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

[Marc Zyngier <maz@kernel.org>]

On Tue, 04 Nov 2025 12:59:00 +0000,
Fuad Tabba <tabba@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@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.

Thanks,

	M.

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