linux-kernel - Re: [PATCH] KVM: arm64: Set/unset vGIC v4 forwarding if direct IRQs are supported

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Message-ID: <aIjcmquPNOdE5l4K@linux.dev>
Date: Tue, 29 Jul 2025 07:37:14 -0700
From: Oliver Upton <oliver.upton@...ux.dev>
To: Raghavendra Rao Ananta <rananta@...gle.com>
Cc: Marc Zyngier <maz@...nel.org>, linux-arm-kernel@...ts.infradead.org,
	kvmarm@...ts.linux.dev, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Subject: Re: [PATCH] KVM: arm64: Set/unset vGIC v4 forwarding if direct IRQs
 are supported

Hi Raghu,

Thanks for reporting this so quickly :)

On Mon, Jul 28, 2025 at 10:37:10PM +0000, Raghavendra Rao Ananta wrote:
> diff --git a/arch/arm64/kvm/vgic/vgic-v4.c b/arch/arm64/kvm/vgic/vgic-v4.c
> index e7e284d47a77..873a190bcff7 100644
> --- a/arch/arm64/kvm/vgic/vgic-v4.c
> +++ b/arch/arm64/kvm/vgic/vgic-v4.c
> @@ -433,7 +433,7 @@ int kvm_vgic_v4_set_forwarding(struct kvm *kvm, int virq,
>  	unsigned long flags;
>  	int ret = 0;
>  
> -	if (!vgic_supports_direct_msis(kvm))
> +	if (!vgic_supports_direct_irqs(kvm))
>  		return 0;
>  
>  	/*
> @@ -533,7 +533,7 @@ int kvm_vgic_v4_unset_forwarding(struct kvm *kvm, int host_irq)
>  	unsigned long flags;
>  	int ret = 0;
>  
> -	if (!vgic_supports_direct_msis(kvm))
> +	if (!vgic_supports_direct_irqs(kvm))
>  		return 0;

I'm not sure this is what we want, since a precondition of actually
doing vLPI injection is the guest having an ITS. Could you try the
following?

Thanks,
Oliver

diff --git a/arch/arm64/kvm/vgic/vgic-mmio-v3.c b/arch/arm64/kvm/vgic/vgic-mmio-v3.c
index a3ef185209e9..70d50c77e5dc 100644
--- a/arch/arm64/kvm/vgic/vgic-mmio-v3.c
+++ b/arch/arm64/kvm/vgic/vgic-mmio-v3.c
@@ -50,6 +50,14 @@ bool vgic_has_its(struct kvm *kvm)
 
 bool vgic_supports_direct_msis(struct kvm *kvm)
 {
+	/*
+	 * Deliberately conflate vLPI and vSGI support on GICv4.1 hardware,
+	 * indirectly allowing userspace to control whether or not vPEs are
+	 * allocated for the VM.
+	 */
+	if (system_supports_direct_sgis() && !vgic_supports_direct_sgis(kvm))
+		return false;
+
 	return kvm_vgic_global_state.has_gicv4 && vgic_has_its(kvm);
 }
 
diff --git a/arch/arm64/kvm/vgic/vgic.h b/arch/arm64/kvm/vgic/vgic.h
index 1384a04c0784..de1c1d3261c3 100644
--- a/arch/arm64/kvm/vgic/vgic.h
+++ b/arch/arm64/kvm/vgic/vgic.h
@@ -396,15 +396,7 @@ bool vgic_supports_direct_sgis(struct kvm *kvm);
 
 static inline bool vgic_supports_direct_irqs(struct kvm *kvm)
 {
-	/*
-	 * Deliberately conflate vLPI and vSGI support on GICv4.1 hardware,
-	 * indirectly allowing userspace to control whether or not vPEs are
-	 * allocated for the VM.
-	 */
-	if (system_supports_direct_sgis())
-		return vgic_supports_direct_sgis(kvm);
-
-	return vgic_supports_direct_msis(kvm);
+	return vgic_supports_direct_msis(kvm) || vgic_supports_direct_sgis(kvm);
 }
 
 int vgic_v4_init(struct kvm *kvm);