virtio: reset device on bad guest index in virtio_load()

If we find a queue with an inconsistent guest index value, explicitly mark the
device as needing a reset - and broken - via virtio_error().

There's at least one driver implementation - the virtio-win NetKVM driver - that
is able to handle a VIRTIO_CONFIG_S_NEEDS_RESET notification and successfully
restore the device to a working state. Other implementations do not correctly
handle this, but as the VQ is not in a functional state anyway, this is still
worth doing.

Signed-off-by: John Levon <john.levon@nutanix.com>
Message-Id: <20201120185103.GA442386@sent>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/virtio/virtio.c

@@ -3161,12 +3161,15 @@ int virtio_load(VirtIODevice *vdev, QEMUFile *f, int version_id)
             nheads = vring_avail_idx(&vdev->vq[i]) - vdev->vq[i].last_avail_idx;
             /* Check it isn't doing strange things with descriptor numbers. */
             if (nheads > vdev->vq[i].vring.num) {
-                qemu_log_mask(LOG_GUEST_ERROR,
-                              "VQ %d size 0x%x Guest index 0x%x "
-                              "inconsistent with Host index 0x%x: delta 0x%x",
-                              i, vdev->vq[i].vring.num,
-                              vring_avail_idx(&vdev->vq[i]),
-                              vdev->vq[i].last_avail_idx, nheads);
+                virtio_error(vdev, "VQ %d size 0x%x Guest index 0x%x "
+                             "inconsistent with Host index 0x%x: delta 0x%x",
+                             i, vdev->vq[i].vring.num,
+                             vring_avail_idx(&vdev->vq[i]),
+                             vdev->vq[i].last_avail_idx, nheads);
+                vdev->vq[i].used_idx = 0;
+                vdev->vq[i].shadow_avail_idx = 0;
+                vdev->vq[i].inuse = 0;
+                continue;
             }
             vdev->vq[i].used_idx = vring_used_idx(&vdev->vq[i]);
             vdev->vq[i].shadow_avail_idx = vring_avail_idx(&vdev->vq[i]);