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KVM: X86: Provide a capability to disable MWAIT intercepts

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Allowing a guest to execute MWAIT without interception enables a guest
to put a (physical) CPU into a power saving state, where it takes
longer to return from than what may be desired by the host.

Don't give a guest that power over a host by default. (Especially,
since nothing prevents a guest from using MWAIT even when it is not
advertised via CPUID.)

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Jan H. Schönherr <jschoenh@amazon.de>
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Wanpeng Li 2018-03-12 04:53:02 -07:00 • committed by Paolo Bonzini
parent 4956aa3b8b
commit 4d5422cea3
8 changed files with 53 additions and 25 deletions
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27
Documentation/virtual/kvm/api.txt
View file

@ -4358,6 +4358,24 @@ enables QEMU to build error log and branch to guest kernel registered
machine check handling routine. Without this capability KVM will
branch to guests' 0x200 interrupt vector.
7.13 KVM_CAP_X86_DISABLE_EXITS
Architectures: x86
Parameters: args[0] defines which exits are disabled
Returns: 0 on success, -EINVAL when args[0] contains invalid exits
Valid bits in args[0] are
#define KVM_X86_DISABLE_EXITS_MWAIT (1 << 0)
Enabling this capability on a VM provides userspace with a way to no
longer intercept some instructions for improved latency in some
workloads, and is suggested when vCPUs are associated to dedicated
physical CPUs. More bits can be added in the future; userspace can
just pass the KVM_CHECK_EXTENSION result to KVM_ENABLE_CAP to disable
all such vmexits.
8. Other capabilities.
----------------------
@ -4470,15 +4488,6 @@ reserved.
Both registers and addresses are 64-bits wide.
It will be possible to run 64-bit or 32-bit guest code.
8.8 KVM_CAP_X86_GUEST_MWAIT
Architectures: x86
This capability indicates that guest using memory monotoring instructions
(MWAIT/MWAITX) to stop the virtual CPU will not cause a VM exit. As such time
spent while virtual CPU is halted in this way will then be accounted for as
guest running time on the host (as opposed to e.g. HLT).
8.9 KVM_CAP_ARM_USER_IRQ
Architectures: arm, arm64