virtio: reset function

A reset function solves three problems:

1) It allows us to renegotiate features, eg. if we want to upgrade a
   guest driver without rebooting the guest.

2) It gives us a clean way of shutting down virtqueues: after a reset,
   we know that the buffers won't be used by the host, and

3) It helps the guest recover from messed-up drivers.

So we remove the ->shutdown hook, and the only way we now remove
feature bits is via reset.

We leave it to the driver to do the reset before it deletes queues:
the balloon driver, for example, needs to chat to the host in its
remove function.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

1 files changed, 5 insertions, 1 deletions

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index 54a8017..6143337 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -264,12 +264,16 @@ static void virtblk_remove(struct virtio_device *vdev)
 	struct virtio_blk *vblk = vdev->priv;
 	int major = vblk->disk->major;
 
+	/* Nothing should be pending. */
 	BUG_ON(!list_empty(&vblk->reqs));
+
+	/* Stop all the virtqueues. */
+	vdev->config->reset(vdev);
+
 	blk_cleanup_queue(vblk->disk->queue);
 	put_disk(vblk->disk);
 	unregister_blkdev(major, "virtblk");
 	mempool_destroy(vblk->pool);
-	/* There should be nothing in the queue now, so no need to shutdown */
 	vdev->config->del_vq(vblk->vq);
 	kfree(vblk);
 }