[PATCH v2] KVM: arm64: Disabling disabled PMU counters wastes a lot of time

[Alexandru Elisei]

Hi Robin,

On 7/12/21 4:44 PM, Robin Murphy wrote:
> On 2021-07-12 16:17, Alexandre Chartre wrote:
>> In a KVM guest on arm64, performance counters interrupts have an
>> unnecessary overhead which slows down execution when using the "perf
>> record" command and limits the "perf record" sampling period.
>>
>> The problem is that when a guest VM disables counters by clearing the
>> PMCR_EL0.E bit (bit 0), KVM will disable all counters defined in
>> PMCR_EL0 even if they are not enabled in PMCNTENSET_EL0.
>>
>> KVM disables a counter by calling into the perf framework, in particular
>> by calling perf_event_create_kernel_counter() which is a time consuming
>> operation. So, for example, with a Neoverse N1 CPU core which has 6 event
>> counters and one cycle counter, KVM will always disable all 7 counters
>> even if only one is enabled.
>>
>> This typically happens when using the "perf record" command in a guest
>> VM: perf will disable all event counters with PMCNTENTSET_EL0 and only
>> uses the cycle counter. And when using the "perf record" -F option with
>> a high profiling frequency, the overhead of KVM disabling all counters
>> instead of one on every counter interrupt becomes very noticeable.
>>
>> The problem is fixed by having KVM disable only counters which are
>> enabled in PMCNTENSET_EL0. If a counter is not enabled in PMCNTENSET_EL0
>> then KVM will not enable it when setting PMCR_EL0.E and it will remain
>> disabled as long as it is not enabled in PMCNTENSET_EL0. So there is
>> effectively no need to disable a counter when clearing PMCR_EL0.E if it
>> is not enabled PMCNTENSET_EL0.
>>
>> Signed-off-by: Alexandre Chartre <alexandre.chartre at oracle.com>
>> ---
>> The patch is based on
>> https://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git/log/?h=kvm-arm64/pmu/reset-values
>>
>>   arch/arm64/kvm/pmu-emul.c | 8 +++++---
>>   1 file changed, 5 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
>> index fae4e95b586c..1f317c3dac61 100644
>> --- a/arch/arm64/kvm/pmu-emul.c
>> +++ b/arch/arm64/kvm/pmu-emul.c
>> @@ -563,21 +563,23 @@ void kvm_pmu_software_increment(struct kvm_vcpu *vcpu,
>> u64 val)
>>    */
>>   void kvm_pmu_handle_pmcr(struct kvm_vcpu *vcpu, u64 val)
>>   {
>> -    unsigned long mask = kvm_pmu_valid_counter_mask(vcpu);
>> +    unsigned long mask;
>>       int i;
>>         if (val & ARMV8_PMU_PMCR_E) {
>>           kvm_pmu_enable_counter_mask(vcpu,
>>                  __vcpu_sys_reg(vcpu, PMCNTENSET_EL0));
>>       } else {
>> -        kvm_pmu_disable_counter_mask(vcpu, mask);
>> +        kvm_pmu_disable_counter_mask(vcpu,
>> +               __vcpu_sys_reg(vcpu, PMCNTENSET_EL0));
>>       }
>>         if (val & ARMV8_PMU_PMCR_C)
>>           kvm_pmu_set_counter_value(vcpu, ARMV8_PMU_CYCLE_IDX, 0);
>>         if (val & ARMV8_PMU_PMCR_P) {
>> -        mask &= ~BIT(ARMV8_PMU_CYCLE_IDX);
>> +        mask = kvm_pmu_valid_counter_mask(vcpu)
>> +            & BIT(ARMV8_PMU_CYCLE_IDX);
>
> This looks suspiciously opposite of what it replaces; 

It always sets the bit, which goes against the architecture and the code it was
replacing, yes.

> however did we even need to do a bitwise operation here in the first place?
> Couldn't we skip the cycle counter by just limiting the for_each_set_bit
> iteration below to 31 bits?

To quote myself [1]:

"Entertained the idea of restricting the number of bits in for_each_set_bit() to
31 since Linux (and the architecture, to some degree) treats the cycle count
register as the 32nd event counter. Settled on this approach because I think it's
clearer."

To expand on that, incorrectly resetting the cycle counter was introduced by a
refactoring, so I preferred making it very clear that PMCR_EL0.P is not supposed
to clear the cycle counter.

[1] https://lore.kernel.org/kvmarm/20210618105139.83795-1-alexandru.elisei@arm.com/

Thanks,

Alex

>
> Robin.
>
>>           for_each_set_bit(i, &mask, 32)
>>               kvm_pmu_set_counter_value(vcpu, i, 0);
>>       }
>>
>> base-commit: 83f870a663592797c576846db3611e0a1664eda2
>>