Fix incorrect assumption in GIC driver

Before handling an IRQ, we should first be checking that it has
been registered with the GIC by the VMM.

examples/simple/vmm.c

@@ -111,8 +111,6 @@ void init(void) {
         LOG_VMM_ERR("Failed to initialise emulated interrupt controller\n");
         return;
     }
-    // @ivanv: Note that remove this line causes the VMM to fault if we
-    // actually get the interrupt. This should be avoided by making the VGIC driver more stable.
     success = virq_register(GUEST_VCPU_ID, SERIAL_IRQ, &serial_ack, NULL);
     /* Just in case there is already an interrupt available to handle, we ack it here. */
     microkit_irq_ack(SERIAL_IRQ_CH);

src/arch/aarch64/vgic/vdist.h

@@ -172,11 +172,15 @@ static void vgic_dist_disable_irq(vgic_t *vgic, size_t vcpu_id, int irq)
 
 static bool vgic_dist_set_pending_irq(vgic_t *vgic, size_t vcpu_id, int irq)
 {
-    // @ivanv: I believe this function causes a fault in the VMM if the IRQ has not
-    // been registered. This is not good.
     /* STATE c) */
-
+    /* First check that we find vIRQ data in case the vIRQ has not been
+     * registered yet. */
     struct virq_handle *virq_data = virq_find_irq_data(vgic, vcpu_id, irq);
+    assert(virq_data);
+    if (!virq_data) {
+        LOG_VMM_ERR("could not find vIRQ data for vIRQ 0x%lx on vCPU 0x%lx\n", irq, vcpu_id);
+        return false;
+    }
     struct gic_dist_map *dist = vgic_get_dist(vgic->registers);
 
     if (virq_data->virq == VIRQ_INVALID || !vgic_dist_is_enabled(dist) || !is_enabled(dist, irq, vcpu_id)) {