Re: [PATCH v9 bpf-next 07/14] net: xdp: add multi-buff support to xdp_build_skb_from_frame

Lorenzo Bianconi
2021-06-29T18:34:14Z

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> On Mon, Jun 14, 2021 at 5:51 AM Lorenzo Bianconi <lorenzo@kernel.org> wrote:
> >
> > Introduce xdp multi-buff support to
> > __xdp_build_skb_from_frame/xdp_build_skb_from_frame
> > utility routines.
> >
> > Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
> > ---
> > net/core/xdp.c | 13 +++++++++++++
> > 1 file changed, 13 insertions(+)
> >
> > diff --git a/net/core/xdp.c b/net/core/xdp.c
> > index f61c63115c95..71bedf6049a1 100644
> > --- a/net/core/xdp.c
> > +++ b/net/core/xdp.c
> > @@ -582,9 +582,15 @@ struct sk_buff *__xdp_build_skb_from_frame(struct xdp_frame *xdpf,
> > struct sk_buff *skb,
> > struct net_device *dev)
> > {
> > + struct skb_shared_info *sinfo = xdp_get_shared_info_from_frame(xdpf);
> > unsigned int headroom, frame_size;
> > + int i, num_frags = 0;
> > void *hard_start;
> >
> > + /* xdp multi-buff frame */
> > + if (unlikely(xdp_frame_is_mb(xdpf)))
> > + num_frags = sinfo->nr_frags;
> > +
> > /* Part of headroom was reserved to xdpf */
> > headroom = sizeof(*xdpf) + xdpf->headroom;
> >
> > @@ -603,6 +609,13 @@ struct sk_buff *__xdp_build_skb_from_frame(struct xdp_frame *xdpf,
> > if (xdpf->metasize)
> > skb_metadata_set(skb, xdpf->metasize);
> >
> > + for (i = 0; i < num_frags; i++)
> > + skb_add_rx_frag(skb, skb_shinfo(skb)->nr_frags,
> > + skb_frag_page(&sinfo->frags[i]),
> > + skb_frag_off(&sinfo->frags[i]),
> > + skb_frag_size(&sinfo->frags[i]),
> > + xdpf->frame_sz);
> > +
>
> So this is assuming the header frame and all of the frags are using
> the same size. Rather than reading the frags out and then writing them
> back, why not just directly rewrite the nr_frags, add the total size
> to skb->len and skb->data_len, and then update the truesize?

ack, thx. I will look into it.

Regards,
Lorenzo

>
> Actually, I think you might need to store the truesize somewhere in
> addition to the data_len that you were storing in the shared info.
>

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