Re: [PATCH 5 of 5] virtio: expose added descriptors immediately

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

On Thu, Nov 03, 2011 at 06:12:53PM +1030, Rusty Russell wrote:
> A virtio driver does virtqueue_add_buf() multiple times before finally
> calling virtqueue_kick(); previously we only exposed the added buffers
> in the virtqueue_kick() call.  This means we don't need a memory
> barrier in virtqueue_add_buf(), but it reduces concurrency as the
> device (ie. host) can't see the buffers until the kick.
> 
> Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

In the past I played with a patch like this, but I didn't see a
performance gain either way. Do you see any gain?

I'm a bit concerned that with this patch, a buggy driver that
adds more than 2^16 descriptors without a kick
would seem to work sometimes. Let's add WARN_ON(vq->num_added > (1 << 16))?

> ---
>  drivers/virtio/virtio_ring.c |   37 ++++++++++++++++++++++---------------
>  1 file changed, 22 insertions(+), 15 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -227,9 +227,15 @@ add_head:
>  
>  	/* Put entry in available array (but don't update avail->idx until they
>  	 * do sync). */
> -	avail = ((vq->vring.avail->idx + vq->num_added++) & (vq->vring.num-1));
> +	avail = (vq->vring.avail->idx & (vq->vring.num-1));
>  	vq->vring.avail->ring[avail] = head;
>  
> +	/* Descriptors and available array need to be set before we expose the
> +	 * new available array entries. */
> +	virtio_wmb();
> +	vq->vring.avail->idx++;
> +	vq->num_added++;
> +
>  	pr_debug("Added buffer head %i to %p\n", head, vq);
>  	END_USE(vq);
>  
> @@ -248,13 +254,10 @@ bool virtqueue_kick_prepare(struct virtq
>  	 * new available array entries. */
>  	virtio_wmb();
>  
> -	old = vq->vring.avail->idx;
> -	new = vq->vring.avail->idx = old + vq->num_added;
> +	old = vq->vring.avail->idx - vq->num_added;
> +	new = vq->vring.avail->idx;
>  	vq->num_added = 0;
>  
> -	/* Need to update avail index before checking if we should notify */
> -	virtio_mb();
> -
>  	if (vq->event) {
>  		needs_kick = vring_need_event(vring_avail_event(&vq->vring),
>  					      new, old);
> 
> 
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