84 lines
4.7 KiB
Plaintext
84 lines
4.7 KiB
Plaintext
KVM CPUID bits
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Glauber Costa <glommer@redhat.com>, Red Hat Inc, 2010
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=====================================================
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A guest running on a kvm host, can check some of its features using
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cpuid. This is not always guaranteed to work, since userspace can
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mask-out some, or even all KVM-related cpuid features before launching
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a guest.
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KVM cpuid functions are:
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function: KVM_CPUID_SIGNATURE (0x40000000)
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returns : eax = 0x40000001,
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ebx = 0x4b4d564b,
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ecx = 0x564b4d56,
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edx = 0x4d.
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Note that this value in ebx, ecx and edx corresponds to the string "KVMKVMKVM".
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The value in eax corresponds to the maximum cpuid function present in this leaf,
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and will be updated if more functions are added in the future.
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Note also that old hosts set eax value to 0x0. This should
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be interpreted as if the value was 0x40000001.
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This function queries the presence of KVM cpuid leafs.
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function: define KVM_CPUID_FEATURES (0x40000001)
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returns : ebx, ecx
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eax = an OR'ed group of (1 << flag), where each flags is:
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flag || value || meaning
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=============================================================================
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KVM_FEATURE_CLOCKSOURCE || 0 || kvmclock available at msrs
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|| || 0x11 and 0x12.
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------------------------------------------------------------------------------
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KVM_FEATURE_NOP_IO_DELAY || 1 || not necessary to perform delays
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|| || on PIO operations.
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------------------------------------------------------------------------------
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KVM_FEATURE_MMU_OP || 2 || deprecated.
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------------------------------------------------------------------------------
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KVM_FEATURE_CLOCKSOURCE2 || 3 || kvmclock available at msrs
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|| || 0x4b564d00 and 0x4b564d01
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------------------------------------------------------------------------------
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KVM_FEATURE_ASYNC_PF || 4 || async pf can be enabled by
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|| || writing to msr 0x4b564d02
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------------------------------------------------------------------------------
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KVM_FEATURE_STEAL_TIME || 5 || steal time can be enabled by
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|| || writing to msr 0x4b564d03.
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------------------------------------------------------------------------------
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KVM_FEATURE_PV_EOI || 6 || paravirtualized end of interrupt
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|| || handler can be enabled by writing
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|| || to msr 0x4b564d04.
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------------------------------------------------------------------------------
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KVM_FEATURE_PV_UNHALT || 7 || guest checks this feature bit
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|| || before enabling paravirtualized
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|| || spinlock support.
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------------------------------------------------------------------------------
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KVM_FEATURE_PV_TLB_FLUSH || 9 || guest checks this feature bit
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|| || before enabling paravirtualized
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|| || tlb flush.
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------------------------------------------------------------------------------
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KVM_FEATURE_ASYNC_PF_VMEXIT || 10 || paravirtualized async PF VM exit
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|| || can be enabled by setting bit 2
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|| || when writing to msr 0x4b564d02
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------------------------------------------------------------------------------
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KVM_FEATURE_PV_SEND_IPI || 11 || guest checks this feature bit
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|| || before using paravirtualized
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|| || send IPIs.
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------------------------------------------------------------------------------
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KVM_FEATURE_CLOCKSOURCE_STABLE_BIT || 24 || host will warn if no guest-side
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|| || per-cpu warps are expected in
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|| || kvmclock.
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------------------------------------------------------------------------------
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edx = an OR'ed group of (1 << flag), where each flags is:
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flag || value || meaning
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==================================================================================
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KVM_HINTS_REALTIME || 0 || guest checks this feature bit to
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|| || determine that vCPUs are never
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|| || preempted for an unlimited time,
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|| || allowing optimizations
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----------------------------------------------------------------------------------
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