experiment01-tailgrow: add load/store helpers from Cilium

Copy some code from cilium project:
 https://github.com/cilium/cilium/blob/master/bpf/include/bpf/ctx/xdp.h

We are interesting in using xdp_load_bytes() and xdp_store_bytes().

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

Author: netoptimizer

experiment01-tailgrow/xdp_data_access_helpers.h

@@ -0,0 +1,84 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/* Copyright (C) 2020 Authors of Cilium
+ *
+ * Code copied from cilium/bpf/include/bpf/ctx/xdp.h
+ *  https://github.com/cilium/cilium/blob/master/bpf/include/bpf/ctx/xdp.h
+ */
+
+#ifndef _XDP_DATA_ACCESS_HELPERS_H_
+#define _XDP_DATA_ACCESS_HELPERS_H_
+
+/* This must be a mask and all offsets guaranteed to be less than that. */
+//#define __CTX_OFF_MAX			0xff
+#define __CTX_OFF_MAX			0x1fff
+
+#ifndef __maybe_unused
+# define __maybe_unused		__attribute__((__unused__))
+#endif
+
+#ifndef EINVAL
+# define EINVAL	22
+#endif
+
+#ifndef memcpy
+# define memcpy(dest, src, n)   __builtin_memcpy((dest), (src), (n))
+#endif
+
+static __always_inline __maybe_unused int
+xdp_load_bytes(struct xdp_md *ctx, __u64 off, void *to, const __u64 len)
+{
+	void *from;
+	int ret;
+	/* LLVM tends to generate code that verifier doesn't understand,
+	 * so force it the way we want it in order to open up a range
+	 * on the reg.
+	 */
+	asm volatile("r1 = *(u32 *)(%[ctx] +0)\n\t"
+		     "r2 = *(u32 *)(%[ctx] +4)\n\t"
+		     "%[off] &= %[offmax]\n\t"
+		     "r1 += %[off]\n\t"
+		     "%[from] = r1\n\t"
+		     "r1 += %[len]\n\t"
+		     "if r1 > r2 goto +2\n\t"
+		     "%[ret] = 0\n\t"
+		     "goto +1\n\t"
+		     "%[ret] = %[errno]\n\t"
+		     : [ret]"=r"(ret), [from]"=r"(from)
+		     : [ctx]"r"(ctx), [off]"r"(off), [len]"ri"(len),
+		       [offmax]"i"(__CTX_OFF_MAX), [errno]"i"(-EINVAL)
+		     : "r1", "r2");
+	if (!ret)
+		memcpy(to, from, len);
+	return ret;
+}
+
+static __always_inline __maybe_unused int
+xdp_store_bytes(struct xdp_md *ctx, __u64 off, const void *from,
+		const __u64 len, __u64 flags __maybe_unused)
+{
+	void *to;
+	int ret;
+	/* See xdp_load_bytes(). */
+	asm volatile("r1 = *(u32 *)(%[ctx] +0)\n\t"
+		     "r2 = *(u32 *)(%[ctx] +4)\n\t"
+		     "%[off] &= %[offmax]\n\t"
+		     "r1 += %[off]\n\t"
+		     "%[to] = r1\n\t"
+		     "r1 += %[len]\n\t"
+		     "if r1 > r2 goto +2\n\t"
+		     "%[ret] = 0\n\t"
+		     "goto +1\n\t"
+		     "%[ret] = %[errno]\n\t"
+		     : [ret]"=r"(ret), [to]"=r"(to)
+		     : [ctx]"r"(ctx), [off]"r"(off), [len]"ri"(len),
+		       [offmax]"i"(__CTX_OFF_MAX), [errno]"i"(-EINVAL)
+		     : "r1", "r2");
+	if (!ret)
+		memcpy(to, from, len);
+	return ret;
+}
+
+#define ctx_load_bytes			xdp_load_bytes
+#define ctx_store_bytes			xdp_store_bytes
+
+#endif /* _XDP_DATA_ACCESS_HELPERS_H_ */