On Mon, Jun 14, 2021 at 5:51 AM Lorenzo Bianconi [off-list ref] wrote:

quoted

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.