feature bit to
 104                                 determine that vCPUs are never
 105                                 preempted for an unlimited time
 106                                 allowing optimizations
 107 ================== ============ =================================