| .. SPDX-License-Identifier: GPL-2.0 |
| |
| ============== |
| KVM CPUID bits |
| ============== |
| |
| :Author: Glauber Costa <glommer@gmail.com> |
| |
| A guest running on a kvm host, can check some of its features using |
| cpuid. This is not always guaranteed to work, since userspace can |
| mask-out some, or even all KVM-related cpuid features before launching |
| a guest. |
| |
| KVM cpuid functions are: |
| |
| function: KVM_CPUID_SIGNATURE (0x40000000) |
| |
| returns:: |
| |
| eax = 0x40000001 |
| ebx = 0x4b4d564b |
| ecx = 0x564b4d56 |
| edx = 0x4d |
| |
| Note that this value in ebx, ecx and edx corresponds to the string "KVMKVMKVM". |
| The value in eax corresponds to the maximum cpuid function present in this leaf, |
| and will be updated if more functions are added in the future. |
| Note also that old hosts set eax value to 0x0. This should |
| be interpreted as if the value was 0x40000001. |
| This function queries the presence of KVM cpuid leafs. |
| |
| function: define KVM_CPUID_FEATURES (0x40000001) |
| |
| returns:: |
| |
| ebx, ecx |
| eax = an OR'ed group of (1 << flag) |
| |
| where ``flag`` is defined as below: |
| |
| ================================= =========== ================================ |
| flag value meaning |
| ================================= =========== ================================ |
| KVM_FEATURE_CLOCKSOURCE 0 kvmclock available at msrs |
| 0x11 and 0x12 |
| |
| KVM_FEATURE_NOP_IO_DELAY 1 not necessary to perform delays |
| on PIO operations |
| |
| KVM_FEATURE_MMU_OP 2 deprecated |
| |
| KVM_FEATURE_CLOCKSOURCE2 3 kvmclock available at msrs |
| |
| 0x4b564d00 and 0x4b564d01 |
| KVM_FEATURE_ASYNC_PF 4 async pf can be enabled by |
| writing to msr 0x4b564d02 |
| |
| KVM_FEATURE_STEAL_TIME 5 steal time can be enabled by |
| writing to msr 0x4b564d03 |
| |
| KVM_FEATURE_PV_EOI 6 paravirtualized end of interrupt |
| handler can be enabled by |
| writing to msr 0x4b564d04 |
| |
| KVM_FEATURE_PV_UNHAULT 7 guest checks this feature bit |
| before enabling paravirtualized |
| spinlock support |
| |
| KVM_FEATURE_PV_TLB_FLUSH 9 guest checks this feature bit |
| before enabling paravirtualized |
| tlb flush |
| |
| KVM_FEATURE_ASYNC_PF_VMEXIT 10 paravirtualized async PF VM EXIT |
| can be enabled by setting bit 2 |
| when writing to msr 0x4b564d02 |
| |
| KVM_FEATURE_PV_SEND_IPI 11 guest checks this feature bit |
| before enabling paravirtualized |
| sebd IPIs |
| |
| KVM_FEATURE_PV_POLL_CONTROL 12 host-side polling on HLT can |
| be disabled by writing |
| to msr 0x4b564d05. |
| |
| KVM_FEATURE_PV_SCHED_YIELD 13 guest checks this feature bit |
| before using paravirtualized |
| sched yield. |
| |
| KVM_FEATURE_CLOCSOURCE_STABLE_BIT 24 host will warn if no guest-side |
| per-cpu warps are expeced in |
| kvmclock |
| ================================= =========== ================================ |
| |
| :: |
| |
| edx = an OR'ed group of (1 << flag) |
| |
| Where ``flag`` here is defined as below: |
| |
| ================== ============ ================================= |
| flag value meaning |
| ================== ============ ================================= |
| KVM_HINTS_REALTIME 0 guest checks this feature bit to |
| determine that vCPUs are never |
| preempted for an unlimited time |
| allowing optimizations |
| ================== ============ ================================= |
