On Fri, Oct 11, 2019 at 01:39:54PM +0100, Marc Zyngier wrote:
> 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@xxxxxxxxxx>
> ---
Reviewed-by: Andrew Murray <andrew.murray@xxxxxxx>
> virt/kvm/arm/pmu.c | 20 ++++++++++++++++++++
> 1 file changed, 20 insertions(+)
>
> diff --git a/virt/kvm/arm/pmu.c b/virt/kvm/arm/pmu.c
> index f291d4ac3519..8731dfeced8b 100644
> --- a/virt/kvm/arm/pmu.c
> +++ b/virt/kvm/arm/pmu.c
> @@ -8,6 +8,7 @@
> #include <linux/kvm.h>
> #include <linux/kvm_host.h>
> #include <linux/perf_event.h>
> +#include <linux/perf/arm_pmu.h>
> #include <linux/uaccess.h>
> #include <asm/kvm_emulate.h>
> #include <kvm/arm_pmu.h>
> @@ -442,8 +443,25 @@ static void kvm_pmu_perf_overflow(struct perf_event *perf_event,
> struct pt_regs *regs)
> {
> struct kvm_pmc *pmc = perf_event->overflow_handler_context;
> + struct arm_pmu *cpu_pmu = to_arm_pmu(perf_event->pmu);
> struct kvm_vcpu *vcpu = kvm_pmc_to_vcpu(pmc);
> int idx = pmc->idx;
> + u64 period;
> +
> + cpu_pmu->pmu.stop(perf_event, PERF_EF_UPDATE);
> +
> + /*
> + * 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);
> +
> + local64_set(&perf_event->hw.period_left, 0);
> + perf_event->attr.sample_period = period;
> + perf_event->hw.sample_period = period;
>
> __vcpu_sys_reg(vcpu, PMOVSSET_EL0) |= BIT(idx);
>
> @@ -451,6 +469,8 @@ static void kvm_pmu_perf_overflow(struct perf_event *perf_event,
> kvm_make_request(KVM_REQ_IRQ_PENDING, vcpu);
> kvm_vcpu_kick(vcpu);
> }
> +
> + cpu_pmu->pmu.start(perf_event, PERF_EF_RELOAD);
> }
>
> /**
> --
> 2.20.1
>
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