[PATCH v6 34/44] KVM: x86/pmu: Elide WRMSRs when loading guest PMCs if values already match

[Sean Christopherson]

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 <manali.shukla at amd.com>
Tested-by: Xudong Hao <xudong.hao at intel.com>
Signed-off-by: Sean Christopherson <seanjc at google.com>
---
 arch/x86/kvm/pmu.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
index cb07d9b62bee..fdf1df3100eb 100644
--- a/arch/x86/kvm/pmu.c
+++ b/arch/x86/kvm/pmu.c
@@ -1314,13 +1314,15 @@ 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);
 	}
 }
 
-- 
2.52.0.223.gf5cc29aaa4-goog