Updating btf.rst doc with info about .BTF.ids section Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx> --- 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
