On Sun, Nov 13, 2022 at 04:38:20PM +0000, Marc Zyngier wrote: > The PMU architecture makes a subtle difference between a 64bit > counter and a counter that has a 64bit overflow. This is for example > the case of the cycle counter, which can generate an overflow on > a 32bit boundary if PMCR_EL0.LC==0 despite the accumulation being > done on 64 bits. > > Use this distinction in the few cases where it matters in the code, > as we will reuse this with PMUv3p5 long counters. > > Signed-off-by: Marc Zyngier <m...@kernel.org> > --- > arch/arm64/kvm/pmu-emul.c | 43 ++++++++++++++++++++++++++++----------- > 1 file changed, 31 insertions(+), 12 deletions(-) > > diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c > index 69b67ab3c4bf..d050143326b5 100644 > --- a/arch/arm64/kvm/pmu-emul.c > +++ b/arch/arm64/kvm/pmu-emul.c > @@ -50,6 +50,11 @@ static u32 kvm_pmu_event_mask(struct kvm *kvm) > * @select_idx: The counter index > */ > static bool kvm_pmu_idx_is_64bit(struct kvm_vcpu *vcpu, u64 select_idx) > +{ > + return (select_idx == ARMV8_PMU_CYCLE_IDX); > +} > + > +static bool kvm_pmu_idx_has_64bit_overflow(struct kvm_vcpu *vcpu, u64 > select_idx) > { > return (select_idx == ARMV8_PMU_CYCLE_IDX && > __vcpu_sys_reg(vcpu, PMCR_EL0) & ARMV8_PMU_PMCR_LC); > @@ -57,7 +62,8 @@ static bool kvm_pmu_idx_is_64bit(struct kvm_vcpu *vcpu, u64 > select_idx) > > static bool kvm_pmu_counter_can_chain(struct kvm_vcpu *vcpu, u64 idx) > { > - return (!(idx & 1) && (idx + 1) < ARMV8_PMU_CYCLE_IDX); > + return (!(idx & 1) && (idx + 1) < ARMV8_PMU_CYCLE_IDX && > + !kvm_pmu_idx_has_64bit_overflow(vcpu, idx)); > } > > static struct kvm_vcpu *kvm_pmc_to_vcpu(struct kvm_pmc *pmc) > @@ -97,7 +103,7 @@ u64 kvm_pmu_get_counter_value(struct kvm_vcpu *vcpu, u64 > select_idx) > counter += perf_event_read_value(pmc->perf_event, &enabled, > &running); > > - if (select_idx != ARMV8_PMU_CYCLE_IDX) > + if (!kvm_pmu_idx_is_64bit(vcpu, select_idx)) > counter = lower_32_bits(counter); > > return counter; > @@ -423,6 +429,23 @@ static void kvm_pmu_counter_increment(struct kvm_vcpu > *vcpu, > } > } > > +/* Compute the sample period for a given counter value */ > +static u64 compute_period(struct kvm_vcpu *vcpu, u64 select_idx, u64 counter) > +{ > + u64 val; > + > + if (kvm_pmu_idx_is_64bit(vcpu, select_idx)) { > + if (!kvm_pmu_idx_has_64bit_overflow(vcpu, select_idx)) > + val = -(counter & GENMASK(31, 0));
If I understand things correctly, this might be missing another mask: + if (!kvm_pmu_idx_has_64bit_overflow(vcpu, select_idx)) { + val = -(counter & GENMASK(31, 0)); + val &= GENMASK(31, 0); + } else { For example, if the counter is 64-bits wide, it overflows at 32-bits, and it is _one_ sample away from overflowing at 32-bits: 0x01010101_ffffffff Then "val = (-counter) & GENMASK(63, 0)" would return 0xffffffff_00000001. But the right period is 0x00000000_00000001 (it's one sample away from overflowing). > + else > + val = (-counter) & GENMASK(63, 0); > + } else { > + val = (-counter) & GENMASK(31, 0); > + } > + > + return val; > +} > + > /** > * When the perf event overflows, set the overflow status and inform the > vcpu. > */ > @@ -442,10 +465,7 @@ static void kvm_pmu_perf_overflow(struct perf_event > *perf_event, > * Reset the sample period to the architectural limit, > * i.e. the point where the counter overflows. > */ > - period = -(local64_read(&perf_event->count)); > - > - if (!kvm_pmu_idx_is_64bit(vcpu, pmc->idx)) > - period &= GENMASK(31, 0); > + period = compute_period(vcpu, idx, local64_read(&perf_event->count)); > > local64_set(&perf_event->hw.period_left, 0); > perf_event->attr.sample_period = period; > @@ -571,14 +591,13 @@ static void kvm_pmu_create_perf_event(struct kvm_vcpu > *vcpu, u64 select_idx) > > /* > * If counting with a 64bit counter, advertise it to the perf > - * code, carefully dealing with the initial sample period. > + * code, carefully dealing with the initial sample period > + * which also depends on the overflow. > */ > - if (kvm_pmu_idx_is_64bit(vcpu, select_idx)) { > + if (kvm_pmu_idx_is_64bit(vcpu, select_idx)) > attr.config1 |= PERF_ATTR_CFG1_COUNTER_64BIT; > - attr.sample_period = (-counter) & GENMASK(63, 0); > - } else { > - attr.sample_period = (-counter) & GENMASK(31, 0); > - } > + > + attr.sample_period = compute_period(vcpu, select_idx, counter); > > event = perf_event_create_kernel_counter(&attr, -1, current, > kvm_pmu_perf_overflow, pmc); > -- > 2.34.1 > > _______________________________________________ kvmarm mailing list kvmarm@lists.cs.columbia.edu https://lists.cs.columbia.edu/mailman/listinfo/kvmarm