[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 at redhat.com> 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