Re: [Qemu-devel] [PATCHv2] virtio: verify that all outstanding buffers are flushed

[Stefan Hajnoczi <stefanha@gmail.com>]

On Wed, Dec 12, 2012 at 12:51:01PM +0200, Michael S. Tsirkin wrote:
> Add sanity check to address the following concern:
> 
> During migration, all we pass the index of the request;
> the rest can be re-read from the ring.
> 
> This is not generally enough 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>
> ---
> 
> Changes from v1:
>     v1 was against the wrong tree, it didn't build against qemu.git
> 
>  hw/virtio.c | 2 ++
>  1 file changed, 2 insertions(+)

VirtIOBlock->rq can trigger the assertion.

IIUC hw/virtio-blk.c may handle I/O errors by keeping the request
pending and on a list (->rq).  This allows the user to restart them
after, for example, adding more space to the host file system containing
the disk image file.

We keep a list of failed requests and we migrate this list.  So I think
inuse != 0 when migrating with pending failed I/O requests.

Stefan