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

Sean Christopherson
2023-11-10T02:29:19Z

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@google.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