Re: [PATCH v5 4/6] KVM: arm64: vgic-v3: Allow userspace to write GICD_TYPER2.nASSGIcap

[Oliver Upton <oliver.upton@linux.dev>]

On Fri, Jul 25, 2025 at 03:53:23PM -0700, Raghavendra Rao Ananta wrote:
> Hi Oliver,
> 
> On Wed, Jul 23, 2025 at 11:28 PM Oliver Upton <oliver.upton@linux.dev> wrote:
> >
> > From: Raghavendra Rao Ananta <rananta@google.com>
> >
> > KVM unconditionally advertises GICD_TYPER2.nASSGIcap (which internally
> > implies vSGIs) on GICv4.1 systems. Allow userspace to change whether a
> > VM supports the feature. Only allow changes prior to VGIC initialization
> > as at that point vPEs need to be allocated for the VM.
> >
> > For convenience, bundle support for vLPIs and vSGIs behind this feature,
> > allowing userspace to control vPE allocation for VMs in environments
> > that may be constrained on vPE IDs.
> >
> Is the "bundling vLPI and vSGI" part missing from this series? Unless
> I'm using a wrong base, we call vgic_v4_init() if
> vgic_supports_direct_irqs(), which will be true even if userspace
> turned off GICD_TYPER2.nASSGIcap. Am I missing something?

Nope, squashing this into the patch:

diff --git a/arch/arm64/kvm/vgic/vgic.h b/arch/arm64/kvm/vgic/vgic.h
index 96048ec59d0d..fb6d7c098ae6 100644
--- a/arch/arm64/kvm/vgic/vgic.h
+++ b/arch/arm64/kvm/vgic/vgic.h
@@ -375,7 +375,15 @@ bool vgic_supports_direct_sgis(struct kvm *kvm);
 
 static inline bool vgic_supports_direct_irqs(struct kvm *kvm)
 {
-	return vgic_supports_direct_msis(kvm) || vgic_supports_direct_sgis(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);
 }
 
 int vgic_v4_init(struct kvm *kvm);