Subject: Re: [RFC PATCH net-next 10/12] vhost_net: build xdp buff

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 22 May 2018 01:21:11 +0300

On Mon, May 21, 2018 at 09:56:11AM -0700, Jesse Brandeburg wrote:
> On Mon, 21 May 2018 17:04:31 +0800 Jason wrote:
> > This patch implement build XDP buffers in vhost_net. The idea is do
> > userspace copy in vhost_net and build XDP buff based on the
> > page. Vhost_net can then submit one or an array of XDP buffs to
> > underlayer socket (e.g TUN). TUN can choose to do XDP or call
> > build_skb() to build skb. To support build skb, vnet header were also
> > stored into the header of the XDP buff.
> > 
> > This userspace copy and XDP buffs building is key to achieve XDP
> > batching in TUN, since TUN does not need to care about userspace copy
> > and then can disable premmption for several XDP buffs to achieve
> > batching from XDP.
> > 
> > TODO: reserve headroom based on the TUN XDP.
> > 
> > Signed-off-by: Jason Wang <[email-masked]>
> > ---
> >  drivers/vhost/net.c | 74 +++++++++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 74 insertions(+)
> > 
> > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> > index f0639d7..1209e84 100644
> > --- a/drivers/vhost/net.c
> > +++ b/drivers/vhost/net.c
> > @@ -492,6 +492,80 @@ static bool vhost_has_more_pkts(struct vhost_net *net,
> >  	       likely(!vhost_exceeds_maxpend(net));
> >  }
> >  
> > +#define VHOST_NET_HEADROOM 256
> > +#define VHOST_NET_RX_PAD (NET_IP_ALIGN + NET_SKB_PAD)
> > +
> > +static int vhost_net_build_xdp(struct vhost_net_virtqueue *nvq,
> > +			       struct iov_iter *from,
> > +			       struct xdp_buff *xdp)
> > +{
> > +	struct vhost_virtqueue *vq = &nvq->vq;
> > +	struct page_frag *alloc_frag = &current->task_frag;
> > +	struct virtio_net_hdr *gso;
> > +	size_t len = iov_iter_count(from);
> > +	int buflen = SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
> > +	int pad = SKB_DATA_ALIGN(VHOST_NET_RX_PAD + VHOST_NET_HEADROOM
> > +				 + nvq->sock_hlen);
> > +	int sock_hlen = nvq->sock_hlen;
> > +	void *buf;
> > +	int copied;
> > +
> > +	if (len < nvq->sock_hlen)
> > +		return -EFAULT;
> > +
> > +	if (SKB_DATA_ALIGN(len + pad) +
> > +	    SKB_DATA_ALIGN(sizeof(struct skb_shared_info)) > PAGE_SIZE)
> > +		return -ENOSPC;
> > +
> > +	buflen += SKB_DATA_ALIGN(len + pad);
> 
> maybe store the result of SKB_DATA_ALIGN in a local instead of doing
> the work twice?

I don't mind, but I guess gcc can always do it itself?

> > +	alloc_frag->offset = ALIGN((u64)alloc_frag->offset, SMP_CACHE_BYTES);
> > +	if (unlikely(!skb_page_frag_refill(buflen, alloc_frag, GFP_KERNEL)))
> > +		return -ENOMEM;
> > +
> > +	buf = (char *)page_address(alloc_frag->page) + alloc_frag->offset;
> > +
> > +	/* We store two kinds of metadata in the header which will be
> > +	 * used for XDP_PASS to do build_skb():
> > +	 * offset 0: buflen
> > +	 * offset sizeof(int): vnet header
> > +	 */
> > +	copied = copy_page_from_iter(alloc_frag->page,
> > +				     alloc_frag->offset + sizeof(int), sock_hlen, from);
> > +	if (copied != sock_hlen)
> > +		return -EFAULT;
> > +
> > +	gso = (struct virtio_net_hdr *)(buf + sizeof(int));
> > +
> > +	if ((gso->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) &&
> > +	    vhost16_to_cpu(vq, gso->csum_start) +
> > +	    vhost16_to_cpu(vq, gso->csum_offset) + 2 >
> > +	    vhost16_to_cpu(vq, gso->hdr_len)) {
> > +		gso->hdr_len = cpu_to_vhost16(vq,
> > +			       vhost16_to_cpu(vq, gso->csum_start) +
> > +			       vhost16_to_cpu(vq, gso->csum_offset) + 2);
> > +
> > +		if (vhost16_to_cpu(vq, gso->hdr_len) > len)
> > +			return -EINVAL;
> > +	}
> > +
> > +	len -= sock_hlen;
> > +	copied = copy_page_from_iter(alloc_frag->page,
> > +				     alloc_frag->offset + pad,
> > +				     len, from);
> > +	if (copied != len)
> > +		return -EFAULT;
> > +
> > +	xdp->data_hard_start = buf;
> > +	xdp->data = buf + pad;
> > +	xdp->data_end = xdp->data + len;
> > +	*(int *)(xdp->data_hard_start)= buflen;
> 
> space before =
> 
> > +
> > +	get_page(alloc_frag->page);
> > +	alloc_frag->offset += buflen;
> > +
> > +	return 0;
> > +}
> > +
> >  static void handle_tx_copy(struct vhost_net *net)
> >  {
> >  	struct vhost_net_virtqueue *nvq = &net->vqs[VHOST_NET_VQ_TX];