Subject: Re: [PATCH net-next v4 2/5] virtio_net: enable irq for the control vq

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 24 Jun 2024 07:30:11 -0400

On Thu, Jun 20, 2024 at 12:19:05AM +0800, Heng Qi wrote:
> If the device does not respond to a request for a long time,
> then control vq polling elevates CPU utilization, a problem that
> exacerbates with more command requests.
> 
> Enabling control vq's irq is advantageous for the guest, and
> this still doesn't support concurrent requests.
> 
> Suggested-by: Jason Wang <[email-masked]>
> Signed-off-by: Heng Qi <[email-masked]>
> ---
>  drivers/net/virtio_net.c | 22 +++++++++++++---------
>  1 file changed, 13 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index b45f58a902e3..ed10084997d3 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -372,6 +372,8 @@ struct virtio_net_ctrl_rss {
>  struct control_buf {
>  	struct virtio_net_ctrl_hdr hdr;
>  	virtio_net_ctrl_ack status;
> +	/* Wait for the device to complete the cvq request. */
> +	struct completion completion;
>  };
>  
>  struct virtnet_info {
> @@ -664,6 +666,13 @@ static bool virtqueue_napi_complete(struct napi_struct *napi,
>  	return false;
>  }
>  
> +static void virtnet_cvq_done(struct virtqueue *cvq)
> +{
> +	struct virtnet_info *vi = cvq->vdev->priv;
> +
> +	complete(&vi->ctrl->completion);
> +}
> +
>  static void skb_xmit_done(struct virtqueue *vq)
>  {
>  	struct virtnet_info *vi = vq->vdev->priv;

> @@ -2724,14 +2733,8 @@ static bool virtnet_send_command_reply(struct virtnet_info *vi,
>  	if (unlikely(!virtqueue_kick(vi->cvq)))
>  		goto unlock;
>  
> -	/* Spin for a response, the kick causes an ioport write, trapping
> -	 * into the hypervisor, so the request should be handled immediately.
> -	 */
> -	while (!virtqueue_get_buf(vi->cvq, &tmp) &&
> -	       !virtqueue_is_broken(vi->cvq)) {
> -		cond_resched();
> -		cpu_relax();
> -	}
> +	wait_for_completion(&ctrl->completion);
> +	virtqueue_get_buf(vi->cvq, &tmp);
>  
>  unlock:
>  	ok = ctrl->status == VIRTIO_NET_OK;

Hmm no this is not a good idea, code should be robust in case
of spurious interrupts.

Also suprise removal is now broken ...

> @@ -5312,7 +5315,7 @@ static int virtnet_find_vqs(struct virtnet_info *vi)
>  
>  	/* Parameters for control virtqueue, if any */
>  	if (vi->has_cvq) {
> -		callbacks[total_vqs - 1] = NULL;
> +		callbacks[total_vqs - 1] = virtnet_cvq_done;
>  		names[total_vqs - 1] = "control";
>  	}
>  
> @@ -5832,6 +5835,7 @@ static int virtnet_probe(struct virtio_device *vdev)
>  	if (vi->has_rss || vi->has_rss_hash_report)
>  		virtnet_init_default_rss(vi);
>  
> +	init_completion(&vi->ctrl->completion);
>  	enable_rx_mode_work(vi);
>  
>  	/* serialize netdev register + virtio_device_ready() with ndo_open() */
> -- 
> 2.32.0.3.g01195cf9f