Re: [PATCH 1/9] KVM: arm64: PMU: Align chained counter implementation with architecture pseudocode

[Marc Zyngier <maz@kernel.org>]

Hi Reiji,

On 2022-10-27 15:33, Reiji Watanabe wrote:
> Hi Marc,
> 
>> > > +static void kvm_pmu_counter_increment(struct kvm_vcpu *vcpu,
>> > > +                                     unsigned long mask, u32 event)
>> > > +{
>> > > +       int i;
>> > > +
>> > > +       if (!kvm_vcpu_has_pmu(vcpu))
>> > > +               return;
>> > > +
>> > > +       if (!(__vcpu_sys_reg(vcpu, PMCR_EL0) & ARMV8_PMU_PMCR_E))
>> > > +               return;
>> > > +
>> > > +       /* Weed out disabled counters */
>> > > +       mask &= __vcpu_sys_reg(vcpu, PMCNTENSET_EL0);
>> > > +
>> > > +       for_each_set_bit(i, &mask, ARMV8_PMU_CYCLE_IDX) {
>> > > +               u64 type, reg;
>> > > +
>> > > +               /* Filter on event type */
>> > > +               type = __vcpu_sys_reg(vcpu, PMEVTYPER0_EL0 + i);
>> > > +               type &= kvm_pmu_event_mask(vcpu->kvm);
>> > > +               if (type != event)
>> > > +                       continue;
>> > > +
>> > > +               /* Increment this counter */
>> > > +               reg = __vcpu_sys_reg(vcpu, PMEVCNTR0_EL0 + i) + 1;
>> > > +               reg = lower_32_bits(reg);
>> > > +               __vcpu_sys_reg(vcpu, PMEVCNTR0_EL0 + i) = reg;
>> > > +
>> > > +               if (reg) /* No overflow? move on */
>> > > +                       continue;
>> > > +
>> > > +               /* Mark overflow */
>> > > +               __vcpu_sys_reg(vcpu, PMOVSSET_EL0) |= BIT(i);
>> >
>> > Perhaps it might be useful to create another helper that takes
>> > care of just one counter (it would essentially do the code above
>> > in the loop). The helper could be used (in addition to the above
>> > loop) from the code below for the CHAIN event case and from
>> > kvm_pmu_perf_overflow(). Then unnecessary execution of
>> > for_each_set_bit() could be avoided for these two cases.
>> 
>> I'm not sure it really helps. We would still need to check whether the
>> counter is enabled, and we'd need to bring that into the helper
>> instead of keeping it outside of the loop.
> 
> That's true. It seems that I overlooked that.
> Although it appears checking with kvm_vcpu_has_pmu() is unnecessary
> (redundant), the check with PMCR_EL0.E is necessary.

Ah indeed, I'll drop the kvm_vcpu_has_pmu() check.

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...

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