KVM: nSVM: Skip writes to MSR_AMD64_TSC_RATIO if guest state isn't loaded

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.

Link: https://lore.kernel.org/r/20230729011608.1065019-7-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 2542066..6d8932e 100644 (file)
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -1149,7 +1149,8 @@ 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();
 }