[PATCH bpf-next v2 6/6] docs, resolve_btfids: Document kfunc BTF annotation emission

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.

Reviewed-by: Emil Tsalapatis <[email protected]>
Signed-off-by: Ihor Solodrai <[email protected]>
---
 Documentation/bpf/kfuncs.rst      |  8 ++++++++
 Documentation/process/changes.rst |  7 +++----
 scripts/Makefile.btf              |  3 +++
 tools/bpf/resolve_btfids/main.c   | 11 +++++++++++
 4 files changed, 25 insertions(+), 4 deletions(-)

diff --git a/Documentation/bpf/kfuncs.rst b/Documentation/bpf/kfuncs.rst
index cbde86d082cc..c60fc574e8b0 100644
--- a/Documentation/bpf/kfuncs.rst
+++ b/Documentation/bpf/kfuncs.rst
@@ -472,6 +472,14 @@ 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/Documentation/process/changes.rst b/Documentation/process/changes.rst
index 1ca8c5f73ad0..6d1dbe4abf0f 100644
--- a/Documentation/process/changes.rst
+++ b/Documentation/process/changes.rst
@@ -147,10 +147,9 @@ Since Linux 5.2, if CONFIG_DEBUG_INFO_BTF is selected, the build system
 generates BTF (BPF Type Format) from DWARF in vmlinux, a bit later from kernel
 modules as well.  This requires pahole v1.22 or later.
 
-Since Linux 7.0, kfuncs annotated with KF_IMPLICIT_ARGS require pahole v1.26
-or later.  Without it, such kfuncs will have incorrect BTF prototypes in
-vmlinux, causing BPF programs to fail to load with a "func_proto incompatible
-with vmlinux" error.  Many sched_ext kfuncs are affected.
+Kfunc BTF annotations (the bpf_kfunc and bpf_fastcall decl tags and the arena
+address_space(1) type attribute) are emitted in-tree by resolve_btfids from the
+BTF_KFUNCS sets, so they no longer depend on a specific pahole version.
 
 It is found in the 'dwarves' or 'pahole' distro packages or from
 https://fedorapeople.org/~acme/dwarves/.
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 ea25ca34aa00..66f4014e6aa1 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
-- 
2.55.0