Re: [Qemu-devel] [PATCH 2/2] virtio-blk: fix length calculations for write operations.

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

On Wed, Mar 11, 2015 at 04:29:32PM +1030, Rusty Russell wrote:
> We only fill in the 'req->qiov.size' bytes on a (successful) read,
> not on a write.
> 
> Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
> ---
>  hw/block/virtio-blk.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
> index 258bb4c..98d87a9 100644
> --- a/hw/block/virtio-blk.c
> +++ b/hw/block/virtio-blk.c
> @@ -50,11 +50,19 @@ static void virtio_blk_complete_request(VirtIOBlockReq *req,
>  {
>      VirtIOBlock *s = req->dev;
>      VirtIODevice *vdev = VIRTIO_DEVICE(s);
> +    int type = virtio_ldl_p(VIRTIO_DEVICE(req->dev), &req->out.type);
>  
>      trace_virtio_blk_req_complete(req, status);
>  
>      stb_p(&req->in->status, status);
> -    virtqueue_push(s->vq, &req->elem, req->qiov.size + sizeof(*req->in));
> +
> +    /* If we didn't succeed, we *may* have written more, but don't
> +     * count on it. */

I wonder about this.
So length as you specify it is <= actually written length.
What are the advantages of this approach?
How about we do the reverse, specify that the length in descriptor
is >= the size actually written?

If we do this, all these buggy hosts suddenly become correct,
which seems better.


> +    if (type == VIRTIO_BLK_T_IN && status == VIRTIO_BLK_S_OK) {
> +        virtqueue_push(s->vq, &req->elem, req->qiov.size + sizeof(*req->in));
> +    } else {
> +        virtqueue_push(s->vq, &req->elem, sizeof(*req->in));
> +    }
>      virtio_notify(vdev, s->vq);
>  }
>  
> -- 
> 2.1.0