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- KVM CPUID bits
- Glauber Costa <glommer@redhat.com>, Red Hat Inc, 2010
- =====================================================
- 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 each flags is:
- 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_UNHALT || 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 using paravirtualized
- || || send IPIs.
- ------------------------------------------------------------------------------
- KVM_FEATURE_CLOCKSOURCE_STABLE_BIT || 24 || host will warn if no guest-side
- || || per-cpu warps are expected in
- || || kvmclock.
- ------------------------------------------------------------------------------
- edx = an OR'ed group of (1 << flag), where each flags is:
- 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
- ----------------------------------------------------------------------------------
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