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

[Andrew Murray <andrew.murray@arm.com>]

On Thu, Dec 05, 2019 at 02:52:26PM +0000, 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?

Indeed - we don't want to indicate an overflow on a disabled counter.


> 
> 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.

Does this mean that we don't care if the low counter is enabled or not
when deciding if the pair is chained?

I would find the code easier to follow if we had an explicit 'is the
high counter enabled here' check (at the point of deciding where to
put the overflow).


> 
> 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)) {

I.e. here we don't care what the state of enablement is for the low counter.

Also at present, this may break the following use-case

 - User creates and uses a pair of chained counters
 - User disables odd/high counter
 - User reads values of both counters
 - User rewrites CHAIN event to odd/high counter OR user re-enables just the even/low counter
 - User reads value of both counters <- this may now different to the last read

Thanks,

Andrew Murray

>  		/*
>  		 * During promotion from !chained to chained we must ensure
>  		 * the adjacent counter is stopped and its event destroyed
> 
> What do you think?
> 
>         M.
> -- 
> Jazz is not dead. It just smells funny...