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 54 0x4b564d00 and 0x4b564d01 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_UNHAULT 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 sebd IPIs 80 81KVM_FEATURE_PV_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_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 98Where ``flag`` here is defined as below: 99 100================== ============ ================================= 101flag value meaning 102================== ============ ================================= 103KVM_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================== ============ ================================= 108