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KVM: x86/pmu: Check eventsel first when emulating (branch) insns retired

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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

Link: https://lore.kernel.org/r/20231110022857.1273836-10-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
This commit is contained in:
Sean Christopherson 2023-11-09 18:28:56 -08:00
parent afda2d7666
commit e35529fb4a
1 changed files with 3 additions and 6 deletions
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9
arch/x86/kvm/pmu.c
View file

@ -857,9 +857,6 @@ 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
@ -874,11 +871,11 @@ 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);