Re: [PATCH net-next 10/11] tap: accept an array of XDP buffs through sendmsg()

[Jason Wang]

On 2018å09æ07æ 02:00, Michael S. Tsirkin wrote:
> On Thu, Sep 06, 2018 at 12:05:25PM +0800, Jason Wang wrote:
> > This patch implement TUN_MSG_PTR msg_control type. This type allows
> > the caller to pass an array of XDP buffs to tuntap through ptr field
> > of the tun_msg_control. Tap will build skb through those XDP buffers.
> >
> > This will avoid lots of indirect calls thus improves the icache
> > utilization and allows to do XDP batched flushing when doing XDP
> > redirection.
> >
> > Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> > ---
> >   drivers/net/tap.c | 73 +++++++++++++++++++++++++++++++++++++++++++++--
> >   1 file changed, 71 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/net/tap.c b/drivers/net/tap.c
> > index 7996ed7cbf18..50eb7bf22225 100644
> > --- a/drivers/net/tap.c
> > +++ b/drivers/net/tap.c
> > @@ -1146,14 +1146,83 @@ static const struct file_operations tap_fops = {
> >   #endif
> >   };
> >   
> > +static int tap_get_user_xdp(struct tap_queue *q, struct xdp_buff *xdp)
> > +{
> > +	struct virtio_net_hdr *gso = xdp->data_hard_start + sizeof(int);
> > +	int buflen = *(int *)xdp->data_hard_start;
> > +	int vnet_hdr_len = 0;
> > +	struct tap_dev *tap;
> > +	struct sk_buff *skb;
> > +	int err, depth;
> > +
> > +	if (q->flags & IFF_VNET_HDR)
> > +		vnet_hdr_len = READ_ONCE(q->vnet_hdr_sz);
> > +
> > +	skb = build_skb(xdp->data_hard_start, buflen);
> > +	if (!skb) {
> > +		err = -ENOMEM;
> > +		goto err;
> > +	}
> So fundamentally why is it called XDP?
> We just build and skb, don't we?

The reason is that the function accepts a pointer to XDP. And also the 
for the future development, I think the name is ok:

- we will probably do XDP offloading in this function.
- we may have a chance to call lower device's ndo_xdp_xmit() in the future.

Thanks

> > +
> > +	skb_reserve(skb, xdp->data - xdp->data_hard_start);
> > +	skb_put(skb, xdp->data_end - xdp->data);
> > +
> > +	skb_set_network_header(skb, ETH_HLEN);
> > +	skb_reset_mac_header(skb);
> > +	skb->protocol = eth_hdr(skb)->h_proto;
> > +
> > +	if (vnet_hdr_len) {
> > +		err = virtio_net_hdr_to_skb(skb, gso, tap_is_little_endian(q));
> > +		if (err)
> > +			goto err_kfree;
> > +	}
> > +
> > +	skb_probe_transport_header(skb, ETH_HLEN);
> > +
> > +	/* Move network header to the right position for VLAN tagged packets */
> > +	if ((skb->protocol == htons(ETH_P_8021Q) ||
> > +	     skb->protocol == htons(ETH_P_8021AD)) &&
> > +	    __vlan_get_protocol(skb, skb->protocol, &depth) != 0)
> > +		skb_set_network_header(skb, depth);
> > +
> > +	rcu_read_lock();
> > +	tap = rcu_dereference(q->tap);
> > +	if (tap) {
> > +		skb->dev = tap->dev;
> > +		dev_queue_xmit(skb);
> > +	} else {
> > +		kfree_skb(skb);
> > +	}
> > +	rcu_read_unlock();
> > +
> > +	return 0;
> > +
> > +err_kfree:
> > +	kfree_skb(skb);
> > +err:
> > +	rcu_read_lock();
> > +		tap = rcu_dereference(q->tap);
> > +	if (tap && tap->count_tx_dropped)
> > +		tap->count_tx_dropped(tap);
> > +	rcu_read_unlock();
> > +	return err;
> > +}
> > +
> >   static int tap_sendmsg(struct socket *sock, struct msghdr *m,
> >   		       size_t total_len)
> >   {
> >   	struct tap_queue *q = container_of(sock, struct tap_queue, sock);
> >   	struct tun_msg_ctl *ctl = m->msg_control;
> > +	struct xdp_buff *xdp;
> > +	int i;
> >   
> > -	if (ctl && ctl->type != TUN_MSG_UBUF)
> > -		return -EINVAL;
> > +	if (ctl && ((ctl->type & 0xF) == TUN_MSG_PTR)) {
> > +		for (i = 0; i < ctl->type >> 16; i++) {
> > +			xdp = &((struct xdp_buff *)ctl->ptr)[i];
> > +			tap_get_user_xdp(q, xdp);
> > +		}
> > +		return 0;
> > +	}
> >   
> >   	return tap_get_user(q, ctl ? ctl->ptr : NULL, &m->msg_iter,
> >   			    m->msg_flags & MSG_DONTWAIT);
> > --
> > 2.17.1