linux-kernel - [PATCH 09/10] KVM: x86/pmu: Check eventsel first when emulating (branch) insns retired

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Date:   Thu,  9 Nov 2023 18:28:56 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Konstantin Khorenko <khorenko@...tuozzo.com>,
        Jim Mattson <jmattson@...gle.com>
Subject: [PATCH 09/10] KVM: x86/pmu: Check eventsel first when emulating
 (branch) insns retired

When triggering events, i.e. emulating PMC events in software, check for
a matching event selector before checking the event is allowed.  The "is
allowed" check *might* be cheap, but it could also be very costly, e.g. if
userspace has defined a large PMU event filter.  The event selector check
on the other hand is all but guaranteed to be <10 uops, e.g. looks
something like:

   0xffffffff8105e615 <+5>:	movabs $0xf0000ffff,%rax
   0xffffffff8105e61f <+15>:	xor    %rdi,%rsi
   0xffffffff8105e622 <+18>:	test   %rax,%rsi
   0xffffffff8105e625 <+21>:	sete   %al

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/pmu.c | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
index ba561849fd3e..a5ea729b16f2 100644
--- a/arch/x86/kvm/pmu.c
+++ b/arch/x86/kvm/pmu.c
@@ -847,9 +847,6 @@ void kvm_pmu_trigger_event(struct kvm_vcpu *vcpu, u64 eventsel)
 		return;
 
 	kvm_for_each_pmc(pmu, pmc, i, bitmap) {
-		if (!pmc_event_is_allowed(pmc))
-			continue;
-
 		/*
 		 * Ignore checks for edge detect (all events currently emulated
 		 * but KVM are always rising edges), pin control (unsupported
@@ -864,11 +861,11 @@ void kvm_pmu_trigger_event(struct kvm_vcpu *vcpu, u64 eventsel)
 		 * might be wrong if they are defined in the future, but so
 		 * could ignoring them, so do the simple thing for now.
 		 */
-		if ((pmc->eventsel ^ eventsel) & AMD64_RAW_EVENT_MASK_NB)
+		if (((pmc->eventsel ^ eventsel) & AMD64_RAW_EVENT_MASK_NB) ||
+		    !pmc_event_is_allowed(pmc) || !cpl_is_matched(pmc))
 			continue;
 
-		if (cpl_is_matched(pmc))
-			kvm_pmu_incr_counter(pmc);
+		kvm_pmu_incr_counter(pmc);
 	}
 }
 EXPORT_SYMBOL_GPL(kvm_pmu_trigger_event);
-- 
2.42.0.869.gea05f2083d-goog