Subject: Re: [PATCH 08/18] virtio_ring: support for used_event idx feature

From: Rusty Russell <rusty-8n+1lVoiYb80n/[email-masked]>
Date: Mon, 09 May 2011 13:47:32 +0930

On Wed, 4 May 2011 23:51:38 +0300, "Michael S. Tsirkin" <[email-masked]> wrote:
> Add support for the used_event idx feature: when enabling
> interrupts, publish the current avail index value to
> the host so that we get interrupts on the next update.
> 
> Signed-off-by: Michael S. Tsirkin <[email-masked]>
> ---
>  drivers/virtio/virtio_ring.c |   14 ++++++++++++++
>  1 files changed, 14 insertions(+), 0 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 507d6eb..3a3ed75 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -320,6 +320,14 @@ void *virtqueue_get_buf(struct virtqueue *_vq, unsigned int *len)
>  	ret = vq->data[i];
>  	detach_buf(vq, i);
>  	vq->last_used_idx++;
> +	/* If we expect an interrupt for the next entry, tell host
> +	 * by writing event index and flush out the write before
> +	 * the read in the next get_buf call. */
> +	if (!(vq->vring.avail->flags & VRING_AVAIL_F_NO_INTERRUPT)) {
> +		vring_used_event(&vq->vring) = vq->last_used_idx;
> +		virtio_mb();
> +	}
> +

Hmm, so you're still using the avail->flags; it's just if thresholding
is enabled the host will ignore it?

It's a little subtle, but it keeps this patch small.  Perhaps we'll want
to make it more explicit later.

Thanks,
Rusty.