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

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>

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index e961633182f88279137ed2b09e24465f647b980e..b447d66f44e60d881e9d5610d459abb3c34fe998 100644 (file)
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -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)