2019-07-10 23:30:53 +08:00
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.. SPDX-License-Identifier: GPL-2.0
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==============
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KVM CPUID bits
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==============
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:Author: Glauber Costa <glommer@gmail.com>
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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::
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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::
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ebx, ecx
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eax = an OR'ed group of (1 << flag)
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where ``flag`` is defined as below:
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2020-08-18 23:24:29 +08:00
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================================== =========== ================================
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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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2019-07-10 23:30:53 +08:00
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2020-08-18 23:24:29 +08:00
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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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2019-07-10 23:30:53 +08:00
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2020-08-18 23:24:29 +08:00
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KVM_FEATURE_MMU_OP 2 deprecated
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2019-07-10 23:30:53 +08:00
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2020-08-18 23:24:29 +08:00
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KVM_FEATURE_CLOCKSOURCE2 3 kvmclock available at msrs
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0x4b564d00 and 0x4b564d01
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2020-04-16 23:59:13 +08:00
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2020-08-18 23:24:29 +08:00
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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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2019-07-10 23:30:53 +08:00
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2020-08-18 23:24:29 +08:00
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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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2019-07-10 23:30:53 +08:00
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2020-08-18 23:24:29 +08:00
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KVM_FEATURE_PV_EOI 6 paravirtualized end of interrupt
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handler can be enabled by
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writing to msr 0x4b564d04
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2019-07-10 23:30:53 +08:00
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2020-08-18 23:24:29 +08:00
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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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2019-07-10 23:30:53 +08:00
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2020-08-18 23:24:29 +08:00
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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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2019-07-10 23:30:53 +08:00
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2020-08-18 23:24:29 +08:00
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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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2019-07-10 23:30:53 +08:00
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2020-08-18 23:24:29 +08:00
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KVM_FEATURE_PV_SEND_IPI 11 guest checks this feature bit
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before enabling paravirtualized
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send IPIs
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2019-07-10 23:30:53 +08:00
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2020-10-24 02:17:56 +08:00
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KVM_FEATURE_POLL_CONTROL 12 host-side polling on HLT can
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2020-08-18 23:24:29 +08:00
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be disabled by writing
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to msr 0x4b564d05.
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2019-07-10 23:30:53 +08:00
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2020-08-18 23:24:29 +08:00
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KVM_FEATURE_PV_SCHED_YIELD 13 guest checks this feature bit
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before using paravirtualized
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sched yield.
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2019-07-10 23:30:53 +08:00
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2020-08-18 23:24:29 +08:00
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KVM_FEATURE_ASYNC_PF_INT 14 guest checks this feature bit
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before using the second async
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pf control msr 0x4b564d06 and
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async pf acknowledgment msr
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0x4b564d07.
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2020-05-25 22:41:22 +08:00
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2020-10-19 22:55:56 +08:00
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KVM_FEATURE_MSI_EXT_DEST_ID 15 guest checks this feature bit
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before using extended destination
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ID bits in MSI address bits 11-5.
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2020-08-18 23:24:29 +08:00
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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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2019-07-10 23:30:53 +08:00
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::
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edx = an OR'ed group of (1 << flag)
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Where ``flag`` here is defined as below:
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================== ============ =================================
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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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