cpuid.rst

  1.. SPDX-License-Identifier: GPL-2.0
  2
  3==============
  4KVM CPUID bits
  5==============
  6
  7:Author: Glauber Costa <glommer@gmail.com>
  8
  9A guest running on a kvm host, can check some of its features using
 10cpuid. This is not always guaranteed to work, since userspace can
 11mask-out some, or even all KVM-related cpuid features before launching
 12a guest.
 13
 14KVM cpuid functions are:
 15
 16function: KVM_CPUID_SIGNATURE (0x40000000)
 17
 18returns::
 19
 20   eax = 0x40000001
 21   ebx = 0x4b4d564b
 22   ecx = 0x564b4d56
 23   edx = 0x4d
 24
 25Note that this value in ebx, ecx and edx corresponds to the string "KVMKVMKVM".
 26The value in eax corresponds to the maximum cpuid function present in this leaf,
 27and will be updated if more functions are added in the future.
 28Note also that old hosts set eax value to 0x0. This should
 29be interpreted as if the value was 0x40000001.
 30This function queries the presence of KVM cpuid leafs.
 31
 32function: define KVM_CPUID_FEATURES (0x40000001)
 33
 34returns::
 35
 36          ebx, ecx
 37          eax = an OR'ed group of (1 << flag)
 38
 39where ``flag`` is defined as below:
 40
 41================================== =========== ================================
 42flag                               value       meaning
 43================================== =========== ================================
 44KVM_FEATURE_CLOCKSOURCE            0           kvmclock available at msrs
 45                                               0x11 and 0x12
 46
 47KVM_FEATURE_NOP_IO_DELAY           1           not necessary to perform delays
 48                                               on PIO operations
 49
 50KVM_FEATURE_MMU_OP                 2           deprecated
 51
 52KVM_FEATURE_CLOCKSOURCE2           3           kvmclock available at msrs
 53                                               0x4b564d00 and 0x4b564d01
 54
 55KVM_FEATURE_ASYNC_PF               4           async pf can be enabled by
 56                                               writing to msr 0x4b564d02
 57
 58KVM_FEATURE_STEAL_TIME             5           steal time can be enabled by
 59                                               writing to msr 0x4b564d03
 60
 61KVM_FEATURE_PV_EOI                 6           paravirtualized end of interrupt
 62                                               handler can be enabled by
 63                                               writing to msr 0x4b564d04
 64
 65KVM_FEATURE_PV_UNHALT              7           guest checks this feature bit
 66                                               before enabling paravirtualized
 67                                               spinlock support
 68
 69KVM_FEATURE_PV_TLB_FLUSH           9           guest checks this feature bit
 70                                               before enabling paravirtualized
 71                                               tlb flush
 72
 73KVM_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
 77KVM_FEATURE_PV_SEND_IPI            11          guest checks this feature bit
 78                                               before enabling paravirtualized
 79                                               send IPIs
 80
 81KVM_FEATURE_POLL_CONTROL           12          host-side polling on HLT can
 82                                               be disabled by writing
 83                                               to msr 0x4b564d05.
 84
 85KVM_FEATURE_PV_SCHED_YIELD         13          guest checks this feature bit
 86                                               before using paravirtualized
 87                                               sched yield.
 88
 89KVM_FEATURE_ASYNC_PF_INT           14          guest checks this feature bit
 90                                               before using the second async
 91                                               pf control msr 0x4b564d06 and
 92                                               async pf acknowledgment msr
 93                                               0x4b564d07.
 94
 95KVM_FEATURE_MSI_EXT_DEST_ID        15          guest checks this feature bit
 96                                               before using extended destination
 97                                               ID bits in MSI address bits 11-5.
 98
 99KVM_FEATURE_HC_MAP_GPA_RANGE       16          guest checks this feature bit before
100                                               using the map gpa range hypercall
101                                               to notify the page state change
102
103KVM_FEATURE_MIGRATION_CONTROL      17          guest checks this feature bit before
104                                               using MSR_KVM_MIGRATION_CONTROL
105
106KVM_FEATURE_CLOCKSOURCE_STABLE_BIT 24          host will warn if no guest-side
107                                               per-cpu warps are expected in
108                                               kvmclock
109================================== =========== ================================
110
111::
112
113      edx = an OR'ed group of (1 << flag)
114
115Where ``flag`` here is defined as below:
116
117================== ============ =================================
118flag               value        meaning
119================== ============ =================================
120KVM_HINTS_REALTIME 0            guest checks this feature bit to
121                                determine that vCPUs are never
122                                preempted for an unlimited time
123                                allowing optimizations
124================== ============ =================================