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================== ============ =================================
125