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[34.82.181.220]) by smtp.gmail.com with ESMTPSA id ij19-20020a170902ab5300b001769e6d4fafsm3865136plb.57.2022.12.01.08.51.49 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 01 Dec 2022 08:51:49 -0800 (PST) Date: Thu, 1 Dec 2022 08:51:46 -0800 From: Ricardo Koller To: Marc Zyngier Cc: linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, kvmarm@lists.linux.dev, kvm@vger.kernel.org, James Morse , Suzuki K Poulose , Alexandru Elisei , Oliver Upton , Reiji Watanabe Subject: Re: [PATCH v4 04/16] KVM: arm64: PMU: Distinguish between 64bit counter and 64bit overflow Message-ID: References: <20221113163832.3154370-1-maz@kernel.org> <20221113163832.3154370-5-maz@kernel.org> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20221201_085152_744114_3462272B X-CRM114-Status: GOOD ( 32.22 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Thu, Dec 01, 2022 at 08:47:47AM -0800, Ricardo Koller wrote: > 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 > > --- > > 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. Sorry, this should be: Then "val = -(counter & GENMASK(31, 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 > > > > _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel