Re: [PATCH v5 08/12] KVM: arm64: PMU: Allow userspace to limit PMCR_EL0.N for the guest

[Oliver Upton <oliver.upton@linux.dev>]

On Tue, Aug 22, 2023 at 11:26:23AM +0800, Shaoqin Huang wrote:

[...]

> > > > +static int set_pmcr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r,
> > > > +                 u64 val)
> > > > +{
> > > > +     struct kvm *kvm = vcpu->kvm;
> > > > +     u64 new_n, mutable_mask;
> > > > +     int ret = 0;
> > > > +
> > > > +     new_n = FIELD_GET(ARMV8_PMU_PMCR_N, val);
> > > > +
> > > > +     mutex_lock(&kvm->arch.config_lock);
> > > > +     if (unlikely(new_n != kvm->arch.pmcr_n)) {
> > > > +             /*
> > > > +              * The vCPU can't have more counters than the PMU
> > > > +              * hardware implements.
> > > > +              */
> > > > +             if (new_n <= kvm->arch.pmcr_n_limit)
> > > > +                     kvm->arch.pmcr_n = new_n;
> > > > +             else
> > > > +                     ret = -EINVAL;
> > > > +     }
> > > 
> > > Since we have set the default value of pmcr_n, if we want to set a new
> > > pmcr_n, shouldn't it be a different value?
> > > 
> > > So how about change the checking to:
> > > 
> > > if (likely(new_n <= kvm->arch.pmcr_n_limit)
> > >          kvm->arch.pmcr_n = new_n;
> > > else
> > >          ret = -EINVAL;
> > > 
> > > what do you think?
> > > 
> > Sorry, I guess I didn't fully understand your suggestion. Are you
> > saying that it's 'likely' that userspace would configure the correct
> > value?
> > 
> It depends on how userspace use this api to limit the number of pmcr. I
> think what you mean in the code is that userspace need to set every vcpu's
> pmcr to the same value, so the `unlikely` here is right, only one vcpu can
> change the kvm->arch.pmcr.n, it saves the cpu cycles.
> 
> What suggest above might be wrong. Since I think when userspace want to
> limit the number of pmcr, it may just set the new_n on one vcpu, since the
> kvm->arch.pmcr_n is a VM-local value, every vcpu can see it, so it's
> `likely` the (new_n <= kvm->arch.pmcr_n_limit), it can decrease one checking
> statement.

How about we just do away with branch hints in the first place? This is
_not_ a hot path.

-- 
Thanks,
Oliver