diff mbox series

[v2,06/21] KVM: nSVM: Skip writes to MSR_AMD64_TSC_RATIO if guest state isn't loaded

Message ID 20230729011608.1065019-7-seanjc@google.com
State New
Headers
Series KVM: x86: Add "governed" X86_FEATURE framework |

Commit Message

Sean Christopherson July 29, 2023, 1:15 a.m. UTC 
  Skip writes to MSR_AMD64_TSC_RATIO that are done in the context of a vCPU
if guest state isn't loaded, i.e. if KVM will update MSR_AMD64_TSC_RATIO
during svm_prepare_switch_to_guest() before entering the guest.  Checking
guest_state_loaded may or may not be a net positive for performance as
the current_tsc_ratio cache will optimize away duplicate WRMSRs in the
vast majority of scenarios.  However, the cost of the check is negligible,
and the real motivation is to document that KVM needs to load the vCPU's
value only when running the vCPU.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/svm/svm.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
diff mbox series

Patch

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index c786c8e9108f..64092df06f94 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -1109,7 +1109,8 @@  static void svm_write_tsc_offset(struct kvm_vcpu *vcpu)
 void svm_write_tsc_multiplier(struct kvm_vcpu *vcpu)
 {
 	preempt_disable();
-	__svm_write_tsc_multiplier(vcpu->arch.tsc_scaling_ratio);
+	if (to_svm(vcpu)->guest_state_loaded)
+		__svm_write_tsc_multiplier(vcpu->arch.tsc_scaling_ratio);
 	preempt_enable();
 }