Re: [PATCH] KVM: arm64: nv: Fix MI line level calculation in vgic_v3_nested_update_mi()

[Marc Zyngier <maz@kernel.org>]

On Wed, 25 Jun 2025 09:47:09 +0100,
Wei-Lin Chang <r09922117@csie.ntu.edu.tw> wrote:
> 
> The state of the vcpu's MI line should be asserted when its
> ICH_HCR_EL2.En is set and ICH_MISR_EL2 is non-zero. Using bitwise AND
> (&=) directly for this calculation will not give us the correct result
> when the LSB of the vcpu's ICH_MISR_EL2 isn't set. Correct this by first
> adjusting the return value of vgic_v3_get_misr() into 1 if it is
> non-zero.
> 
> Signed-off-by: Wei-Lin Chang <r09922117@csie.ntu.edu.tw>
> ---
>  arch/arm64/kvm/vgic/vgic-v3-nested.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/vgic/vgic-v3-nested.c b/arch/arm64/kvm/vgic/vgic-v3-nested.c
> index 4f6954c30674..ebffad632fd2 100644
> --- a/arch/arm64/kvm/vgic/vgic-v3-nested.c
> +++ b/arch/arm64/kvm/vgic/vgic-v3-nested.c
> @@ -400,7 +400,7 @@ void vgic_v3_nested_update_mi(struct kvm_vcpu *vcpu)
>  
>  	level  = __vcpu_sys_reg(vcpu, ICH_HCR_EL2) & ICH_HCR_EL2_En;
>  	if (level)
> -		level &= vgic_v3_get_misr(vcpu);
> +		level &= !!vgic_v3_get_misr(vcpu);
>  	kvm_vgic_inject_irq(vcpu->kvm, vcpu,
>  			    vcpu->kvm->arch.vgic.mi_intid, level, vcpu);
>  }

Very well spotted, once more. Where were you when I posted all these
patches? ;-)

We could make it even clearer with this:

diff --git a/arch/arm64/kvm/vgic/vgic-v3-nested.c b/arch/arm64/kvm/vgic/vgic-v3-nested.c
index a50fb7e6841f7..679aafe77de2e 100644
--- a/arch/arm64/kvm/vgic/vgic-v3-nested.c
+++ b/arch/arm64/kvm/vgic/vgic-v3-nested.c
@@ -401,9 +401,7 @@ void vgic_v3_nested_update_mi(struct kvm_vcpu *vcpu)
 {
 	bool level;
 
-	level  = __vcpu_sys_reg(vcpu, ICH_HCR_EL2) & ICH_HCR_EL2_En;
-	if (level)
-		level &= vgic_v3_get_misr(vcpu);
+	level = (__vcpu_sys_reg(vcpu, ICH_HCR_EL2) & ICH_HCR_EL2_En) && vgic_v3_get_misr(vcpu);
 	kvm_vgic_inject_irq(vcpu->kvm, vcpu,
 			    vcpu->kvm->arch.vgic.mi_intid, level, vcpu);
 }

If you're OK with it, I'll use this, keeping your authorship of
course.

Thanks,

	M.

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