.. 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
================== ============ =================================