[PATCH v4 bpf-next 09/14] bpf: Add info about .BTF.ids section to btf.rst

[Jiri Olsa <jolsa@kernel.org>]

Updating btf.rst doc with info about .BTF.ids section

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
---
 Documentation/bpf/btf.rst | 53 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 53 insertions(+)

diff --git a/Documentation/bpf/btf.rst b/Documentation/bpf/btf.rst
index 4d565d202ce3..60cfb3b06f34 100644
--- a/Documentation/bpf/btf.rst
+++ b/Documentation/bpf/btf.rst
@@ -691,6 +691,59 @@ kernel API, the ``insn_off`` is the instruction offset in the unit of ``struct
 bpf_insn``. For ELF API, the ``insn_off`` is the byte offset from the
 beginning of section (``btf_ext_info_sec->sec_name_off``).
 
+4.2 .BTF.ids section
+====================
+
+The .BTF.ids section encodes BTF ID values that are used within the kernel.
+
+This section is created during the kernel compilation with the help of
+macros defined in ``include/linux/btf_ids.h`` header file. Kernel code can
+use them to create lists and sets (sorted lists) of BTF ID values.
+
+The ``BTF_ID_LIST`` macro defines unsorted list of BTF ID values,
+with following syntax::
+
+  BTF_ID_LIST(list)
+  BTF_ID(type1, name1)
+  BTF_ID(type2, name2)
+
+resulting in following layout in .BTF.ids section::
+
+  __BTF_ID__type1__name1__1:
+  .zero 4
+  __BTF_ID__type2__name2__2:
+  .zero 4
+
+The ``int list[];`` variable is defined to access the list.
+
+The ``BTF_SET_START/END`` macros pair defines sorted list of BTF ID values
+and their count, with following syntax::
+
+  BTF_SET_START(set)
+  BTF_ID(type1, name1)
+  BTF_ID(type2, name2)
+  BTF_SET_END(set)
+
+resulting in following layout in .BTF.ids section::
+
+  __BTF_ID__set__set:
+  .zero 4
+  __BTF_ID__type1__name1__3:
+  .zero 4
+  __BTF_ID__type2__name2__4:
+  .zero 4
+
+The ``struct btf_id_set set;`` variable is defined to access the list.
+
+The ``typeX`` name can be one of following::
+
+   struct, union, typedef, func
+
+and is used as a filter whem resolving the BTF ID value.
+
+All the BTF ID lists and sets are compiled in the .BTF.ids section and
+resolved during the linking phase of kernel build by ``resolve_btfids`` tool.
+
 5. Using BTF
 ************
 
-- 
2.25.4