KVM: arm64: pmu: Resync EL0 state on counter rotation

Huang Shijie reports that, when profiling a guest from the host
with a number of events that exceeds the number of available
counters, the reported counts are wildly inaccurate. Without
the counter oversubscription, the reported counts are correct.

Their investigation indicates that upon counter rotation (which
takes place on the back of a timer interrupt), we fail to
re-apply the guest EL0 enabling, leading to the counting of host
events instead of guest events.

In order to solve this, add yet another hook between the host PMU
driver and KVM, re-applying the guest EL0 configuration if the
right conditions apply (the host is VHE, we are in interrupt
context, and we interrupted a running vcpu). This triggers a new
vcpu request which will apply the correct configuration on guest
reentry.

With this, we have the correct counts, even when the counters are
oversubscribed.

Reported-by: Huang Shijie <shijie@os.amperecomputing.com>
Suggested-by: Oliver Upton <oliver.upton@linux.dev>
Tested_by: Huang Shijie <shijie@os.amperecomputing.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20230809013953.7692-1-shijie@os.amperecomputing.com
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20230820090108.177817-1-maz@kernel.org

1 files changed, 3 insertions, 0 deletions

diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index 72dc53a75d1c..978b0411082f 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -803,6 +803,9 @@ static int check_vcpu_requests(struct kvm_vcpu *vcpu)
 			kvm_pmu_handle_pmcr(vcpu,
 					    __vcpu_sys_reg(vcpu, PMCR_EL0));
 
+		if (kvm_check_request(KVM_REQ_RESYNC_PMU_EL0, vcpu))
+			kvm_vcpu_pmu_restore_guest(vcpu);
+
 		if (kvm_check_request(KVM_REQ_SUSPEND, vcpu))
 			return kvm_vcpu_suspend(vcpu);