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

[Marc Zyngier <maz@kernel.org>]

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.

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