linux-kernel - Re: [PATCH] virtio_net: Enable MSI-X vector for ctrl queue

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Date:   Fri, 18 Jun 2021 08:38:10 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Keiichi Watanabe <keiichiw@...omium.org>
Cc:     netdev@...r.kernel.org, chirantan@...omium.org,
        Jason Wang <jasowang@...hat.com>,
        "David S . Miller" <davem@...emloft.net>,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] virtio_net: Enable MSI-X vector for ctrl queue

On Fri, Jun 18, 2021 at 04:26:25PM +0900, Keiichi Watanabe wrote:
> When we use vhost-user backend on the host, MSI-X vector should be set
> so that the vmm can get an irq FD and send it to the backend device
> process with vhost-user protocol.
> Since whether the vector is set for a queue is determined depending on
> the queue has a callback, this commit sets an empty callback for
> virtio-net's control queue.
> 
> Signed-off-by: Keiichi Watanabe <keiichiw@...omium.org>

I'm confused by this explanation. If the vmm wants to get
an interrupt it can do so - why change the guest driver?

> ---
>  drivers/net/virtio_net.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 11f722460513..002e3695d4b3 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -2696,6 +2696,11 @@ static void virtnet_del_vqs(struct virtnet_info *vi)
>  	virtnet_free_queues(vi);
>  }
>  
> +static void virtnet_ctrlq_done(struct virtqueue *rvq)
> +{
> +	/* Do nothing */
> +}
> +
>  /* How large should a single buffer be so a queue full of these can fit at
>   * least one full packet?
>   * Logic below assumes the mergeable buffer header is used.
> @@ -2748,7 +2753,7 @@ 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_ctrlq_done;
>  		names[total_vqs - 1] = "control";
>  	}
>  
> -- 
> 2.32.0.288.g62a8d224e6-goog