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

[Raghavendra Rao Ananta]

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.

Thank you.
Raghavendra
>
> > +     if (kvm_vm_has_ran_once(vcpu->kvm)) {
> > +             mutex_unlock(&kvm->arch.config_lock);
> > +             return 0;
> > +     }
>