From: Sean Christopherson Date: Sat, 6 Dec 2025 00:17:10 +0000 (-0800) Subject: KVM: x86/pmu: Elide WRMSRs when loading guest PMCs if values already match X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=b0b6a8d3be16ea742bf835407e9968378c0c753c;p=thirdparty%2Flinux.git KVM: x86/pmu: Elide WRMSRs when loading guest PMCs if values already match When loading a mediated PMU state, elide the WRMSRs to load PMCs with the guest's value if the value in hardware already matches the guest's value. For the relatively common case where neither the guest nor the host is actively using the PMU, i.e. when all/many counters are '0', eliding the WRMSRs reduces the latency of handling VM-Exit by a measurable amount (WRMSR is significantly more expensive than RDPMC). As measured by KVM-Unit-Tests' CPUID VM-Exit testcase, this provides a a ~25% reduction in latency (4k => 3k cycles) on Intel Emerald Rapids, and a ~13% reduction (6.2k => 5.3k cycles) on AMD Turin. Cc: Manali Shukla Tested-by: Xudong Hao Tested-by: Manali Shukla Link: https://patch.msgid.link/20251206001720.468579-35-seanjc@google.com Signed-off-by: Sean Christopherson --- diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c index b78ad897886dd..954622f8f8178 100644 --- a/arch/x86/kvm/pmu.c +++ b/arch/x86/kvm/pmu.c @@ -1312,13 +1312,15 @@ static void kvm_pmu_load_guest_pmcs(struct kvm_vcpu *vcpu) for (i = 0; i < pmu->nr_arch_gp_counters; i++) { pmc = &pmu->gp_counters[i]; - wrmsrl(gp_counter_msr(i), pmc->counter); + if (pmc->counter != rdpmc(i)) + wrmsrl(gp_counter_msr(i), pmc->counter); wrmsrl(gp_eventsel_msr(i), pmc->eventsel_hw); } for (i = 0; i < pmu->nr_arch_fixed_counters; i++) { pmc = &pmu->fixed_counters[i]; - wrmsrl(fixed_counter_msr(i), pmc->counter); + if (pmc->counter != rdpmc(INTEL_PMC_FIXED_RDPMC_BASE | i)) + wrmsrl(fixed_counter_msr(i), pmc->counter); } }