Re: [PATCH] KVM: arm64: Disabling disabled PMU counters wastes a lot of time

[Alexandre Chartre <alexandre.chartre@xxxxxxxxxx>]

On 6/29/21 3:47 PM, Marc Zyngier wrote:
> On Tue, 29 Jun 2021 14:16:55 +0100,
> Alexandre Chartre <alexandre.chartre@xxxxxxxxxx> wrote:
> > Hi Marc,
> >
> > On 6/29/21 11:06 AM, Marc Zyngier wrote:
> > > Hi Alexandre,
>
> [...]
>
> > > So the sysreg is the only thing we should consider, and I think we
> > > should drop the useless masking. There is at least another instance of
> > > this in the PMU code (kvm_pmu_overflow_status()), and apart from
> > > kvm_pmu_vcpu_reset(), only the sysreg accessors should care about the
> > > masking to sanitise accesses.
> > >
> > > What do you think?
> >
> > I think you are right. PMCNTENSET_EL0 is already masked with
> > kvm_pmu_valid_counter_mask() so there's effectively no need to mask
> > it again when we use it. I will send an additional patch (on top of
> > this one) to remove useless masking. Basically, changes would be:
> >
> > diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
> > index bab4b735a0cf..e0dfd7ce4ba0 100644
> > --- a/arch/arm64/kvm/pmu-emul.c
> > +++ b/arch/arm64/kvm/pmu-emul.c
> > @@ -373,7 +373,6 @@ static u64 kvm_pmu_overflow_status(struct kvm_vcpu *vcpu)
> >                  reg = __vcpu_sys_reg(vcpu, PMOVSSET_EL0);
> >                  reg &= __vcpu_sys_reg(vcpu, PMCNTENSET_EL0);
> >                  reg &= __vcpu_sys_reg(vcpu, PMINTENSET_EL1);
> > -               reg &= kvm_pmu_valid_counter_mask(vcpu);
> >          }
> >           return reg;
> > @@ -564,21 +563,22 @@ void kvm_pmu_software_increment(struct kvm_vcpu *vcpu, u64 val)
> >    */
> >   void kvm_pmu_handle_pmcr(struct kvm_vcpu *vcpu, u64 val)
> >   {
> > -       unsigned long mask = kvm_pmu_valid_counter_mask(vcpu);
> > +       unsigned long mask;
> >          int i;
> >           if (val & ARMV8_PMU_PMCR_E) {
> >                  kvm_pmu_enable_counter_mask(vcpu,
> > -                      __vcpu_sys_reg(vcpu, PMCNTENSET_EL0) & mask);
> > +                      __vcpu_sys_reg(vcpu, PMCNTENSET_EL0));
> >          } else {
> >                  kvm_pmu_disable_counter_mask(vcpu,
> > -                      __vcpu_sys_reg(vcpu, PMCNTENSET_EL0) & mask);
> > +                      __vcpu_sys_reg(vcpu, PMCNTENSET_EL0));
> >          }
> >           if (val & ARMV8_PMU_PMCR_C)
> >                  kvm_pmu_set_counter_value(vcpu, ARMV8_PMU_CYCLE_IDX, 0);
> >           if (val & ARMV8_PMU_PMCR_P) {
> > +               mask = kvm_pmu_valid_counter_mask(vcpu);
>
> Careful here, this clashes with a fix from Alexandru that is currently
> in -next (PMCR_EL0.P shouldn't reset the cycle counter) and aimed at
> 5.14. And whilst you're at it, consider moving the 'mask' declaration
> here too.
>
> >                  for_each_set_bit(i, &mask, 32)
> >                          kvm_pmu_set_counter_value(vcpu, i, 0);
> >          }
> > diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> > index 1a7968ad078c..2e406905760e 100644
> > --- a/arch/arm64/kvm/sys_regs.c
> > +++ b/arch/arm64/kvm/sys_regs.c
> > @@ -845,7 +845,7 @@ static bool access_pmcnten(struct kvm_vcpu *vcpu, struct sys_reg_params *p,
> >                          kvm_pmu_disable_counter_mask(vcpu, val);
> >                  }
> >          } else {
> > -               p->regval = __vcpu_sys_reg(vcpu, PMCNTENSET_EL0) & mask;
> > +               p->regval = __vcpu_sys_reg(vcpu, PMCNTENSET_EL0);
> >          }
> >           return true;
>
> If you are cleaning up the read-side of sysregs, access_pminten() and
> access_pmovs() could have some of your attention too.

Ok, so for now, I will just resubmit the initial patch with the commit
comment fixes. Then, look at all the mask cleanup on top of Alexandru
changes and prepare another patch.

alex.