Re: [PATCH v7 08/12] KVM: arm64: PMU: Allow userspace to limit PMCR_EL0.N for the guest

[Raghavendra Rao Ananta]

On Thu, Oct 19, 2023 at 3:45 AM Sebastian Ott <sebott@xxxxxxxxxx> wrote:
>
> On Tue, 17 Oct 2023, Raghavendra Rao Ananta wrote:
> > On Tue, Oct 17, 2023 at 8:52 AM Sebastian Ott <sebott@xxxxxxxxxx> wrote:
> >>
> >> On Mon, 9 Oct 2023, Raghavendra Rao Ananta 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;
> >>> +
> >>> +     mutex_lock(&kvm->arch.config_lock);
> >>> +
> >>> +     /*
> >>> +      * Make PMCR immutable once the VM has started running, but do
> >>> +      * not return an error (-EBUSY) to meet the existing expectations.
> >>> +      */
> >>
> >> Why should we mention which error we're _not_ returning?
> >>
> > Oh, it's not to break the existing userspace expectations. Before this
> > series, any 'write' from userspace was possible. Returning -EBUSY all
> > of a sudden might tamper with this expectation.
>
> Yes I get that part. What I've meant is why specifically mention -EBUSY?
> You're also not returning -EFAULT nor -EINVAL.
>
> /*
>   * Make PMCR immutable once the VM has started running, but do
>   * not return an error to meet the existing expectations.
>   */
> IMHO provides the same info to the reader and is less confusing
>
Sounds good. I'll apply this.

Thank you.
Raghavendra
> Sebastian