xdp: fix xdp_bench01 action XDP_TX, such packets reach the TX wire

Simply bouncing the packet via XDP_TX is getting blocked by the
NIC hardware, in this case the ConnectX-4 driver mlx5.

The MAC-addresses need to be swapped, before the NIC HW allows
these packets to reach the wire.

For now, allow keeping the "invalid" case to allow further testing.
But print a big WARNING that packets might not reach the wire.

The correct use is enabled via using option --readmem and action XDP_TX.

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

kernel/samples/bpf/xdp_bench01_mem_access_cost_kern.c

@@ -25,6 +25,22 @@ struct bpf_map_def SEC("maps") touch_memory = {
 	.max_entries = 1,
 };
 
+static void swap_src_dst_mac(void *data)
+{
+	unsigned short *p = data;
+	unsigned short dst[3];
+
+	dst[0] = p[0];
+	dst[1] = p[1];
+	dst[2] = p[2];
+	p[0] = p[3];
+	p[1] = p[4];
+	p[2] = p[5];
+	p[3] = dst[0];
+	p[4] = dst[1];
+	p[5] = dst[2];
+}
+
 SEC("xdp_bench01")
 int xdp_prog(struct xdp_md *ctx)
 {
@@ -57,6 +73,13 @@ int xdp_prog(struct xdp_md *ctx)
 		/* Avoid compile removing this: e.g Drop non 802.3 Ethertypes */
 		if (ntohs(eth_type) < ETH_P_802_3_MIN)
 			return XDP_DROP;
+
+		/* If touch_mem, also swap MACs for XDP_TX.  This is
+		 * needed for action XDP_TX, else HW will not TX packet
+		 * (this was observed with mlx5 driver).
+		 */
+		if (*action == XDP_TX)
+			swap_src_dst_mac(data);
 	}
 
 	value = bpf_map_lookup_elem(&rx_cnt, &key);

kernel/samples/bpf/xdp_bench01_mem_access_cost_user.c

@@ -366,6 +366,11 @@ int main(int argc, char **argv)
 	set_xdp_action(action);
 	set_touch_mem(touch_mem);
 
+	/* Some NIC drop packets on XDP_TX if MAC-addr isn't changed */
+	if ((action == XDP_TX) && !(touch_mem))
+		fprintf(stderr, "\n **WARNING** "
+		 "XDP_TX without --readmem, might not TX to wire\n\n");
+
 	/* Remove XDP program when program is interrupted */
 	signal(SIGINT, int_exit);