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

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

On Wed, Dec 05, 2012 at 01:08:07PM +0200, Michael S. Tsirkin wrote:
> 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>

Doh sent a wrong patch sorry. I'll resend the right one shortly.
Pls disregard.

> ---
> 
> 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