Subject: Re: [PATCH] virtio: Introduce a new kick interface virtqueue_kick_try()

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 19 May 2021 08:44:01 -0400

On Wed, May 19, 2021 at 07:47:57PM +0800, Xuan Zhuo wrote:
> Unlike virtqueue_kick(), virtqueue_kick_try() returns true only when the
> kick is successful. In virtio-net, we want to count the number of kicks.
> So we need an interface that can perceive whether the kick is actually
> executed.
> 
> Signed-off-by: Xuan Zhuo <[email-masked]>
> ---
>  drivers/net/virtio_net.c     |  8 ++++----
>  drivers/virtio/virtio_ring.c | 20 ++++++++++++++++++++
>  include/linux/virtio.h       |  2 ++
>  3 files changed, 26 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 9b6a4a875c55..167697030cb6 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -617,7 +617,7 @@ static int virtnet_xdp_xmit(struct net_device *dev,
>  	ret = nxmit;
>  
>  	if (flags & XDP_XMIT_FLUSH) {
> -		if (virtqueue_kick_prepare(sq->vq) && virtqueue_notify(sq->vq))
> +		if (virtqueue_kick_try(sq->vq))
>  			kicks = 1;
>  	}
>  out:
> @@ -1325,7 +1325,7 @@ static bool try_fill_recv(struct virtnet_info *vi, struct receive_queue *rq,
>  		if (err)
>  			break;
>  	} while (rq->vq->num_free);
> -	if (virtqueue_kick_prepare(rq->vq) && virtqueue_notify(rq->vq)) {
> +	if (virtqueue_kick_try(rq->vq)) {
>  		unsigned long flags;
>  
>  		flags = u64_stats_update_begin_irqsave(&rq->stats.syncp);
> @@ -1533,7 +1533,7 @@ static int virtnet_poll(struct napi_struct *napi, int budget)
>  
>  	if (xdp_xmit & VIRTIO_XDP_TX) {
>  		sq = virtnet_xdp_get_sq(vi);
> -		if (virtqueue_kick_prepare(sq->vq) && virtqueue_notify(sq->vq)) {
> +		if (virtqueue_kick_try(sq->vq)) {
>  			u64_stats_update_begin(&sq->stats.syncp);
>  			sq->stats.kicks++;
>  			u64_stats_update_end(&sq->stats.syncp);
> @@ -1710,7 +1710,7 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
>  	}
>  
>  	if (kick || netif_xmit_stopped(txq)) {
> -		if (virtqueue_kick_prepare(sq->vq) && virtqueue_notify(sq->vq)) {
> +		if (virtqueue_kick_try(sq->vq)) {
>  			u64_stats_update_begin(&sq->stats.syncp);
>  			sq->stats.kicks++;
>  			u64_stats_update_end(&sq->stats.syncp);
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 71e16b53e9c1..1462be756875 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -1874,6 +1874,26 @@ bool virtqueue_kick(struct virtqueue *vq)
>  }
>  EXPORT_SYMBOL_GPL(virtqueue_kick);
>  
> +/**
> + * virtqueue_kick_try - try update after add_buf
> + * @vq: the struct virtqueue
> + *
> + * After one or more virtqueue_add_* calls, invoke this to kick
> + * the other side.
> + *
> + * Caller must ensure we don't call this with other virtqueue
> + * operations at the same time (except where noted).
> + *
> + * Returns true if kick success, otherwise false.

so what is the difference between this and virtqueue_kick
which says

 * Returns false if kick failed, otherwise true.

> + */
> +bool virtqueue_kick_try(struct virtqueue *vq)
> +{
> +	if (virtqueue_kick_prepare(vq) && virtqueue_notify(vq))
> +		return true;
> +	return false;
> +}
> +EXPORT_SYMBOL_GPL(virtqueue_kick_try);
> +
>  /**
>   * virtqueue_get_buf - get the next used buffer
>   * @_vq: the struct virtqueue we're talking about.
> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> index b1894e0323fa..45cd6a0af24d 100644
> --- a/include/linux/virtio.h
> +++ b/include/linux/virtio.h
> @@ -59,6 +59,8 @@ int virtqueue_add_sgs(struct virtqueue *vq,
>  
>  bool virtqueue_kick(struct virtqueue *vq);
>  
> +bool virtqueue_kick_try(struct virtqueue *vq);
> +
>  bool virtqueue_kick_prepare(struct virtqueue *vq);
>  
>  bool virtqueue_notify(struct virtqueue *vq);
> -- 
> 2.31.0