[Qemu-devel] [PATCH] virtio: verify that all outstanding buffers are flushed (was Re: vmstate conversion for virtio?)

["Michael S. Tsirkin" <mst@redhat.com>]

Add sanity check to address the following concern:

On Wed, Dec 05, 2012 at 09:47:22AM +1030, Rusty Russell wrote:
> All we need is the index of the request; the rest can be re-read from
> the ring.

I'd like to point out that this is not generally
true if any available requests are outstanding.
Imagine a ring of size 4.
Below A means available U means used.

A 1
A 2
U 2
A 2
U 2
A 2
U 2
A 2
U 2

At this point available ring has wrapped around, the only
way to know head 1 is outstanding is because backend
has stored this info somewhere.

The reason we manage to migrate without tracking this in migration
state is because we flush outstanding requests before
migration.
This flush is device-specific though, let's add
a safeguard in virtio core to ensure it's done properly.

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

---

diff --git a/hw/virtio.c b/hw/virtio.c
index f40a8c5..b80a5a9 100644
--- a/hw/virtio.c
+++ b/hw/virtio.c
@@ -788,6 +788,8 @@ void virtio_save(VirtIODevice *vdev, QEMUFile *f)
         if (vdev->vq[i].vring.num == 0)
             break;
 
+        assert(!vq->inuse);
+
         qemu_put_be32(f, vdev->vq[i].vring.num);
         qemu_put_be64(f, vdev->vq[i].pa);
         qemu_put_be16s(f, &vdev->vq[i].last_avail_idx);

-- 
MST