Re: [Qemu-devel] [PATCH for-2.7 1/4] virtio: recalculate vq->inuse after migration

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

On Fri, Aug 12, 2016 at 04:32:55PM +0100, Stefan Hajnoczi wrote:
> The vq->inuse field is not migrated.  Many devices don't hold
> VirtQueueElements across migration so it doesn't matter that vq->inuse
> starts at 0 on the destination QEMU.
> 
> At least virtio-serial, virtio-blk, and virtio-balloon migrate while
> holding VirtQueueElements.  For these devices we need to recalculate
> vq->inuse upon load so the value is correct.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>


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

> ---
>  hw/virtio/virtio.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index 15ee3a7..4df8274 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -1648,6 +1648,20 @@ int virtio_load(VirtIODevice *vdev, QEMUFile *f, int version_id)
>              }
>              vdev->vq[i].used_idx = vring_used_idx(&vdev->vq[i]);
>              vdev->vq[i].shadow_avail_idx = vring_avail_idx(&vdev->vq[i]);
> +
> +            /* Some devices migrate VirtQueueElements that have been popped
> +             * from the avail ring but not yet returned to the used ring.
> +             */
> +            vdev->vq[i].inuse = vdev->vq[i].last_avail_idx -
> +                                vdev->vq[i].used_idx;
> +            if (vdev->vq[i].inuse > vdev->vq[i].vring.num) {
> +                error_report("VQ %d size 0x%x < last_avail_idx 0x%x - "
> +                             "used_idx 0x%x"
> +                             i, vdev->vq[i].vring.num,
> +                             vdev->vq[i].last_avail_idx,
> +                             vdev->vq[i].used_idx);
> +                return -1;
> +            }
>          }
>      }
>  
> -- 
> 2.7.4