KVM: arm64: Unmap vLPIs affected by changes to GSI routing information

JIRA: https://issues.redhat.com/browse/RHEL-93666

KVM's interrupt infrastructure is dodgy at best, allowing for some ugly
'off label' usage of the various UAPIs. In one example, userspace can
change the routing entry of a particular "GSI" after configuring
irqbypass with KVM_IRQFD. KVM/arm64 is oblivious to this, and winds up
preserving the stale translation in cases where vLPIs are configured.

Honor userspace's intentions and tear down the vLPI mapping if affected
by a "GSI" routing change. Make no attempt to reconstruct vLPIs if the
new target is an MSI and just fall back to software injection.

Tested-by: Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20250523194722.4066715-5-oliver.upton@linux.dev
Signed-off-by: Marc Zyngier <maz@kernel.org>
(cherry picked from commit 4bf3693)
Signed-off-by: Gavin Shan <gshan@redhat.com>

Author: Gavin Shan

arch/arm64/kvm/arm.c

@@ -2741,6 +2741,7 @@ int kvm_arch_irq_bypass_add_producer(struct irq_bypass_consumer *cons,
 	return kvm_vgic_v4_set_forwarding(irqfd->kvm, prod->irq,
 					  &irqfd->irq_entry);
 }
+
 void kvm_arch_irq_bypass_del_producer(struct irq_bypass_consumer *cons,
 				      struct irq_bypass_producer *prod)
 {
@@ -2754,6 +2755,28 @@ void kvm_arch_irq_bypass_del_producer(struct irq_bypass_consumer *cons,
 	kvm_vgic_v4_unset_forwarding(irqfd->kvm, prod->irq);
 }
 
+bool kvm_arch_irqfd_route_changed(struct kvm_kernel_irq_routing_entry *old,
+				  struct kvm_kernel_irq_routing_entry *new)
+{
+	if (new->type != KVM_IRQ_ROUTING_MSI)
+		return true;
+
+	return memcmp(&old->msi, &new->msi, sizeof(new->msi));
+}
+
+int kvm_arch_update_irqfd_routing(struct kvm *kvm, unsigned int host_irq,
+				  uint32_t guest_irq, bool set)
+{
+	/*
+	 * Remapping the vLPI requires taking the its_lock mutex to resolve
+	 * the new translation. We're in spinlock land at this point, so no
+	 * chance of resolving the translation.
+	 *
+	 * Unmap the vLPI and fall back to software LPI injection.
+	 */
+	return kvm_vgic_v4_unset_forwarding(kvm, host_irq);
+}
+
 void kvm_arch_irq_bypass_stop(struct irq_bypass_consumer *cons)
 {
 	struct kvm_kernel_irqfd *irqfd =