bpf: Add documentation for libbpf including API autogen

This patch is meant to start the initiative to document libbpf.
It includes .rst files which are text documentation describing building,
API naming convention, as well as an index to generated API documentation.

In this approach the generated API documentation is enabled by the kernels
existing kernel documentation system which uses sphinx. The resulting docs
would then be synced to kernel.org/doc

You can test this by running `make htmldocs` and serving the html in
Documentation/output. Since libbpf does not yet have comments in kernel
doc format, see kernel.org/doc/html/latest/doc-guide/kernel-doc.html for
an example so you can test this.

The advantage of this approach is to use the existing sphinx
infrastructure that the kernel has, and have libbpf docs in
the same place as everything else.

The current plan is to have the libbpf mirror sync the generated docs
and version them based on the libbpf releases which are cut on github.

This patch includes the addition of libbpf_api.rst which pulls comment
documentation from header files in libbpf under tools/lib/bpf/. The comment
docs would be of the standard kernel doc format.

Signed-off-by: Grant Seltzer <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

Documentation/bpf/index.rst

···
 The Cilium project also maintains a `BPF and XDP Reference Guide`_
 that goes into great technical depth about the BPF Architecture.
 
+libbpf
+======
+
+Libbpf is a userspace library for loading and interacting with bpf programs.
+
+.. toctree::
+   :maxdepth: 1
+
+   libbpf/libbpf
+   libbpf/libbpf_api
+   libbpf/libbpf_build
+   libbpf/libbpf_naming_convention
+
 BPF Type Format (BTF)
 =====================
 

Documentation/bpf/libbpf/libbpf.rst

···
+.. SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)
+
+libbpf
+======
+
+This is documentation for libbpf, a userspace library for loading and
+interacting with bpf programs.
+
+All general BPF questions, including kernel functionality, libbpf APIs and
+their application, should be sent to [e-mail] mailing list.
+You can `subscribe <http://vger.kernel.org/vger-lists.html#bpf>`_ to the
+mailing list search its `archive <https://lore.kernel.org/bpf/>`_.
+Please search the archive before asking new questions. It very well might
+be that this was already addressed or answered before.

Documentation/bpf/libbpf/libbpf_api.rst

···
+.. SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)
+
+API
+===
+
+This documentation is autogenerated from header files in libbpf, tools/lib/bpf
+
+.. kernel-doc:: tools/lib/bpf/libbpf.h
+   :internal:
+
+.. kernel-doc:: tools/lib/bpf/bpf.h
+   :internal:
+
+.. kernel-doc:: tools/lib/bpf/btf.h
+   :internal:
+
+.. kernel-doc:: tools/lib/bpf/xsk.h
+   :internal:
+
+.. kernel-doc:: tools/lib/bpf/bpf_tracing.h
+   :internal:
+
+.. kernel-doc:: tools/lib/bpf/bpf_core_read.h
+   :internal:
+
+.. kernel-doc:: tools/lib/bpf/bpf_endian.h
+   :internal:

Documentation/bpf/libbpf/libbpf_build.rst

···
+.. SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)
+
+Building libbpf
+===============
+
+libelf and zlib are internal dependencies of libbpf and thus are required to link
+against and must be installed on the system for applications to work.
+pkg-config is used by default to find libelf, and the program called
+can be overridden with PKG_CONFIG.
+
+If using pkg-config at build time is not desired, it can be disabled by
+setting NO_PKG_CONFIG=1 when calling make.
+
+To build both static libbpf.a and shared libbpf.so:
+
+.. code-block:: bash
+
+    $ cd src
+    $ make
+
+To build only static libbpf.a library in directory build/ and install them
+together with libbpf headers in a staging directory root/:
+
+.. code-block:: bash
+
+    $ cd src
+    $ mkdir build root
+    $ BUILD_STATIC_ONLY=y OBJDIR=build DESTDIR=root make install
+
+To build both static libbpf.a and shared libbpf.so against a custom libelf
+dependency installed in /build/root/ and install them together with libbpf
+headers in a build directory /build/root/:
+
+.. code-block:: bash
+
+    $ cd src
+    $ PKG_CONFIG_PATH=/build/root/lib64/pkgconfig DESTDIR=/build/root make

tools/lib/bpf/README.rst Documentation/bpf/libbpf/libbpf_naming_convention.rst

···
 .. SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)
 
-libbpf API naming convention
-============================
+API naming convention
+=====================
 
 libbpf API provides access to a few logically separated groups of
 functions and types. Every group has its own naming convention
···
 
 All types and functions provided by libbpf API should have one of the
 following prefixes: ``bpf_``, ``btf_``, ``libbpf_``, ``xsk_``,
-``perf_buffer_``.
+``btf_dump_``, ``ring_buffer_``, ``perf_buffer_``.
 
 System call wrappers
 --------------------
 
 System call wrappers are simple wrappers for commands supported by
 sys_bpf system call. These wrappers should go to ``bpf.h`` header file
-and map one-on-one to corresponding commands.
+and map one to one to corresponding commands.
 
 For example ``bpf_map_lookup_elem`` wraps ``BPF_MAP_LOOKUP_ELEM``
 command of sys_bpf, ``bpf_prog_attach`` wraps ``BPF_PROG_ATTACH``, etc.
···
 purpose of the function to open ELF file and create ``bpf_object`` from
 it.
 
-Another example: ``bpf_program__load`` is named for corresponding
-object, ``bpf_program``, that is separated from other part of the name
-by double underscore.
-
 All objects and corresponding functions other than BTF related should go
 to ``libbpf.h``. BTF types and functions should go to ``btf.h``.
 
···
 functions. These can be mixed and matched. Note that these functions
 are not reentrant for performance reasons.
 
-Please take a look at Documentation/networking/af_xdp.rst in the Linux
-kernel source tree on how to use XDP sockets and for some common
-mistakes in case you do not get any traffic up to user space.
-
-libbpf ABI
+ABI
 ==========
 
 libbpf can be both linked statically or used as DSO. To avoid possible
···
 
 For example, if current state of ``libbpf.map`` is:
 
-.. code-block::
+.. code-block:: c
+
         LIBBPF_0.0.1 {
         	global:
                         bpf_func_a;
···
 , and a new symbol ``bpf_func_c`` is being introduced, then
 ``libbpf.map`` should be changed like this:
 
-.. code-block::
+.. code-block:: c
+
         LIBBPF_0.0.1 {
         	global:
                         bpf_func_a;
···
 incompatible ones, described in details in [1].
 
 Stand-alone build
-=================
+-------------------
 
 Under https://github.com/libbpf/libbpf there is a (semi-)automated
 mirror of the mainline's version of libbpf for a stand-alone build.
···
 the mainline kernel tree.
 
 License
-=======
+-------------------
 
 libbpf is dual-licensed under LGPL 2.1 and BSD 2-Clause.
 
 Links
-=====
+-------------------
 
 [1] https://www.akkadia.org/drepper/dsohowto.pdf
     (Chapter 3. Maintaining APIs and ABIs).