[PATCH v3 5/5] KVM: arm64: GICv3: Force exit to sync ICH_HCR_EL2.En

[Fuad Tabba]

On Mon, 17 Nov 2025 at 11:42, Marc Zyngier <maz at kernel.org> wrote:
>
> On Mon, 17 Nov 2025 11:35:18 +0000,
> Fuad Tabba <tabba at google.com> wrote:
> >
> > Hi Marc,
> >
> > On Mon, 17 Nov 2025 at 09:22, Marc Zyngier <maz at kernel.org> wrote:
> > >
> > > FEAT_NV2 is pretty terrible for anything that tries to enforce immediate
> > > effects, and writing to ICH_HCR_EL2 in the hope to disable a maintenance
> > > interrupt is vain. This only hits memory, and the guest hasn't cleared
> > > anything -- the MI will fire.
> > >
> > > For example, running the vgic_irq test under NV results in about 800
> > > maintenance interrupts being actually handled by the L1 guest,
> > > when none were expected.
> > >
> > > As a cheap workaround, read back ICH_MISR_EL2 after writing 0 to
> > > ICH_HCR_EL2. This is very cheap on real HW, and causes a trap to
> > > the host in NV, giving it the opportunity to retire the pending
> > > MI. With this, the above test tuns to completion without any MI
> > > being actually handled.
> >
> > nit: tuns->runs
> >
> >
> > >
> > > Yes, this is really poor...
> > >
> > > Signed-off-by: Marc Zyngier <maz at kernel.org>
> > > ---
> > >  arch/arm64/kvm/hyp/vgic-v3-sr.c      | 7 +++++++
> > >  arch/arm64/kvm/vgic/vgic-v3-nested.c | 6 ++++--
> > >  2 files changed, 11 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/arch/arm64/kvm/hyp/vgic-v3-sr.c b/arch/arm64/kvm/hyp/vgic-v3-sr.c
> > > index 99342c13e1794..f503cf01ac82c 100644
> > > --- a/arch/arm64/kvm/hyp/vgic-v3-sr.c
> > > +++ b/arch/arm64/kvm/hyp/vgic-v3-sr.c
> > > @@ -244,6 +244,13 @@ void __vgic_v3_save_state(struct vgic_v3_cpu_if *cpu_if)
> > >         }
> > >
> > >         write_gicreg(0, ICH_HCR_EL2);
> > > +
> > > +       /*
> > > +        * Hack alert: On NV, this results in a trap so that the above
> > > +        * write actually takes effect...
> > > +        */
> > > +       isb();
> > > +       read_gicreg(ICH_MISR_EL2);
> > >  }
> >
> > nit: is it worth gating this with "ARM64_HAS_NESTED_VIRT"?
>
> This is in a *guest*, which knows nothing about being virtualised!

Nested makes my head hurt :D

Cheers,
/fuad

>
> > Otherwise,
> > Reviewed-by: Fuad Tabba <tabba at google.com>
>
> Thanks!
>
>         M.
>
> --
> Without deviation from the norm, progress is not possible.