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

["Emil Tsalapatis" <emil@etsalapatis.com>]

On Mon Jun 1, 2026 at 6:18 PM EDT, Ihor Solodrai wrote:
> 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 <emil@etsalapatis.com>

Nit: Replace address_space(1) with arena here and in any other occurences.

>
> 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