docs/bpf: Document BPF ARRAY_OF_MAPS and HASH_OF_MAPS

Add documentation for the ARRAY_OF_MAPS and HASH_OF_MAPS map types,
including usage and examples.
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20221108102215.47297-1-donald.hunter@gmail.com

Documentation/bpf/map_of_maps.rst

+.. SPDX-License-Identifier: GPL-2.0-only
+.. Copyright (C) 2022 Red Hat, Inc.
+
+========================================================
+BPF_MAP_TYPE_ARRAY_OF_MAPS and BPF_MAP_TYPE_HASH_OF_MAPS
+========================================================
+
+.. note::
+   - ``BPF_MAP_TYPE_ARRAY_OF_MAPS`` and ``BPF_MAP_TYPE_HASH_OF_MAPS`` were
+     introduced in kernel version 4.12
+
+``BPF_MAP_TYPE_ARRAY_OF_MAPS`` and ``BPF_MAP_TYPE_HASH_OF_MAPS`` provide general
+purpose support for map in map storage. One level of nesting is supported, where
+an outer map contains instances of a single type of inner map, for example
+``array_of_maps->sock_map``.
+
+When creating an outer map, an inner map instance is used to initialize the
+metadata that the outer map holds about its inner maps. This inner map has a
+separate lifetime from the outer map and can be deleted after the outer map has
+been created.
+
+The outer map supports element lookup, update and delete from user space using
+the syscall API. A BPF program is only allowed to do element lookup in the outer
+map.
+
+.. note::
+   - Multi-level nesting is not supported.
+   - Any BPF map type can be used as an inner map, except for
+     ``BPF_MAP_TYPE_PROG_ARRAY``.
+   - A BPF program cannot update or delete outer map entries.
+
+For ``BPF_MAP_TYPE_ARRAY_OF_MAPS`` the key is an unsigned 32-bit integer index
+into the array. The array is a fixed size with ``max_entries`` elements that are
+zero initialized when created.
+
+For ``BPF_MAP_TYPE_HASH_OF_MAPS`` the key type can be chosen when defining the
+map. The kernel is responsible for allocating and freeing key/value pairs, up to
+the max_entries limit that you specify. Hash maps use pre-allocation of hash
+table elements by default. The ``BPF_F_NO_PREALLOC`` flag can be used to disable
+pre-allocation when it is too memory expensive.
+
+Usage
+=====
+
+Kernel BPF Helper
+-----------------
+
+.. c:function::
+   void *bpf_map_lookup_elem(struct bpf_map *map, const void *key)
+
+Inner maps can be retrieved using the ``bpf_map_lookup_elem()`` helper. This
+helper returns a pointer to the inner map, or ``NULL`` if no entry was found.
+
+Examples
+========
+
+Kernel BPF Example
+------------------
+
+This snippet shows how to create and initialise an array of devmaps in a BPF
+program. Note that the outer array can only be modified from user space using
+the syscall API.
+
+.. code-block:: c
+
+    struct inner_map {
+            __uint(type, BPF_MAP_TYPE_DEVMAP);
+            __uint(max_entries, 10);
+            __type(key, __u32);
+            __type(value, __u32);
+    } inner_map1 SEC(".maps"), inner_map2 SEC(".maps");
+
+    struct {
+            __uint(type, BPF_MAP_TYPE_ARRAY_OF_MAPS);
+            __uint(max_entries, 2);
+            __type(key, __u32);
+            __array(values, struct inner_map);
+    } outer_map SEC(".maps") = {
+            .values = { &inner_map1,
+                        &inner_map2 }
+    };
+
+See ``progs/test_btf_map_in_map.c`` in ``tools/testing/selftests/bpf`` for more
+examples of declarative initialisation of outer maps.
+
+User Space
+----------
+
+This snippet shows how to create an array based outer map:
+
+.. code-block:: c
+
+    int create_outer_array(int inner_fd) {
+            LIBBPF_OPTS(bpf_map_create_opts, opts, .inner_map_fd = inner_fd);
+            int fd;
+
+            fd = bpf_map_create(BPF_MAP_TYPE_ARRAY_OF_MAPS,
+                                "example_array",       /* name */
+                                sizeof(__u32),         /* key size */
+                                sizeof(__u32),         /* value size */
+                                256,                   /* max entries */
+                                &opts);                /* create opts */
+            return fd;
+    }
+
+
+This snippet shows how to add an inner map to an outer map:
+
+.. code-block:: c
+
+    int add_devmap(int outer_fd, int index, const char *name) {
+            int fd;
+
+            fd = bpf_map_create(BPF_MAP_TYPE_DEVMAP, name,
+                                sizeof(__u32), sizeof(__u32), 256, NULL);
+            if (fd < 0)
+                    return fd;
+
+            return bpf_map_update_elem(outer_fd, &index, &fd, BPF_ANY);
+    }
+
+References
+==========
+
+- https://lore.kernel.org/netdev/20170322170035.923581-3-kafai@fb.com/
+- https://lore.kernel.org/netdev/20170322170035.923581-4-kafai@fb.com/