Re: [PATCH 6/7] KVM: arm64: Expose FEAT_RASv1p1 in a canonical manner

[Cornelia Huck <cohuck@redhat.com>]

On Mon, Jul 21 2025, Marc Zyngier <maz@kernel.org> wrote:

> On Mon, 21 Jul 2025 13:32:08 +0100,
> Cornelia Huck <cohuck@redhat.com> wrote:
>> 
>> On Mon, Jul 21 2025, Marc Zyngier <maz@kernel.org> wrote:
>> 
>> > If we have RASv1p1 on the host, advertise it to the guest in the
>> > "canonical way", by setting ID_AA64PFR0_EL1 to V1P1, rather than
>> > the convoluted RAS+RAS_frac method.
>> 
>> Don't the two methods have slightly different semantics with RAS == V1P1
>> possibly implying FEAT_DoubleFault, and RAS+RAS_frac not?
>
> Ah, that's an interesting point -- I definitely had glanced over that.
>
> But I'm not sure a guest can actually distinguish between these two
> configurations, given that FEAT_DoubleFault is essentially an EL3
> feature (as indicated in the RAS == V1P1 section, and further
> confirmed in R_GRJVN), making it invisible to the guest.
>
> FEAT_DoubleFault2 is, on the contrary, totally visible from the guest,
> and independent of EL3.
>
> Does this make sense to you?

It does; but it might make sense to add a comment explaining that.

Userspace should hopefully be able to just map everything to RAS == V1P1
and be done with it.