[PATCH v1 2/8] KVM: arm64: Trap access to ALLINT if FEAT_NMI not supported by the guest

[Fuad Tabba]

Hi Marc,

On Tue, 4 Nov 2025 at 15:15, Marc Zyngier <maz at kernel.org> wrote:
>
> On Tue, 04 Nov 2025 12:59:00 +0000,
> Fuad Tabba <tabba at google.com> wrote:
> >
> > Access to ALLINT is part of FEAT_NMI. If a guest does not support this
> > feature, any access to this register must be trapped to the hypervisor
> > (EL2).
> >
> > KVM didn't configure this trap, potentially allowing a guest to toggle
> > all interrupt mask when it doesn't support FEAT_NMI. Fix this by
> > checking if the guest has FEAT_NMI support.
> >
> > Signed-off-by: Fuad Tabba <tabba at google.com>
> > ---
> >  arch/arm64/include/asm/kvm_emulate.h | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/arch/arm64/include/asm/kvm_emulate.h b/arch/arm64/include/asm/kvm_emulate.h
> > index 0f8311263edf..3fc62808c548 100644
> > --- a/arch/arm64/include/asm/kvm_emulate.h
> > +++ b/arch/arm64/include/asm/kvm_emulate.h
> > @@ -688,6 +688,9 @@ static inline void vcpu_set_hcrx(struct kvm_vcpu *vcpu)
> >               else
> >                       vcpu->arch.hcrx_el2 |= HCRX_EL2_MCE2;
> >
> > +             if (!kvm_has_feat(kvm, ID_AA64PFR1_EL1, NMI, IMP))
> > +                     vcpu->arch.hcrx_el2 |= HCRX_EL2_TALLINT;
> > +
> >               if (kvm_has_tcr2(kvm))
> >                       vcpu->arch.hcrx_el2 |= HCRX_EL2_TCR2En;
> >
>
> I think this is moving in the wrong direction. We have for quite some
> time now tried to automatically derive these behaviours from the guest
> config, as we do for FGUs.
>
> I would like to see a similar behaviour being introduced for non-FGT
> bits so that we don't have to worry about these things anymore.

I agree that we should make this more like FGT, but since that will
likely require a bit more time (and probably a couple of respins),
should we fix the HCRX traps as they are now?

Thanks,
/fuad

> Thanks,
>
>         M.
>
> --
> Without deviation from the norm, progress is not possible.