bpf, docs: Reformat BPF maps page to be more readable

Add a more complete introduction, with links to man pages.
Move toctree of map types above usage notes.
Format usage notes to improve readability.
Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20221012152715.25073-1-donald.hunter@gmail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

Documentation/bpf/maps.rst

 
-=========
-eBPF maps
+========
+BPF maps
+========
+
+BPF 'maps' provide generic storage of different types for sharing data between
+kernel and user space. There are several storage types available, including
+hash, array, bloom filter and radix-tree. Several of the map types exist to
+support specific BPF helpers that perform actions based on the map contents. The
+maps are accessed from BPF programs via BPF helpers which are documented in the
+`man-pages`_ for `bpf-helpers(7)`_.
+
+BPF maps are accessed from user space via the ``bpf`` syscall, which provides
+commands to create maps, lookup elements, update elements and delete
+elements. More details of the BPF syscall are available in
+:doc:`/userspace-api/ebpf/syscall` and in the `man-pages`_ for `bpf(2)`_.
+
+Map Types
 =========
 
-'maps' is a generic storage of different types for sharing data between kernel
-and userspace.
+.. toctree::
+   :maxdepth: 1
+   :glob:
 
-The maps are accessed from user space via BPF syscall, which has commands:
+   map_*
 
-- create a map with given type and attributes
-  ``map_fd = bpf(BPF_MAP_CREATE, union bpf_attr *attr, u32 size)``
-  using attr->map_type, attr->key_size, attr->value_size, attr->max_entries
-  returns process-local file descriptor or negative error
+Usage Notes
+===========
 
-- lookup key in a given map
-  ``err = bpf(BPF_MAP_LOOKUP_ELEM, union bpf_attr *attr, u32 size)``
-  using attr->map_fd, attr->key, attr->value
-  returns zero and stores found elem into value or negative error
+.. c:function::
+   int bpf(int command, union bpf_attr *attr, u32 size)
 
-- create or update key/value pair in a given map
-  ``err = bpf(BPF_MAP_UPDATE_ELEM, union bpf_attr *attr, u32 size)``
-  using attr->map_fd, attr->key, attr->value
-  returns zero or negative error
+Use the ``bpf()`` system call to perform the operation specified by
+``command``. The operation takes parameters provided in ``attr``. The ``size``
+argument is the size of the ``union bpf_attr`` in ``attr``.
 
-- find and delete element by key in a given map
-  ``err = bpf(BPF_MAP_DELETE_ELEM, union bpf_attr *attr, u32 size)``
-  using attr->map_fd, attr->key
+**BPF_MAP_CREATE**
 
-- to delete map: close(fd)
-  Exiting process will delete maps automatically
+Create a map with the desired type and attributes in ``attr``:
 
-userspace programs use this syscall to create/access maps that eBPF programs
-are concurrently updating.
+.. code-block:: c
 
-maps can have different types: hash, array, bloom filter, radix-tree, etc.
+    int fd;
+    union bpf_attr attr = {
+            .map_type = BPF_MAP_TYPE_ARRAY;  /* mandatory */
+            .key_size = sizeof(__u32);       /* mandatory */
+            .value_size = sizeof(__u32);     /* mandatory */
+            .max_entries = 256;              /* mandatory */
+            .map_flags = BPF_F_MMAPABLE;
+            .map_name = "example_array";
+    };
 
-The map is defined by:
+    fd = bpf(BPF_MAP_CREATE, &attr, sizeof(attr));
 
-  - type
-  - max number of elements
-  - key size in bytes
-  - value size in bytes
+Returns a process-local file descriptor on success, or negative error in case of
+failure. The map can be deleted by calling ``close(fd)``. Maps held by open
+file descriptors will be deleted automatically when a process exits.
 
-Map Types
-=========
+.. note:: Valid characters for ``map_name`` are ``A-Z``, ``a-z``, ``0-9``,
+   ``'_'`` and ``'.'``.
 
-.. toctree::
-   :maxdepth: 1
-   :glob:
+**BPF_MAP_LOOKUP_ELEM**
 
-   map_*
\ No newline at end of file
+Lookup key in a given map using ``attr->map_fd``, ``attr->key``,
+``attr->value``. Returns zero and stores found elem into ``attr->value`` on
+success, or negative error on failure.
+
+**BPF_MAP_UPDATE_ELEM**
+
+Create or update key/value pair in a given map using ``attr->map_fd``, ``attr->key``,
+``attr->value``. Returns zero on success or negative error on failure.
+
+**BPF_MAP_DELETE_ELEM**
+
+Find and delete element by key in a given map using ``attr->map_fd``,
+``attr->key``. Returns zero on success or negative error on failure.
+
+.. Links:
+.. _man-pages: https://www.kernel.org/doc/man-pages/
+.. _bpf(2): https://man7.org/linux/man-pages/man2/bpf.2.html
+.. _bpf-helpers(7): https://man7.org/linux/man-pages/man7/bpf-helpers.7.html