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SubjectRe: [PATCH 5 of 5] virtio: expose added descriptors immediately
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);
>
>
> _______________________________________________
> Virtualization mailing list
> Virtualization@lists.linux-foundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/virtualization


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