Re: [PATCH v3 1/5] KVM: arm64: GICv3: Don't advertise ICH_HCR_EL2.En==1 when no vgic is configured

[Marc Zyngier <maz@kernel.org>]

On Mon, 17 Nov 2025 10:34:36 +0000,
Fuad Tabba <tabba@google.com> wrote:
> 
> Hi Marc,
> 
> On Mon, 17 Nov 2025 at 09:15, Marc Zyngier <maz@kernel.org> wrote:
> >
> > Configuring GICv3 to deal with the lack of GIC in the guest relies
> > on not setting ICH_HCR_EL2.En in the shadow register, as this is
> > an indication of the fact that we want to trap all system registers
> > to report an UNDEF in the guest.
> >
> > Make sure we leave vgic_hcr untouched in this case.
> >
> > Reported-by: Mark Brown <broonie@kernel.org>
> > Tested-by: Mark Brown <broonie@kernel.org>
> > Closes: https://lore.kernel.org/r/72e1e8b5-e397-4dc5-9cd6-a32b6af3d739@sirena.org.uk
> > Fixes: 877324a1b5415 ("KVM: arm64: Revamp vgic maintenance interrupt configuration")
> > Signed-off-by: Marc Zyngier <maz@kernel.org>
> > ---
> >  arch/arm64/kvm/vgic/vgic-v3.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/arch/arm64/kvm/vgic/vgic-v3.c b/arch/arm64/kvm/vgic/vgic-v3.c
> > index 598621b14a30d..1d6dd1b545bdd 100644
> > --- a/arch/arm64/kvm/vgic/vgic-v3.c
> > +++ b/arch/arm64/kvm/vgic/vgic-v3.c
> > @@ -26,6 +26,9 @@ void vgic_v3_configure_hcr(struct kvm_vcpu *vcpu,
> >  {
> >         struct vgic_v3_cpu_if *cpuif = &vcpu->arch.vgic_cpu.vgic_v3;
> >
> > +       if (!irqchip_in_kernel(vcpu->kvm))
> > +               return;
> > +
> 
> Bear with me, since I'm not too familiar with this code. This is the
> only function that initializes cpuif->vgic_hcr. Should we be
> explicitly setting it to 0, or warn if ICH_HCR_EL2_En is set?

It must be zero by construction, as the vcpu structure (which this is
part of) is always zeroed at allocation time, and the only way to
populate this field to have created an irqchip and start populating
the shadow structure, right after this test.

If we start doubting the state of bits of the vcpu structure that
should never be written to because of some particular configuration,
then we should address this globally, not piecemeal.

Thanks,

	M.

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