samples/bpf: xdp_rxq_info action XDP_TX must adjust MAC-addrs

XDP_TX requires also changing the MAC-addrs, else some hardware
may drop the TX packet before reaching the wire.  This was
observed with driver mlx5.

If xdp_rxq_info select --action XDP_TX the swapmac functionality
is activated.  It is also possible to manually enable via cmdline
option --swapmac.  This is practical if wanting to measure the
overhead of writing/updating payload for other action types.
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

samples/bpf/xdp_rxq_info_kern.c

@@ -21,6 +21,7 @@ struct config {
 enum cfg_options_flags {
 	NO_TOUCH = 0x0U,
 	READ_MEM = 0x1U,
+	SWAP_MAC = 0x2U,
 };
 struct bpf_map_def SEC("maps") config_map = {
 	.type		= BPF_MAP_TYPE_ARRAY,
@@ -52,6 +53,23 @@ struct bpf_map_def SEC("maps") rx_queue_index_map = {
 	.max_entries	= MAX_RXQs + 1,
 };
 
+static __always_inline
+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_prog0")
 int  xdp_prognum0(struct xdp_md *ctx)
 {
@@ -98,7 +116,7 @@ int  xdp_prognum0(struct xdp_md *ctx)
 		rxq_rec->issue++;
 
 	/* Default: Don't touch packet data, only count packets */
-	if (unlikely(config->options & READ_MEM)) {
+	if (unlikely(config->options & (READ_MEM|SWAP_MAC))) {
 		struct ethhdr *eth = data;
 
 		if (eth + 1 > data_end)
@@ -107,6 +125,12 @@ int  xdp_prognum0(struct xdp_md *ctx)
 		/* Avoid compiler removing this: Drop non 802.3 Ethertypes */
 		if (ntohs(eth->h_proto) < ETH_P_802_3_MIN)
 			return XDP_ABORTED;
+
+		/* XDP_TX requires changing MAC-addrs, else HW may drop.
+		 * Can also be enabled with --swapmac (for test purposes)
+		 */
+		if (unlikely(config->options & SWAP_MAC))
+			swap_src_dst_mac(data);
 	}
 
 	return config->action;

samples/bpf/xdp_rxq_info_user.c

@@ -51,6 +51,7 @@ static const struct option long_options[] = {
 	{"no-separators", no_argument,		NULL, 'z' },
 	{"action",	required_argument,	NULL, 'a' },
 	{"readmem", 	no_argument,		NULL, 'r' },
+	{"swapmac", 	no_argument,		NULL, 'm' },
 	{0, 0, NULL,  0 }
 };
 
@@ -72,6 +73,7 @@ struct config {
 enum cfg_options_flags {
 	NO_TOUCH = 0x0U,
 	READ_MEM = 0x1U,
+	SWAP_MAC = 0x2U,
 };
 #define XDP_ACTION_MAX (XDP_TX + 1)
 #define XDP_ACTION_MAX_STRLEN 11
@@ -119,6 +121,8 @@ static char* options2str(enum cfg_options_flags flag)
 {
 	if (flag == NO_TOUCH)
 		return "no_touch";
+	if (flag & SWAP_MAC)
+		return "swapmac";
 	if (flag & READ_MEM)
 		return "read";
 	fprintf(stderr, "ERR: Unknown config option flags");
@@ -517,6 +521,9 @@ int main(int argc, char **argv)
 		case 'r':
 			cfg_options |= READ_MEM;
 			break;
+		case 'm':
+			cfg_options |= SWAP_MAC;
+			break;
 		case 'h':
 		error:
 		default:
@@ -543,6 +550,10 @@ int main(int argc, char **argv)
 		}
 	}
 	cfg.action = action;
+
+	/* XDP_TX requires changing MAC-addrs, else HW may drop */
+	if (action == XDP_TX)
+		cfg_options |= SWAP_MAC;
 	cfg.options = cfg_options;
 
 	/* Trick to pretty printf with thousands separators use %' */