[PATCH 3/3] KVM: arm64: pmu: Reset sample period on overflow handling

[maz@kernel.org]

From: Marc Zyngier <maz@kernel.org>

The PMU emulation code uses the perf event sample period to trigger
the overflow detection. This works fine  for the *first* overflow
handling, but results in a huge number of interrupts on the host,
unrelated to the number of interrupts handled in the guest (a x20
factor is pretty common for the cycle counter). On a slow system
(such as a SW model), this can result in the guest only making
forward progress at a glacial pace.

It turns out that the clue is in the name. The sample period is
exactly that: a period. And once the an overflow has occured,
the following period should be the full width of the associated
counter, instead of whatever the guest had initially programed.

Reset the sample period to the architected value in the overflow
handler, which now results in a number of host interrupts that is
much closer to the number of interrupts in the guest.

Fixes: b02386eb7dac ("arm64: KVM: Add PMU overflow interrupt routing")
Signed-off-by: Marc Zyngier <maz@kernel.org>
---
 virt/kvm/arm/pmu.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/virt/kvm/arm/pmu.c b/virt/kvm/arm/pmu.c
index c30c3a74fc7f..3ca4761fc0f5 100644
--- a/virt/kvm/arm/pmu.c
+++ b/virt/kvm/arm/pmu.c
@@ -444,6 +444,18 @@ static void kvm_pmu_perf_overflow(struct perf_event *perf_event,
 	struct kvm_pmc *pmc = perf_event->overflow_handler_context;
 	struct kvm_vcpu *vcpu = kvm_pmc_to_vcpu(pmc);
 	int idx = pmc->idx;
+	u64 val, period;
+
+	/* Start by resetting the sample period to the architectural limit */
+	val = kvm_pmu_get_pair_counter_value(vcpu, pmc);
+
+	if (kvm_pmu_idx_is_64bit(vcpu, pmc->idx))
+		period = (-val) & GENMASK(63, 0);
+	else
+		period = (-val) & GENMASK(31, 0);
+
+	pmc->perf_event->attr.sample_period = period;
+	pmc->perf_event->hw.sample_period = period;
 
 	__vcpu_sys_reg(vcpu, PMOVSSET_EL0) |= BIT(idx);
 
-- 
2.20.1


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