| David Brazdil | 0f672f6 | 2019-12-10 10:32:29 +0000 | [diff] [blame^] | 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 | ================== ============ ================================= |