From: Sean Christopherson Date: Sat, 29 Jul 2023 01:15:53 +0000 (-0700) Subject: KVM: nSVM: Skip writes to MSR_AMD64_TSC_RATIO if guest state isn't loaded X-Git-Url: http://git.maquefel.me/?a=commitdiff_plain;h=223f93d4d88aedc02bdcc92b3734e76f707812f3;p=linux.git 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 --- diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c index 2542066652a34..6d8932e057536 100644 --- 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(); }