AF_XDP-interaction: Import AF_XDP example from XDP-tutorial

https://github.com/xdp-project/xdp-tutorial/blob/master/advanced03-AF_XDP/

Give Eelco Chaudron credit as code Author.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>

AF_XDP-interaction/af_xdp_kern.c

@@ -0,0 +1,43 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#include <linux/bpf.h>
+
+#include <bpf/bpf_helpers.h>
+
+struct bpf_map_def SEC("maps") xsks_map = {
+	.type = BPF_MAP_TYPE_XSKMAP,
+	.key_size = sizeof(int),
+	.value_size = sizeof(int),
+	.max_entries = 64,  /* Assume netdev has no more than 64 queues */
+};
+
+struct bpf_map_def SEC("maps") xdp_stats_map = {
+	.type        = BPF_MAP_TYPE_PERCPU_ARRAY,
+	.key_size    = sizeof(int),
+	.value_size  = sizeof(__u32),
+	.max_entries = 64,
+};
+
+SEC("xdp_sock")
+int xdp_sock_prog(struct xdp_md *ctx)
+{
+	int index = ctx->rx_queue_index;
+	__u32 *pkt_count;
+
+	pkt_count = bpf_map_lookup_elem(&xdp_stats_map, &index);
+	if (pkt_count) {
+
+		/* We pass every other packet */
+		if ((*pkt_count)++ & 1)
+			return XDP_PASS;
+	}
+
+	/* 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;
+}
+
+char _license[] SEC("license") = "GPL";