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

[Sebastian Ott]

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

Sebastian