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1.. SPDX-License-Identifier: GPL-2.02 3==============4KVM CPUID bits5==============6 7:Author: Glauber Costa <glommer@gmail.com>8 9A guest running on a kvm host, can check some of its features using10cpuid. This is not always guaranteed to work, since userspace can11mask-out some, or even all KVM-related cpuid features before launching12a guest.13 14KVM cpuid functions are:15 16function: KVM_CPUID_SIGNATURE (0x40000000)17 18returns::19 20 eax = 0x4000000121 ebx = 0x4b4d564b22 ecx = 0x564b4d5623 edx = 0x4d24 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 should29be 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, ecx37 eax = an OR'ed group of (1 << flag)38 39where ``flag`` is defined as below:40 41================================== =========== ================================42flag value meaning43================================== =========== ================================44KVM_FEATURE_CLOCKSOURCE 0 kvmclock available at msrs45 0x11 and 0x1246 47KVM_FEATURE_NOP_IO_DELAY 1 not necessary to perform delays48 on PIO operations49 50KVM_FEATURE_MMU_OP 2 deprecated51 52KVM_FEATURE_CLOCKSOURCE2 3 kvmclock available at msrs53 0x4b564d00 and 0x4b564d0154 55KVM_FEATURE_ASYNC_PF 4 async pf can be enabled by56 writing to msr 0x4b564d0257 58KVM_FEATURE_STEAL_TIME 5 steal time can be enabled by59 writing to msr 0x4b564d0360 61KVM_FEATURE_PV_EOI 6 paravirtualized end of interrupt62 handler can be enabled by63 writing to msr 0x4b564d0464 65KVM_FEATURE_PV_UNHALT 7 guest checks this feature bit66 before enabling paravirtualized67 spinlock support68 69KVM_FEATURE_PV_TLB_FLUSH 9 guest checks this feature bit70 before enabling paravirtualized71 tlb flush72 73KVM_FEATURE_ASYNC_PF_VMEXIT 10 paravirtualized async PF VM EXIT74 can be enabled by setting bit 275 when writing to msr 0x4b564d0276 77KVM_FEATURE_PV_SEND_IPI 11 guest checks this feature bit78 before enabling paravirtualized79 send IPIs80 81KVM_FEATURE_POLL_CONTROL 12 host-side polling on HLT can82 be disabled by writing83 to msr 0x4b564d05.84 85KVM_FEATURE_PV_SCHED_YIELD 13 guest checks this feature bit86 before using paravirtualized87 sched yield.88 89KVM_FEATURE_ASYNC_PF_INT 14 guest checks this feature bit90 before using the second async91 pf control msr 0x4b564d06 and92 async pf acknowledgment msr93 0x4b564d07.94 95KVM_FEATURE_MSI_EXT_DEST_ID 15 guest checks this feature bit96 before using extended destination97 ID bits in MSI address bits 11-5.98 99KVM_FEATURE_HC_MAP_GPA_RANGE 16 guest checks this feature bit before100 using the map gpa range hypercall101 to notify the page state change102 103KVM_FEATURE_MIGRATION_CONTROL 17 guest checks this feature bit before104 using MSR_KVM_MIGRATION_CONTROL105 106KVM_FEATURE_CLOCKSOURCE_STABLE_BIT 24 host will warn if no guest-side107 per-cpu warps are expected in108 kvmclock109================================== =========== ================================110 111::112 113 edx = an OR'ed group of (1 << flag)114 115Where ``flag`` here is defined as below:116 117================== ============ =================================118flag value meaning119================== ============ =================================120KVM_HINTS_REALTIME 0 guest checks this feature bit to121 determine that vCPUs are never122 preempted for an unlimited time123 allowing optimizations124================== ============ =================================125