Subject: Re: [bpf-next V2 PATCH 07/15] virtio_net: convert to use generic xdp_frame and xdp_return_frame API

From: Jesper Dangaard Brouer <[email-masked]>
Date: Fri, 9 Mar 2018 10:44:58 +0100

On Fri, 9 Mar 2018 16:03:28 +0800
Jason Wang <[email-masked]> wrote:

> On 2018年03月08日 21:08, Jesper Dangaard Brouer wrote:
> > The virtio_net driver assumes XDP frames are always released based on
> > page refcnt (via put_page).  Thus, is only queues the XDP data pointer
> > address and uses virt_to_head_page() to retrieve struct page.
> >
> > Use the XDP return API to get away from such assumptions. Instead
> > queue an xdp_frame, which allow us to use the xdp_return_frame API,
> > when releasing the frame.
> >
> > Signed-off-by: Jesper Dangaard Brouer <[email-masked]>
> > ---
> >   drivers/net/virtio_net.c |   31 +++++++++++++++++++++----------
> >   1 file changed, 21 insertions(+), 10 deletions(-)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index 23374603e4d9..6c4220450506 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -419,30 +419,41 @@ static bool __virtnet_xdp_xmit(struct virtnet_info *vi,
> >   			       struct xdp_buff *xdp)
> >   {
> >   	struct virtio_net_hdr_mrg_rxbuf *hdr;
> > -	unsigned int len;
> > +	struct xdp_frame *xdpf, *xdpf_sent;
> >   	struct send_queue *sq;
> > +	unsigned int len;
> >   	unsigned int qp;
> > -	void *xdp_sent;
> >   	int err;
> >   
> >   	qp = vi->curr_queue_pairs - vi->xdp_queue_pairs + smp_processor_id();
> >   	sq = &vi->sq[qp];
> >   
> >   	/* Free up any pending old buffers before queueing new ones. */
> > -	while ((xdp_sent = virtqueue_get_buf(sq->vq, &len)) != NULL) {
> > -		struct page *sent_page = virt_to_head_page(xdp_sent);
> > +	while ((xdpf_sent = virtqueue_get_buf(sq->vq, &len)) != NULL)
> > +		xdp_return_frame(xdpf_sent->data, &xdpf_sent->mem);
> >   
> > -		put_page(sent_page);
> > -	}
> > +	xdpf = convert_to_xdp_frame(xdp);
> > +	if (unlikely(!xdpf))
> > +		return -EOVERFLOW;
> > +
> > +	/* virtqueue want to use data area in-front of packet */
> > +	if (unlikely(xdpf->metasize > 0))
> > +		return -EOPNOTSUPP;
> > +
> > +	if (unlikely(xdpf->headroom < vi->hdr_len))
> > +		return -EOVERFLOW;  
> 
> I think this need another independent patch. For now we can simply drop 
> the packet, but we probably need to think more to address it completely, 
> allocate header part either dynamically or statically.

Okay, so we can followup later if we want to handle this case better
than drop.

> >   
> > -	xdp->data -= vi->hdr_len;
> > +	/* Make room for virtqueue hdr (also change xdpf->headroom?) */
> > +	xdpf->data -= vi->hdr_len;
> >   	/* Zero header and leave csum up to XDP layers */
> > -	hdr = xdp->data;
> > +	hdr = xdpf->data;
> >   	memset(hdr, 0, vi->hdr_len);
> > +	hdr->hdr.hdr_len = xdpf->len; /* Q: is this needed? */  
> 
> Maybe another patch but not a must, hdr_len is hint for the linear part 
> of skb used in host. Backend implementation may simply ignore this value.

So, I should leave it out for now?
Or it is okay to keep it?

> > +	xdpf->len   += vi->hdr_len;
> >   
> > -	sg_init_one(sq->sg, xdp->data, xdp->data_end - xdp->data);
> > +	sg_init_one(sq->sg, xdpf->data, xdpf->len);

When _later_ introducing bulking, can we use something else than
sg_init_one() to send/queue multiple raw XDP frames for sending?
(I'm asking as I don't know this "sg_*" API usage)

> > -	err = virtqueue_add_outbuf(sq->vq, sq->sg, 1, xdp->data,
> > GFP_ATOMIC);
> > +	err = virtqueue_add_outbuf(sq->vq, sq->sg, 1, xdpf,
> > GFP_ATOMIC); if (unlikely(err))
> >   		return false; /* Caller handle free/refcnt */
> >   

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer