Subject: Re: [PATCH net-next 21/33] virtio_net: add XDP frame size in two code paths

From: Jesper Dangaard Brouer <[email-masked]>
Date: Mon, 27 Apr 2020 16:32:24 +0200

On Mon, 27 Apr 2020 15:21:02 +0800
Jason Wang <[email-masked]> wrote:

> On 2020/4/23 上午12:09, Jesper Dangaard Brouer wrote:
> > The virtio_net driver is running inside the guest-OS. There are two
> > XDP receive code-paths in virtio_net, namely receive_small() and
> > receive_mergeable(). The receive_big() function does not support XDP.
> >
> > In receive_small() the frame size is available in buflen. The buffer
> > backing these frames are allocated in add_recvbuf_small() with same
> > size, except for the headroom, but tailroom have reserved room for
> > skb_shared_info. The headroom is encoded in ctx pointer as a value.
> >
> > In receive_mergeable() the frame size is more dynamic. There are two
> > basic cases: (1) buffer size is based on a exponentially weighted
> > moving average (see DECLARE_EWMA) of packet length. Or (2) in case
> > virtnet_get_headroom() have any headroom then buffer size is
> > PAGE_SIZE. The ctx pointer is this time used for encoding two values;
> > the buffer len "truesize" and headroom. In case (1) if the rx buffer
> > size is underestimated, the packet will have been split over more
> > buffers (num_buf info in virtio_net_hdr_mrg_rxbuf placed in top of
> > buffer area). If that happens the XDP path does a xdp_linearize_page
> > operation.
> >
> > Cc: Jason Wang <[email-masked]>
> > Signed-off-by: Jesper Dangaard Brouer <[email-masked]>
> > ---
> >   drivers/net/virtio_net.c |   15 ++++++++++++---
> >   1 file changed, 12 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index 11f722460513..1df3676da185 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -689,6 +689,7 @@ static struct sk_buff *receive_small(struct net_device *dev,
> >   		xdp.data_end = xdp.data + len;
> >   		xdp.data_meta = xdp.data;
> >   		xdp.rxq = &rq->xdp_rxq;
> > +		xdp.frame_sz = buflen;
> >   		orig_data = xdp.data;
> >   		act = bpf_prog_run_xdp(xdp_prog, &xdp);
> >   		stats->xdp_packets++;
> > @@ -797,10 +798,11 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
> >   	int offset = buf - page_address(page);
> >   	struct sk_buff *head_skb, *curr_skb;
> >   	struct bpf_prog *xdp_prog;
> > -	unsigned int truesize;
> > +	unsigned int truesize = mergeable_ctx_to_truesize(ctx);
> >   	unsigned int headroom = mergeable_ctx_to_headroom(ctx);
> > -	int err;
> >   	unsigned int metasize = 0;
> > +	unsigned int frame_sz;
> > +	int err;
> >   
> >   	head_skb = NULL;
> >   	stats->bytes += len - vi->hdr_len;
> > @@ -821,6 +823,11 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
> >   		if (unlikely(hdr->hdr.gso_type))
> >   			goto err_xdp;
> >   
> > +		/* Buffers with headroom use PAGE_SIZE as alloc size,
> > +		 * see add_recvbuf_mergeable() + get_mergeable_buf_len()
> > +		 */
> > +		frame_sz = headroom ? PAGE_SIZE : truesize;
> > +
> >   		/* This happens when rx buffer size is underestimated
> >   		 * or headroom is not enough because of the buffer
> >   		 * was refilled before XDP is set. This should only
> > @@ -834,6 +841,8 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
> >   						      page, offset,
> >   						      VIRTIO_XDP_HEADROOM,
> >   						      &len);
> > +			frame_sz = PAGE_SIZE;  
> 
> 
> Should this be PAGE_SIZE -  SKB_DATA_ALIGN(sizeof(struct skb_shared_info))?

No, frame_sz include the SKB_DATA_ALIGN(sizeof(struct skb_shared_info)) length.

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