[PATCH bpf-next v1 14/14] docs, resolve_btfids: Document kfunc BTF annotation emission

[Ihor Solodrai <ihor.solodrai@linux.dev>]

resolve_btfids now emits the bpf_kfunc and bpf_fastcall BTF decl tags
and the arena address_space(1) type attribute for kfuncs, which were
previously produced by pahole.

Reflect this in the in-tree comments and documentation.

Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
---
 Documentation/bpf/kfuncs.rst    |  8 ++++++++
 scripts/Makefile.btf            |  3 +++
 tools/bpf/resolve_btfids/main.c | 11 +++++++++++
 3 files changed, 22 insertions(+)

diff --git a/Documentation/bpf/kfuncs.rst b/Documentation/bpf/kfuncs.rst
index 4c814ff6061e..d98e666f03a3 100644
--- a/Documentation/bpf/kfuncs.rst
+++ b/Documentation/bpf/kfuncs.rst
@@ -437,6 +437,14 @@ type. An example is shown below::
         }
         late_initcall(init_subsystem);
 
+At kernel build time the ``resolve_btfids`` tool discovers all kfuncs from the
+registered ``BTF_SET8_KFUNCS`` sets and emits their BTF annotations into the
+kernel's BTF; these annotations were historically produced by pahole. For each
+discovered kfunc ``resolve_btfids`` emits a ``bpf_kfunc`` BTF decl tag, a
+``bpf_fastcall`` decl tag when the kfunc is flagged ``KF_FASTCALL``, and the
+``address_space(1)`` type attribute on the return value and/or arguments flagged
+``KF_ARENA_RET``, ``KF_ARENA_ARG1`` or ``KF_ARENA_ARG2`` (see section 2.8).
+
 2.7  Specifying no-cast aliases with ___init
 --------------------------------------------
 
diff --git a/scripts/Makefile.btf b/scripts/Makefile.btf
index a1812985a61a..717e76ce96a7 100644
--- a/scripts/Makefile.btf
+++ b/scripts/Makefile.btf
@@ -14,6 +14,9 @@ pahole-flags-$(call test-ge, $(pahole-ver), 125)	+= --skip_encoding_btf_inconsis
 else
 
 # Switch to using --btf_features for v1.26 and later.
+#
+# kfunc BTF annotations (bpf_kfunc/bpf_fastcall decl tags and the arena
+# address_space(1) type attribute) are emitted by resolve_btfids, not pahole.
 pahole-flags-$(call test-ge, $(pahole-ver), 126)  = -j$(JOBS) --btf_features=encode_force,var,float,enum64,decl_tag,type_tag,optimized_func,consistent_func
 
 pahole-flags-$(call test-ge, $(pahole-ver), 131) += --btf_features=layout
diff --git a/tools/bpf/resolve_btfids/main.c b/tools/bpf/resolve_btfids/main.c
index aafbcfec755b..5a4683d11426 100644
--- a/tools/bpf/resolve_btfids/main.c
+++ b/tools/bpf/resolve_btfids/main.c
@@ -58,6 +58,17 @@
  *             __BTF_ID__func__vfs_fallocate__5:
  *             .zero 4
  *	       .word (1 << 3) | (1 << 1) | (1 << 2)
+ *
+ * In addition to resolving BTF IDs, resolve_btfids performs kernel-specific
+ * BTF-to-BTF transformations for kfuncs found in BTF_SET8_KFUNCS sets. For
+ * each such kfunc it:
+ *
+ *   - emits a "bpf_kfunc" decl tag, and "bpf_fastcall" when KF_FASTCALL is set;
+ *   - wraps the return value and/or arguments flagged KF_ARENA_RET,
+ *     KF_ARENA_ARG1 or KF_ARENA_ARG2 with the "address_space(1)" type attribute;
+ *   - rewrites the prototype of KF_IMPLICIT_ARGS kfuncs.
+ *
+ * These kfunc annotations were historically produced by pahole.
  */
 
 #define  _GNU_SOURCE
-- 
2.54.0