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

[Oliver Upton <oupton@kernel.org>]

Hey Marc,

On Mon, Nov 17, 2025 at 09:15:27AM +0000, Marc Zyngier 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.

Just to make sure I'm following, the scenario you're talking about is
we've already put the vgic into a non-nested state, populated an LR with
the pending MI at the time of that switch and L0 has no signal for when
it can drop the LR / pending state.

> Yes, this is really poor...

+1 :-/

> Signed-off-by: Marc Zyngier <maz@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);

I'm all for writing correct code but since we don't actually care about
the value of MISR_EL2 do we need the ISB? There's no need for a CSE for
non-NV and you'd get it 'for free' by way of exception entry in the NV
case.

Thanks,
Oliver