Re: [PATCHv2 11/14] virtio: don't delay avail index update

Rusty Russell
2011-05-21T02:26:32Z

On Fri, 20 May 2011 02:12:19 +0300, "Michael S. Tsirkin" <mst-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> wrote:
> Update avail index immediately instead of upon kick:
> for virtio-net RX this helps parallelism with the host.
>
> Signed-off-by: Michael S. Tsirkin <mst-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
> ---
> drivers/virtio/virtio_ring.c | 28 +++++++++++++++++++---------
> 1 files changed, 19 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index eed5f29..8218fe6 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -89,7 +89,7 @@ struct vring_virtqueue
> unsigned int num_free;
> /* Head of free buffer list. */
> unsigned int free_head;
> - /* Number we've added since last sync. */
> + /* Number we've added since last kick. */
> unsigned int num_added;

I always like to see obsolescent nomenclature cleaned up like this.
Thanks.

> /* Last used index we've seen. */
> @@ -174,6 +174,13 @@ int virtqueue_add_buf_gfp(struct virtqueue *_vq,
>
> BUG_ON(data == NULL);
>
> + /* Prevent drivers from adding more than num bufs without a kick. */
> + if (vq->num_added == vq->vring.num) {
> + printk(KERN_ERR "gaaa!!!\n");
> + END_USE(vq);
> + return -ENOSPC;
> + }
> +

I like "gaaa!" but it won't tell us which driver. How about the more
conventional:

if (WARN_ON(vq->num_added >= vq->vring.num)) {
END_USE(vq);
return -ENOSPC;
}

I'd really like to see the results of this patch. It's useless for
outgoing net traffic (we deal with one packet at a time) but perhaps a
flood of incoming packets would show something.

Thanks,
Rusty.