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SubjectRe: [PATCH] KVM: ARM64: Update perf event when setting PMU count value
On Sun, May 19, 2019 at 06:05:59PM +0800, Xiang Zheng wrote:
> Guest will adjust the sample period and set PMU counter value when
> it takes a long time to handle the PMU interrupts.
>
> However, we don't have a corresponding change on the virtual PMU
> which is emulated via a perf event. It could cause a large number
> of PMU interrupts injected to guest. Then guest will get hang for
> handling these interrupts.

Yes this is indeed an issue. I believe I've addressed this in my 'chained
pmu' series - the relevant patch is here...

https://lists.cs.columbia.edu/pipermail/kvmarm/2019-May/035933.html

Some other comments below.

>
> So update the sample_period of perf event if the counter value is
> changed to avoid this case.
>
> Signed-off-by: Xiang Zheng <zhengxiang9@huawei.com>
> ---
> virt/kvm/arm/pmu.c | 54 +++++++++++++++++++++++++++++++++++++++++++++---------
> 1 file changed, 45 insertions(+), 9 deletions(-)
>
> diff --git a/virt/kvm/arm/pmu.c b/virt/kvm/arm/pmu.c
> index 1c5b76c..cbad3ec 100644
> --- a/virt/kvm/arm/pmu.c
> +++ b/virt/kvm/arm/pmu.c
> @@ -24,6 +24,11 @@
> #include <kvm/arm_pmu.h>
> #include <kvm/arm_vgic.h>
>
> +static void kvm_pmu_stop_counter(struct kvm_vcpu *vcpu, struct kvm_pmc *pmc);
> +static struct perf_event *kvm_pmu_create_perf_event(struct kvm_vcpu *vcpu,
> + struct kvm_pmc *pmc,
> + struct perf_event_attr *attr);
> +
> /**
> * kvm_pmu_get_counter_value - get PMU counter value
> * @vcpu: The vcpu pointer
> @@ -57,11 +62,29 @@ u64 kvm_pmu_get_counter_value(struct kvm_vcpu *vcpu, u64 select_idx)
> */
> void kvm_pmu_set_counter_value(struct kvm_vcpu *vcpu, u64 select_idx, u64 val)
> {
> - u64 reg;
> + u64 reg, counter, old_sample_period;
> + struct kvm_pmu *pmu = &vcpu->arch.pmu;
> + struct kvm_pmc *pmc = &pmu->pmc[select_idx];
> + struct perf_event *event;
> + struct perf_event_attr attr;
>
> reg = (select_idx == ARMV8_PMU_CYCLE_IDX)
> ? PMCCNTR_EL0 : PMEVCNTR0_EL0 + select_idx;
> __vcpu_sys_reg(vcpu, reg) += (s64)val - kvm_pmu_get_counter_value(vcpu, select_idx);
> +
> + if (pmc->perf_event) {
> + attr = pmc->perf_event->attr;
> + old_sample_period = attr.sample_period;
> + counter = kvm_pmu_get_counter_value(vcpu, select_idx);
> + attr.sample_period = (-counter) & pmc->bitmask;
> + if (attr.sample_period == old_sample_period)
> + return;

I'd be interested to know how often this would evaluate to true.

> +
> + kvm_pmu_stop_counter(vcpu, pmc);
> + event = kvm_pmu_create_perf_event(vcpu, pmc, &attr);

I'm not sure it's necessary to change the prototype of kvm_pmu_create_perf_event
as this function will recalculate the sample period based on the updated counter
value anyway.

Thanks,

Andrew Murray

> + if (event)
> + pmc->perf_event = event;
> + }
> }
>
> /**
> @@ -303,6 +326,24 @@ static void kvm_pmu_perf_overflow(struct perf_event *perf_event,
> }
> }
>
> +static struct perf_event *kvm_pmu_create_perf_event(struct kvm_vcpu *vcpu,
> + struct kvm_pmc *pmc,
> + struct perf_event_attr *attr)
> +{
> + struct perf_event *event;
> +
> + event = perf_event_create_kernel_counter(attr, -1, current,
> + kvm_pmu_perf_overflow, pmc);
> +
> + if (IS_ERR(event)) {
> + pr_err_once("kvm: pmu event creation failed %ld\n",
> + PTR_ERR(event));
> + return NULL;
> + }
> +
> + return event;
> +}
> +
> /**
> * kvm_pmu_software_increment - do software increment
> * @vcpu: The vcpu pointer
> @@ -416,15 +457,10 @@ void kvm_pmu_set_counter_event_type(struct kvm_vcpu *vcpu, u64 data,
> /* The initial sample period (overflow count) of an event. */
> attr.sample_period = (-counter) & pmc->bitmask;
>
> - event = perf_event_create_kernel_counter(&attr, -1, current,
> - kvm_pmu_perf_overflow, pmc);
> - if (IS_ERR(event)) {
> - pr_err_once("kvm: pmu event creation failed %ld\n",
> - PTR_ERR(event));
> - return;
> - }
> + event = kvm_pmu_create_perf_event(vcpu, pmc, &attr);
>
> - pmc->perf_event = event;
> + if (event)
> + pmc->perf_event = event;
> }
>
> bool kvm_arm_support_pmu_v3(void)
> --
> 1.8.3.1
>
>
> _______________________________________________
> kvmarm mailing list
> kvmarm@lists.cs.columbia.edu
> https://lists.cs.columbia.edu/mailman/listinfo/kvmarm

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