Subject: Re: [PATCH 1/3] A device for zero-copy based on KVM virtio-net.

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 7 Apr 2010 14:17:22 +0300

On Wed, Apr 07, 2010 at 05:00:39PM +0800, [email-masked] wrote:
> From: Xin Xiaohui <[email-masked]>
> 
> ---
> 
> Michael,
> Thanks a lot for the explanation. I have drafted a patch for the qemu write
> after I looked into tun driver. Does it do in right way?
> 
> Thanks
> Xiaohui
> 
>  drivers/vhost/mpassthru.c |   45 +++++++++++++++++++++++++++++++++++++++++++++
>  1 files changed, 45 insertions(+), 0 deletions(-)
> 
> diff --git a/drivers/vhost/mpassthru.c b/drivers/vhost/mpassthru.c
> index e9449ac..1cde097 100644
> --- a/drivers/vhost/mpassthru.c
> +++ b/drivers/vhost/mpassthru.c
> @@ -1065,6 +1065,49 @@ static unsigned int mp_chr_poll(struct file *file, poll_table * wait)
>  	return mask;
>  }
>  
> +static ssize_t mp_chr_aio_write(struct kiocb *iocb, const struct iovec *iov,
> +				unsigned long count, loff_t pos)
> +{
> +	struct file *file = iocb->ki_filp;
> +	struct mp_struct *mp = mp_get(file->private_data);
> +	struct sock *sk = mp->socket.sk;
> +	struct sk_buff *skb;
> +	int len, err;
> +	ssize_t result;
> +
> +	if (!mp)
> +		return -EBADFD;
> +

Can this happen? When?

> +	/* currently, async is not supported */
> +	if (!is_sync_kiocb(iocb))
> +		return -EFAULT;

Really necessary? I think do_sync_write handles all this.

> +
> +	len = iov_length(iov, count);
> +	skb = sock_alloc_send_skb(sk, len + NET_IP_ALIGN,
> +				  file->f_flags & O_NONBLOCK, &err);
> +
> +	if (!skb)
> +		return -EFAULT;

Surely not EFAULT. -EAGAIN?

> +
> +	skb_reserve(skb, NET_IP_ALIGN);
> +	skb_put(skb, len);
> +
> +	if (skb_copy_datagram_from_iovec(skb, 0, iov, 0, len)) {
> +		kfree_skb(skb);
> +		return -EFAULT;
> +	}
> +	skb_set_network_header(skb, ETH_HLEN);

Is this really right or necessary? Also,
probably need to check that length is at least ETH_ALEN before
doing this.

> +	skb->protocol = *((__be16 *)(skb->data) + ETH_ALEN);

eth_type_trans?

> +	skb->dev = mp->dev;
> +
> +	dev_queue_xmit(skb);
> +	mp->dev->stats.tx_packets++;
> +	mp->dev->stats.tx_bytes += len;

Doesn't the hard start xmit function for the device
increment the counters?

> +
> +	mp_put(mp);
> +	return result;
> +}
> +
>  static int mp_chr_close(struct inode *inode, struct file *file)
>  {
>  	struct mp_file *mfile = file->private_data;
> @@ -1084,6 +1127,8 @@ static int mp_chr_close(struct inode *inode, struct file *file)
>  static const struct file_operations mp_fops = {
>  	.owner  = THIS_MODULE,
>  	.llseek = no_llseek,
> +	.write  = do_sync_write,
> +	.aio_write = mp_chr_aio_write,
>  	.poll   = mp_chr_poll,
>  	.unlocked_ioctl = mp_chr_ioctl,
>  	.open   = mp_chr_open,
> -- 
> 1.5.4.4