KVM: arm64: Resolve vLPI by host IRQ in vgic_v4_unset_forwarding()

The virtual mapping and "GSI" routing of a particular vLPI is subject to
change in response to the guest / userspace. This can be pretty annoying
to deal with when KVM needs to track the physical state that's managed
for vLPI direct injection.

Make vgic_v4_unset_forwarding() resilient by using the host IRQ to
resolve the vgic IRQ. Since this uses the LPI xarray directly, finding
the ITS by doorbell address + grabbing it's its_lock is no longer
necessary. Note that matching the right ITS / ITE is already handled in
vgic_v4_set_forwarding(), and unless there's a bug in KVM's VGIC ITS
emulation the virtual mapping that should remain stable for the lifetime
of the vLPI mapping.

Tested-by: Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20250523194722.4066715-4-oliver.upton@linux.dev
Signed-off-by: Marc Zyngier <maz@kernel.org>

Author: oupton

arch/arm64/kvm/arm.c

@@ -2800,8 +2800,7 @@ void kvm_arch_irq_bypass_del_producer(struct irq_bypass_consumer *cons,
 	if (irq_entry->type != KVM_IRQ_ROUTING_MSI)
 		return;
 
-	kvm_vgic_v4_unset_forwarding(irqfd->kvm, prod->irq,
-				     &irqfd->irq_entry);
+	kvm_vgic_v4_unset_forwarding(irqfd->kvm, prod->irq);
 }
 
 void kvm_arch_irq_bypass_stop(struct irq_bypass_consumer *cons)

arch/arm64/kvm/vgic/vgic-v4.c

@@ -508,42 +508,47 @@ int kvm_vgic_v4_set_forwarding(struct kvm *kvm, int virq,
 	return ret;
 }
 
-int kvm_vgic_v4_unset_forwarding(struct kvm *kvm, int virq,
-				 struct kvm_kernel_irq_routing_entry *irq_entry)
+static struct vgic_irq *__vgic_host_irq_get_vlpi(struct kvm *kvm, int host_irq)
+{
+	struct vgic_irq *irq;
+	unsigned long idx;
+
+	guard(rcu)();
+	xa_for_each(&kvm->arch.vgic.lpi_xa, idx, irq) {
+		if (!irq->hw || irq->host_irq != host_irq)
+			continue;
+
+		if (!vgic_try_get_irq_kref(irq))
+			return NULL;
+
+		return irq;
+	}
+
+	return NULL;
+}
+
+int kvm_vgic_v4_unset_forwarding(struct kvm *kvm, int host_irq)
 {
-	struct vgic_its *its;
 	struct vgic_irq *irq;
 	unsigned long flags;
 	int ret = 0;
 
 	if (!vgic_supports_direct_msis(kvm))
 		return 0;
 
-	/*
-	 * Get the ITS, and escape early on error (not a valid
-	 * doorbell for any of our vITSs).
-	 */
-	its = vgic_get_its(kvm, irq_entry);
-	if (IS_ERR(its))
+	irq = __vgic_host_irq_get_vlpi(kvm, host_irq);
+	if (!irq)
 		return 0;
 
-	mutex_lock(&its->its_lock);
-
-	ret = vgic_its_resolve_lpi(kvm, its, irq_entry->msi.devid,
-				   irq_entry->msi.data, &irq);
-	if (ret)
-		goto out;
-
 	raw_spin_lock_irqsave(&irq->irq_lock, flags);
-	WARN_ON(irq->hw && irq->host_irq != virq);
+	WARN_ON(irq->hw && irq->host_irq != host_irq);
 	if (irq->hw) {
 		atomic_dec(&irq->target_vcpu->arch.vgic_cpu.vgic_v3.its_vpe.vlpi_count);
 		irq->hw = false;
-		ret = its_unmap_vlpi(virq);
+		ret = its_unmap_vlpi(host_irq);
 	}
 
 	raw_spin_unlock_irqrestore(&irq->irq_lock, flags);
-out:
-	mutex_unlock(&its->its_lock);
+	vgic_put_irq(kvm, irq);
 	return ret;
 }

include/kvm/arm_vgic.h

@@ -434,8 +434,7 @@ struct kvm_kernel_irq_routing_entry;
 int kvm_vgic_v4_set_forwarding(struct kvm *kvm, int irq,
 			       struct kvm_kernel_irq_routing_entry *irq_entry);
 
-int kvm_vgic_v4_unset_forwarding(struct kvm *kvm, int irq,
-				 struct kvm_kernel_irq_routing_entry *irq_entry);
+int kvm_vgic_v4_unset_forwarding(struct kvm *kvm, int host_irq);
 
 int vgic_v4_load(struct kvm_vcpu *vcpu);
 void vgic_v4_commit(struct kvm_vcpu *vcpu);