Re: [RFC PATCH] virtio_net: XDP support for adjust_head

[Jason Wang]

On 2017年01月05日 02:58, John Fastabend wrote:

[...]

@@ -393,34 +397,39 @@ static u32 do_xdp_prog(struct virtnet_info *vi,
                  struct bpf_prog *xdp_prog,
                  void *data, int len)
   {
-    int hdr_padded_len;
       struct xdp_buff xdp;
-    void *buf;
       unsigned int qp;
       u32 act;
   +
       if (vi->mergeable_rx_bufs) {
-        hdr_padded_len = sizeof(struct virtio_net_hdr_mrg_rxbuf);
-        xdp.data = data + hdr_padded_len;
+        int desc_room = sizeof(struct virtio_net_hdr_mrg_rxbuf);
+
+        /* Allow consuming headroom but reserve enough space to push
+         * the descriptor on if we get an XDP_TX return code.
+         */
+        xdp.data_hard_start = data - vi->headroom + desc_room;
+        xdp.data = data + desc_room;
           xdp.data_end = xdp.data + (len - vi->hdr_len);
-        buf = data;
       } else { /* small buffers */
           struct sk_buff *skb = data;
   -        xdp.data = skb->data;
+        xdp.data_hard_start = skb->data;
+        xdp.data = skb->data + vi->headroom;
           xdp.data_end = xdp.data + len;
-        buf = skb->data;
       }
         act = bpf_prog_run_xdp(xdp_prog, &xdp);
       switch (act) {
       case XDP_PASS:
+        if (!vi->mergeable_rx_bufs)
+            __skb_pull((struct sk_buff *) data,
+                   xdp.data - xdp.data_hard_start);

Instead of doing things here and virtnet_xdp_xmit(). How about always making
skb->data point to the buffer head like:

1) reserve headroom in add_recvbuf_small()
2) skb_push(xdp->data - xdp_data_hard_start, skb) if we detect xdp->data was
modified afer bpf_prog_run_xdp()

Then there's no special code in either XDP_PASS or XDP_TX?

Alternatively moving the pull into the receive_small XDP handler also
removes the special case. I'll submit a patch shortly let me know what
you think.

I'm ok with this.

Thanks