Subject: Re: [PATCH v1 1/3] virtio-net: Using single MSIX IRQ for TX/RX Q pair

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 27 Oct 2015 10:38:48 +0200

On Mon, Oct 26, 2015 at 10:52:47AM -0700, Ravi Kerur wrote:
> Ported earlier patch from Jason Wang (dated 12/26/2014).
> 
> This patch tries to reduce the number of MSIX irqs required for
> virtio-net by sharing a MSIX irq for each TX/RX queue pair through
> channels. If transport support channel, about half of the MSIX irqs
> were reduced.
> 
> Signed-off-by: Ravi Kerur <[email-masked]>

Why bother BTW? Looks like this is adding a bunch of overhead
on data path - to what end?

Maybe you have a huge number of these devices ... but in that case, how
about sharing the config interrupt instead?
That's only possible if host supports VIRTIO_1
(so we can detect config interrupt by reading the ISR).

> ---
>  drivers/net/virtio_net.c | 29 ++++++++++++++++++++++++++++-
>  1 file changed, 28 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index d8838ded..d705cce 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -72,6 +72,9 @@ struct send_queue {
>  
>  	/* Name of the send queue: output.$index */
>  	char name[40];
> +
> +	/* Name of the channel, shared with irq. */
> +	char channel_name[40];
>  };
>  
>  /* Internal representation of a receive virtqueue */
> @@ -1529,6 +1532,8 @@ static int virtnet_find_vqs(struct virtnet_info *vi)
>  	int ret = -ENOMEM;
>  	int i, total_vqs;
>  	const char **names;
> +	const char **channel_names;
> +	unsigned *channels;
>  
>  	/* We expect 1 RX virtqueue followed by 1 TX virtqueue, followed by
>  	 * possible N-1 RX/TX queue pairs used in multiqueue mode, followed by
> @@ -1548,6 +1553,17 @@ static int virtnet_find_vqs(struct virtnet_info *vi)
>  	if (!names)
>  		goto err_names;
>  
> +	channel_names = kmalloc_array(vi->max_queue_pairs,
> +				      sizeof(*channel_names),
> +				      GFP_KERNEL);
> +	if (!channel_names)
> +		goto err_channel_names;
> +
> +	channels = kmalloc_array(total_vqs, sizeof(*channels),
> +				 GFP_KERNEL);
> +	if (!channels)
> +		goto err_channels;
> +
>  	/* Parameters for control virtqueue, if any */
>  	if (vi->has_cvq) {
>  		callbacks[total_vqs - 1] = NULL;
> @@ -1562,10 +1578,15 @@ static int virtnet_find_vqs(struct virtnet_info *vi)
>  		sprintf(vi->sq[i].name, "output.%d", i);
>  		names[rxq2vq(i)] = vi->rq[i].name;
>  		names[txq2vq(i)] = vi->sq[i].name;
> +		sprintf(vi->sq[i].channel_name, "txrx.%d", i);
> +		channel_names[i] = vi->sq[i].channel_name;
> +		channels[rxq2vq(i)] = i;
> +		channels[txq2vq(i)] = i;
>  	}
>  
>  	ret = vi->vdev->config->find_vqs(vi->vdev, total_vqs, vqs, callbacks,
> -					 names);
> +					 names, channels, channel_names,
> +					 vi->max_queue_pairs);
>  	if (ret)
>  		goto err_find;
>  
> @@ -1580,6 +1601,8 @@ static int virtnet_find_vqs(struct virtnet_info *vi)
>  		vi->sq[i].vq = vqs[txq2vq(i)];
>  	}
>  
> +	kfree(channels);
> +	kfree(channel_names);
>  	kfree(names);
>  	kfree(callbacks);
>  	kfree(vqs);
> @@ -1587,6 +1610,10 @@ static int virtnet_find_vqs(struct virtnet_info *vi)
>  	return 0;
>  
>  err_find:
> +	kfree(channels);
> +err_channels:
> +	kfree(channel_names);
> +err_channel_names:
>  	kfree(names);
>  err_names:
>  	kfree(callbacks);
> -- 
> 1.9.1