Subject: Re: [PATCH V2 net-next 5/7] tun: support receiving skb through msg_control

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 30 Mar 2017 18:06:28 +0300

On Thu, Mar 30, 2017 at 03:22:28PM +0800, Jason Wang wrote:
> This patch makes tun_recvmsg() can receive from skb from its caller
> through msg_control. Vhost_net will be the first user.
> 
> Signed-off-by: Jason Wang <[email-masked]>

Do we need to bother with tun? I didn't realize one
can even use that with vhost. What would be the point of
all the virtio header stuff dealing with checksums etc?

Even if you see a use-case is it worth optimizing?

> ---
>  drivers/net/tun.c | 18 ++++++++++--------
>  1 file changed, 10 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index 70dd9ec..a82bced 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -1498,9 +1498,8 @@ static struct sk_buff *tun_ring_recv(struct tun_file *tfile, int noblock,
>  
>  static ssize_t tun_do_read(struct tun_struct *tun, struct tun_file *tfile,
>  			   struct iov_iter *to,
> -			   int noblock)
> +			   int noblock, struct sk_buff *skb)
>  {
> -	struct sk_buff *skb;
>  	ssize_t ret;
>  	int err;
>  
> @@ -1509,10 +1508,12 @@ static ssize_t tun_do_read(struct tun_struct *tun, struct tun_file *tfile,
>  	if (!iov_iter_count(to))
>  		return 0;
>  
> -	/* Read frames from ring */
> -	skb = tun_ring_recv(tfile, noblock, &err);
> -	if (!skb)
> -		return err;
> +	if (!skb) {
> +		/* Read frames from ring */
> +		skb = tun_ring_recv(tfile, noblock, &err);
> +		if (!skb)
> +			return err;
> +	}
>  
>  	ret = tun_put_user(tun, tfile, skb, to);
>  	if (unlikely(ret < 0))
> @@ -1532,7 +1533,7 @@ static ssize_t tun_chr_read_iter(struct kiocb *iocb, struct iov_iter *to)
>  
>  	if (!tun)
>  		return -EBADFD;
> -	ret = tun_do_read(tun, tfile, to, file->f_flags & O_NONBLOCK);
> +	ret = tun_do_read(tun, tfile, to, file->f_flags & O_NONBLOCK, NULL);
>  	ret = min_t(ssize_t, ret, len);
>  	if (ret > 0)
>  		iocb->ki_pos = ret;
> @@ -1634,7 +1635,8 @@ static int tun_recvmsg(struct socket *sock, struct msghdr *m, size_t total_len,
>  					 SOL_PACKET, TUN_TX_TIMESTAMP);
>  		goto out;
>  	}
> -	ret = tun_do_read(tun, tfile, &m->msg_iter, flags & MSG_DONTWAIT);
> +	ret = tun_do_read(tun, tfile, &m->msg_iter, flags & MSG_DONTWAIT,
> +			  m->msg_control);
>  	if (ret > (ssize_t)total_len) {
>  		m->msg_flags |= MSG_TRUNC;
>  		ret = flags & MSG_TRUNC ? ret : total_len;
> -- 
> 2.7.4