Re: [PATCH v2 4/5] KVM: arm64: Expose FEAT_RASv1p1 in a canonical manner

[Marc Zyngier <maz@kernel.org>]

On Thu, 07 Aug 2025 13:55:31 +0100,
Joey Gouly <joey.gouly@arm.com> wrote:
> 
> On Wed, Aug 06, 2025 at 05:56:14PM +0100, Marc Zyngier 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.
> > 
> > Note that this also advertises FEAT_DoubleFault, which doesn't
> > affect the guest at all, as only EL3 is concerned by this.
> > 
> > Signed-off-by: Marc Zyngier <maz@kernel.org>
> > ---
> >  arch/arm64/kvm/sys_regs.c | 12 ++++++++++++
> >  1 file changed, 12 insertions(+)
> > 
> > diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> > index 1b4114790024e..66e5a733e9628 100644
> > --- a/arch/arm64/kvm/sys_regs.c
> > +++ b/arch/arm64/kvm/sys_regs.c
> > @@ -1800,6 +1800,18 @@ static u64 sanitise_id_aa64pfr0_el1(const struct kvm_vcpu *vcpu, u64 val)
> >  	if (!vcpu_has_sve(vcpu))
> >  		val &= ~ID_AA64PFR0_EL1_SVE_MASK;
> >  
> > +	/*
> > +	 * Describe RASv1p1 in a canonical way -- ID_AA64PFR1_EL1.RAS_frac
> > +	 * is cleared separately. Note that by advertising RASv1p1 here, we
> 
> Where is it cleared? __kvm_read_sanitised_id_reg() is where I would have
> expected to see it:
> 
>     case SYS_ID_AA64PFR1_EL1:

[...]

Ah crap, it is the nested code that we get rid of it, nowhere else.
Which means that non-nested VMs have already observed RAS_frac. What a
mess. Then RAS_frac must be exposed as writable.

The question is whether we want to allow migration between one flavour
of RASv1p1 and the other.

	M.

-- 
Jazz isn't dead. It just smells funny.