[[RFC xdp-hints] 16/16] samples/bpf: Show XDP hints usage

[Ederson de Souza <ederson.desouza@intel.com>]

An example of how to retrieve XDP hints/metadata from an XDP frame. To
get the xdp_hints struct, one can use:

$ bpftool net xdp show
  xdp:
  enp6s0(2) md_btf_id(44) md_btf_enabled(0)

To get the BTF id, and then:

$ bpftool btf dump id 44 format c > btf.h

But, in this example, to demonstrate BTF and CORE features, a simpler
struct was defined, containing the only field used by the sample.

A lowpoint is that it's not currently possible to use some CORE features
from "samples/bpf" directory, as those samples are currently built
without using "clang -target bpf". This way, it was not possible to use
"bpf_core_field_exists" macro to check, in runtime, the presence of a
given XDP hints field.
---
 samples/bpf/xdp_sample_pkts_kern.c | 21 +++++++++++++++++++++
 samples/bpf/xdp_sample_pkts_user.c |  4 +++-
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/samples/bpf/xdp_sample_pkts_kern.c b/samples/bpf/xdp_sample_pkts_kern.c
index 9cf76b340dd7..9f0b0c5a6237 100644
--- a/samples/bpf/xdp_sample_pkts_kern.c
+++ b/samples/bpf/xdp_sample_pkts_kern.c
@@ -3,6 +3,7 @@
 #include <linux/version.h>
 #include <uapi/linux/bpf.h>
 #include <bpf/bpf_helpers.h>
+#include <bpf/bpf_core_read.h>
 
 #define SAMPLE_SIZE 64ul
 
@@ -12,16 +13,23 @@ struct {
 	__uint(value_size, sizeof(u32));
 } my_map SEC(".maps");
 
+struct xdp_hints {
+	u64 rx_timestamp;
+};
+
 SEC("xdp_sample")
 int xdp_sample_prog(struct xdp_md *ctx)
 {
+	void *meta_data = (void *)(long)ctx->data_meta;
 	void *data_end = (void *)(long)ctx->data_end;
 	void *data = (void *)(long)ctx->data;
+	struct xdp_hints *hints;
 
 	/* Metadata will be in the perf event before the packet data. */
 	struct S {
 		u16 cookie;
 		u16 pkt_len;
+		u64 rx_timestamp;
 	} __packed metadata;
 
 	if (data < data_end) {
@@ -41,9 +49,22 @@ int xdp_sample_prog(struct xdp_md *ctx)
 
 		metadata.cookie = 0xdead;
 		metadata.pkt_len = (u16)(data_end - data);
+		metadata.rx_timestamp = 0;
 		sample_size = min(metadata.pkt_len, SAMPLE_SIZE);
 		flags |= (u64)sample_size << 32;
 
+		if (meta_data < data) {
+			hints = meta_data;
+			/* bpf_core_field_exists doesn't work from samples/bpf,
+			 * as it is only available for "clang -target bpf", which
+			 * is not used on samples/bpf. A program that can use
+			 * the "vmlinux.h" and "clang -target btf" could use this
+			 * call to check the existence of a given field in runtime
+			 */
+			/*if (bpf_core_field_exists(hints->rx_timestamp))*/
+				metadata.rx_timestamp = BPF_CORE_READ(hints, rx_timestamp);
+		}
+
 		ret = bpf_perf_event_output(ctx, &my_map, flags,
 					    &metadata, sizeof(metadata));
 		if (ret)
diff --git a/samples/bpf/xdp_sample_pkts_user.c b/samples/bpf/xdp_sample_pkts_user.c
index 495e09897bd3..b87e0ae8eb3d 100644
--- a/samples/bpf/xdp_sample_pkts_user.c
+++ b/samples/bpf/xdp_sample_pkts_user.c
@@ -76,6 +76,7 @@ static void print_bpf_output(void *ctx, int cpu, void *data, __u32 size)
 	struct {
 		__u16 cookie;
 		__u16 pkt_len;
+		__u64 rx_timestamp;
 		__u8  pkt_data[SAMPLE_SIZE];
 	} __packed *e = data;
 	int i;
@@ -85,7 +86,8 @@ static void print_bpf_output(void *ctx, int cpu, void *data, __u32 size)
 		return;
 	}
 
-	printf("Pkt len: %-5d bytes. Ethernet hdr: ", e->pkt_len);
+	printf("Pkt len: %-5d bytes. RX timestamp: %llu Ethernet hdr: ", e->pkt_len,
+	       e->rx_timestamp);
 	for (i = 0; i < 14 && i < e->pkt_len; i++)
 		printf("%02x ", e->pkt_data[i]);
 	printf("\n");
-- 
2.32.0