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KVM: VMX: Micro-optimize vmexit time when not exposing PMU

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PMU is not exposed to guest by most of products from cloud providers since the
bad performance of PMU emulation and security concern. However, it calls
perf_guest_switch_get_msrs() and clear_atomic_switch_msr() unconditionally
even if PMU is not exposed to the guest before each vmentry.

~2% vmexit time reduced can be observed by kvm-unit-tests/vmexit.flat on my
SKX server.

Before patch:
vmcall 1559

After patch:
vmcall 1529

Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Wanpeng Li 2020-03-13 11:55:18 +08:00 committed by Paolo Bonzini
parent 331b4de9a7
commit 041bc42ce2
1 changed files with 2 additions and 1 deletions
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3
arch/x86/kvm/vmx/vmx.c
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@ -6549,7 +6549,8 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
pt_guest_enter(vmx);
atomic_switch_perf_msrs(vmx);
if (vcpu_to_pmu(vcpu)->version)
atomic_switch_perf_msrs(vmx);
atomic_switch_umwait_control_msr(vmx);
if (enable_preemption_timer)