[PATCH v2] KVM: VMX: Micro-optimize vmexit time when not exposing PMU

[Wanpeng Li]

From: Wanpeng Li <wanpengli@xxxxxxxxxxx>

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@xxxxxxxxxxx>
---
v1 -> v2:
 * move the check before atomic_switch_perf_msrs

 arch/x86/kvm/vmx/vmx.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 40b1e61..b20423c 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6567,7 +6567,8 @@ 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)
-- 
2.7.4