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SubjectRe: [RFC 2/3] KVM: arm64: pmu: Fix chained SW_INCR counters
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Hi Marc,

On 12/5/19 3:52 PM, Marc Zyngier wrote:
> On 2019-12-05 14:06, Auger Eric wrote:
>> Hi Marc,
>>
>> On 12/5/19 10:43 AM, Marc Zyngier wrote:
>>> Hi Eric,
>>>
>>> On 2019-12-04 20:44, Eric Auger wrote:
>>>> At the moment a SW_INCR counter always overflows on 32-bit
>>>> boundary, independently on whether the n+1th counter is
>>>> programmed as CHAIN.
>>>>
>>>> Check whether the SW_INCR counter is a 64b counter and if so,
>>>> implement the 64b logic.
>>>>
>>>> Fixes: 80f393a23be6 ("KVM: arm/arm64: Support chained PMU counters")
>>>> Signed-off-by: Eric Auger <eric.auger@redhat.com>
>>>> ---
>>>>  virt/kvm/arm/pmu.c | 16 +++++++++++++++-
>>>>  1 file changed, 15 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/virt/kvm/arm/pmu.c b/virt/kvm/arm/pmu.c
>>>> index c3f8b059881e..7ab477db2f75 100644
>>>> --- a/virt/kvm/arm/pmu.c
>>>> +++ b/virt/kvm/arm/pmu.c
>>>> @@ -491,6 +491,8 @@ void kvm_pmu_software_increment(struct kvm_vcpu
>>>> *vcpu, u64 val)
>>>>
>>>>      enable = __vcpu_sys_reg(vcpu, PMCNTENSET_EL0);
>>>>      for (i = 0; i < ARMV8_PMU_CYCLE_IDX; i++) {
>>>> +        bool chained = test_bit(i >> 1, vcpu->arch.pmu.chained);
>>>> +
>>>
>>> I'd rather you use kvm_pmu_pmc_is_chained() rather than open-coding
>>> this. But see below:
>>>
>>>>          if (!(val & BIT(i)))
>>>>              continue;
>>>>          type = __vcpu_sys_reg(vcpu, PMEVTYPER0_EL0 + i)
>>>> @@ -500,8 +502,20 @@ void kvm_pmu_software_increment(struct kvm_vcpu
>>>> *vcpu, u64 val)
>>>>              reg = __vcpu_sys_reg(vcpu, PMEVCNTR0_EL0 + i) + 1;
>>>>              reg = lower_32_bits(reg);
>>>>              __vcpu_sys_reg(vcpu, PMEVCNTR0_EL0 + i) = reg;
>>>> -            if (!reg)
>>>> +            if (reg) /* no overflow */
>>>> +                continue;
>>>> +            if (chained) {
>>>> +                reg = __vcpu_sys_reg(vcpu, PMEVCNTR0_EL0 + i + 1) + 1;
>>>> +                reg = lower_32_bits(reg);
>>>> +                __vcpu_sys_reg(vcpu, PMEVCNTR0_EL0 + i + 1) = reg;
>>>> +                if (reg)
>>>> +                    continue;
>>>> +                /* mark an overflow on high counter */
>>>> +                __vcpu_sys_reg(vcpu, PMOVSSET_EL0) |= BIT(i + 1);
>>>> +            } else {
>>>> +                /* mark an overflow */
>>>>                  __vcpu_sys_reg(vcpu, PMOVSSET_EL0) |= BIT(i);
>>>> +            }
>>>>          }
>>>>      }
>>>>  }
>>>
>>> I think the whole function is a bit of a mess, and could be better
>>> structured to treat 64bit counters as a first class citizen.
>>>
>>> I'm suggesting something along those lines, which tries to
>>> streamline things a bit and keep the flow uniform between the
>>> two word sizes. IMHO, it helps reasonning about it and gives
>>> scope to the ARMv8.5 full 64bit counters... It is of course
>>> completely untested.
>>
>> Looks OK to me as well. One remark though, don't we need to test if the
>> n+1th reg is enabled before incrementing it?
>
> Hmmm. I'm not sure. I think we should make sure that we don't flag
> a counter as being chained if the odd counter is disabled, rather
> than checking it here. As long as the odd counter is not chained
> *and* enabled, we shouldn't touch it.>
> Again, untested:
>
> diff --git a/virt/kvm/arm/pmu.c b/virt/kvm/arm/pmu.c
> index cf371f643ade..47366817cd2a 100644
> --- a/virt/kvm/arm/pmu.c
> +++ b/virt/kvm/arm/pmu.c
> @@ -15,6 +15,7 @@
>  #include <kvm/arm_vgic.h>
>
>  static void kvm_pmu_create_perf_event(struct kvm_vcpu *vcpu, u64
> select_idx);
> +static void kvm_pmu_update_pmc_chained(struct kvm_vcpu *vcpu, u64
> select_idx);
>
>  #define PERF_ATTR_CFG1_KVM_PMU_CHAINED 0x1
>
> @@ -298,6 +299,7 @@ void kvm_pmu_enable_counter_mask(struct kvm_vcpu
> *vcpu, u64 val)
>           * For high counters of chained events we must recreate the
>           * perf event with the long (64bit) attribute set.
>           */
> +        kvm_pmu_update_pmc_chained(vcpu, i);
>          if (kvm_pmu_pmc_is_chained(pmc) &&
>              kvm_pmu_idx_is_high_counter(i)) {
>              kvm_pmu_create_perf_event(vcpu, i);
> @@ -645,7 +647,8 @@ static void kvm_pmu_update_pmc_chained(struct
> kvm_vcpu *vcpu, u64 select_idx)
>      struct kvm_pmu *pmu = &vcpu->arch.pmu;
>      struct kvm_pmc *pmc = &pmu->pmc[select_idx];
>
> -    if (kvm_pmu_idx_has_chain_evtype(vcpu, pmc->idx)) {
> +    if (kvm_pmu_idx_has_chain_evtype(vcpu, pmc->idx) &&
> +        kvm_pmu_counter_is_enabled(vcpu, pmc->idx)) {

In create_perf_event(), has_chain_evtype() is used and a 64b sample
period would be chosen even if the counters are disjoined (since the odd
is disabled). We would need to use pmc_is_chained() instead.

With perf_events, the check of whether the odd register is enabled is
properly done (create_perf_event). Then I understand whenever there is a
change in enable state or type we delete the previous perf event and
re-create a new one. Enable state check just is missing for SW_INCR.

Some other questions:
- do we need a perf event to be created even if the counter is not
enabled? For instance on counter resets, create_perf_events get called.
- also actions are made for counters which are not implemented. loop
until ARMV8_PMU_MAX_COUNTERS. Do you think it is valuable to have a
bitmask of supported counters stored before pmu readiness?
I can propose such changes if you think they are valuable.

Thanks

Eric

>          /*
>           * During promotion from !chained to chained we must ensure
>           * the adjacent counter is stopped and its event destroyed
>
> What do you think?
>
>         M.

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