xsk: sample kernel code is now in libbpf

In order to use AF_XDP sockets there are two parts needed. The

user-space application and the XDP program. For a complete setup and

usage example, please refer to the sample application. The user-space

side is xdpsock_user.c and the XDP side xdpsock_kern.c.

side is xdpsock_user.c and the XDP side is part of libbpf.

The XDP code sample included in tools/lib/bpf/xsk.c is the following::

   SEC("xdp_sock") int xdp_sock_prog(struct xdp_md *ctx)

   {

       int index = ctx->rx_queue_index;

       // A set entry here means that the correspnding queue_id

       // has an active AF_XDP socket bound to it.

       if (bpf_map_lookup_elem(&xsks_map, &index))

           return bpf_redirect_map(&xsks_map, index, 0);

       return XDP_PASS;

   }

Naive ring dequeue and enqueue could look like this::