[PATCH bpf-next RFC 3/3] samples/bpf: change xdp_fwd to use new BTF config interface

[Jesper Dangaard Brouer <brouer@xxxxxxxxxx>]

This enable BTF for samples/bpf/xdp_fwd program, and demonstrates
how the BPF-developer can defined their own version of bpf_devmap_val.

Signed-off-by: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>
---
 samples/bpf/xdp_fwd.h      |   24 ++++++++++++++++++++++++
 samples/bpf/xdp_fwd_kern.c |    5 +++--
 samples/bpf/xdp_fwd_user.c |    9 ++++++++-
 3 files changed, 35 insertions(+), 3 deletions(-)
 create mode 100644 samples/bpf/xdp_fwd.h

diff --git a/samples/bpf/xdp_fwd.h b/samples/bpf/xdp_fwd.h
new file mode 100644
index 000000000000..8abb2a417117
--- /dev/null
+++ b/samples/bpf/xdp_fwd.h
@@ -0,0 +1,24 @@
+// SPDX-License-Identifier: GPL-2.0
+#ifndef _SAMPLES_BPF_XDP_FWD_H
+#define _SAMPLES_BPF_XDP_FWD_H
+
+#define ENABLE_BPF_PROG 1
+
+/* Notice XDP prog can redefine this struct, which through BTF affect
+ * what kernel-side config options are available.
+ */
+struct bpf_devmap_val {
+	__u32 ifindex;   /* device index - mandatory */
+#ifdef ENABLE_BPF_PROG
+	union {
+		int   fd;  /* prog fd on map write */
+		__u32 id;  /* prog id on map read */
+	} bpf_prog;
+#endif
+	struct {
+		unsigned char data2[2];
+		__u16 vlan_hdr;
+	} storage;
+};
+
+#endif /* _SAMPLES_BPF_XDP_FWD_H */
diff --git a/samples/bpf/xdp_fwd_kern.c b/samples/bpf/xdp_fwd_kern.c
index 54c099cbd639..bb1ee44dcd60 100644
--- a/samples/bpf/xdp_fwd_kern.c
+++ b/samples/bpf/xdp_fwd_kern.c
@@ -20,13 +20,14 @@
 #include <linux/ipv6.h>
 
 #include <bpf/bpf_helpers.h>
+#include "xdp_fwd.h"
 
 #define IPV6_FLOWINFO_MASK              cpu_to_be32(0x0FFFFFFF)
 
 struct {
 	__uint(type, BPF_MAP_TYPE_DEVMAP);
-	__uint(key_size, sizeof(int));
-	__uint(value_size, sizeof(int));
+	__type(key, u32);
+	__type(value, struct bpf_devmap_val);
 	__uint(max_entries, 64);
 } xdp_tx_ports SEC(".maps");
 
diff --git a/samples/bpf/xdp_fwd_user.c b/samples/bpf/xdp_fwd_user.c
index 74a4583d0d86..4ddf70bcedde 100644
--- a/samples/bpf/xdp_fwd_user.c
+++ b/samples/bpf/xdp_fwd_user.c
@@ -26,13 +26,20 @@
 
 #include <bpf/libbpf.h>
 #include <bpf/bpf.h>
+#include "xdp_fwd.h"
 
 static __u32 xdp_flags = XDP_FLAGS_UPDATE_IF_NOEXIST;
 
 static int do_attach(int idx, int prog_fd, int map_fd, const char *name)
 {
+	struct bpf_devmap_val val = { 0 };
 	int err;
 
+	val.ifindex = idx;
+#ifdef ENABLE_BPF_PROG
+	val.bpf_prog.fd = - 1;
+#endif
+
 	err = bpf_set_link_xdp_fd(idx, prog_fd, xdp_flags);
 	if (err < 0) {
 		printf("ERROR: failed to attach program to %s\n", name);
@@ -40,7 +47,7 @@ static int do_attach(int idx, int prog_fd, int map_fd, const char *name)
 	}
 
 	/* Adding ifindex as a possible egress TX port */
-	err = bpf_map_update_elem(map_fd, &idx, &idx, 0);
+	err = bpf_map_update_elem(map_fd, &idx, &val, 0);
 	if (err)
 		printf("ERROR: failed using device %s as TX-port\n", name);