Re: [RFC 2/3] KVM: arm64: pmu: Fix chained SW_INCR counters

[Auger Eric <eric.auger@redhat.com>]

Hi Marc,
On 1/19/20 6:58 PM, Marc Zyngier wrote:
> On Thu, 5 Dec 2019 20:01:42 +0100
> Auger Eric <eric.auger@redhat.com> wrote:
> 
> Hi Eric,
> 
>> 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.
> 
> Can you please respin this? I'd like to have it queued quickly, if at
> all possible.

Yes I am going to respin quickly.

Thanks

Eric
> 
>>
>> 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.
> 
> It shouldn't be necessary.
> 
>> - 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.
> 
> That would certainly be a performance optimization.
> 
> Thanks,
> 
> 	M.
> 

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