virtio: sanity-check available index

Checking available index upon load instead of
only when vm is running makes is easier to
debug failures.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/virtio.c

@@ -681,6 +681,7 @@ int virtio_load(VirtIODevice *vdev, QEMUFile *f)
     uint32_t features;
     uint32_t supported_features =
         vdev->binding->get_features(vdev->binding_opaque);
+    uint16_t num_heads;
 
     if (vdev->binding->load_config) {
         ret = vdev->binding->load_config(vdev->binding_opaque, f);
@@ -713,6 +714,16 @@ int virtio_load(VirtIODevice *vdev, QEMUFile *f)
         if (vdev->vq[i].pa) {
             virtqueue_init(&vdev->vq[i]);
         }
+	num_heads = vring_avail_idx(&vdev->vq[i]) - vdev->vq[i].last_avail_idx;
+	/* Check it isn't doing very strange things with descriptor numbers. */
+	if (num_heads > vdev->vq[i].vring.num) {
+		fprintf(stderr, "VQ %d size 0x%x Guest index 0x%x "
+                        "inconsistent with Host index 0x%x: delta 0x%x\n",
+			i, vdev->vq[i].vring.num,
+                        vring_avail_idx(&vdev->vq[i]),
+                        vdev->vq[i].last_avail_idx, num_heads);
+		return -1;
+	}
         if (vdev->binding->load_queue) {
             ret = vdev->binding->load_queue(vdev->binding_opaque, i, f);
             if (ret)