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

[Marc Zyngier <maz@xxxxxxxxxx>]

On 2021-06-29 15:17, Alexandre Chartre wrote:
> 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.

Please send this as a series rather than individual patches. I'm only
queuing critical fixes at the moment (this is the merge window).
If you post the series after -rc1, I'll queue it and let it simmer
in -next.

Thanks,

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