1 .. SPDX-License-Identifier: GPL-2.0-only
2 .. Copyright (C) 2022 Red Hat, Inc.
3
4 ========================================================
5 BPF_MAP_TYPE_ARRAY_OF_MAPS and BPF_MAP_TYPE_HASH_OF_MAPS
6 ========================================================
7
8 .. note::
9 - ``BPF_MAP_TYPE_ARRAY_OF_MAPS`` and ``BPF_MAP_TYPE_HASH_OF_MAPS`` were
10 introduced in kernel version 4.12
11
12 ``BPF_MAP_TYPE_ARRAY_OF_MAPS`` and ``BPF_MAP_TYPE_HASH_OF_MAPS`` provide general
13 purpose support for map in map storage. One level of nesting is supported, where
14 an outer map contains instances of a single type of inner map, for example
15 ``array_of_maps->sock_map``.
16
17 When creating an outer map, an inner map instance is used to initialize the
18 metadata that the outer map holds about its inner maps. This inner map has a
19 separate lifetime from the outer map and can be deleted after the outer map has
20 been created.
21
22 The outer map supports element lookup, update and delete from user space using
23 the syscall API. A BPF program is only allowed to do element lookup in the outer
24 map.
25
26 .. note::
27 - Multi-level nesting is not supported.
28 - Any BPF map type can be used as an inner map, except for
29 ``BPF_MAP_TYPE_PROG_ARRAY``.
30 - A BPF program cannot update or delete outer map entries.
31
32 For ``BPF_MAP_TYPE_ARRAY_OF_MAPS`` the key is an unsigned 32-bit integer index
33 into the array. The array is a fixed size with ``max_entries`` elements that are
34 zero initialized when created.
35
36 For ``BPF_MAP_TYPE_HASH_OF_MAPS`` the key type can be chosen when defining the
37 map. The kernel is responsible for allocating and freeing key/value pairs, up to
38 the max_entries limit that you specify. Hash maps use pre-allocation of hash
39 table elements by default. The ``BPF_F_NO_PREALLOC`` flag can be used to disable
40 pre-allocation when it is too memory expensive.
41
42 Usage
43 =====
44
45 Kernel BPF Helper
46 -----------------
47
48 bpf_map_lookup_elem()
49 ~~~~~~~~~~~~~~~~~~~~~
50
51 .. code-block:: c
52
53 void *bpf_map_lookup_elem(struct bpf_map *map, const void *key)
54
55 Inner maps can be retrieved using the ``bpf_map_lookup_elem()`` helper. This
56 helper returns a pointer to the inner map, or ``NULL`` if no entry was found.
57
58 Examples
59 ========
60
61 Kernel BPF Example
62 ------------------
63
64 This snippet shows how to create and initialise an array of devmaps in a BPF
65 program. Note that the outer array can only be modified from user space using
66 the syscall API.
67
68 .. code-block:: c
69
70 struct inner_map {
71 __uint(type, BPF_MAP_TYPE_DEVMAP);
72 __uint(max_entries, 10);
73 __type(key, __u32);
74 __type(value, __u32);
75 } inner_map1 SEC(".maps"), inner_map2 SEC(".maps");
76
77 struct {
78 __uint(type, BPF_MAP_TYPE_ARRAY_OF_MAPS);
79 __uint(max_entries, 2);
80 __type(key, __u32);
81 __array(values, struct inner_map);
82 } outer_map SEC(".maps") = {
83 .values = { &inner_map1,
84 &inner_map2 }
85 };
86
87 See ``progs/test_btf_map_in_map.c`` in ``tools/testing/selftests/bpf`` for more
88 examples of declarative initialisation of outer maps.
89
90 User Space
91 ----------
92
93 This snippet shows how to create an array based outer map:
94
95 .. code-block:: c
96
97 int create_outer_array(int inner_fd) {
98 LIBBPF_OPTS(bpf_map_create_opts, opts, .inner_map_fd = inner_fd);
99 int fd;
100
101 fd = bpf_map_create(BPF_MAP_TYPE_ARRAY_OF_MAPS,
102 "example_array", /* name */
103 sizeof(__u32), /* key size */
104 sizeof(__u32), /* value size */
105 256, /* max entries */
106 &opts); /* create opts */
107 return fd;
108 }
109
110
111 This snippet shows how to add an inner map to an outer map:
112
113 .. code-block:: c
114
115 int add_devmap(int outer_fd, int index, const char *name) {
116 int fd;
117
118 fd = bpf_map_create(BPF_MAP_TYPE_DEVMAP, name,
119 sizeof(__u32), sizeof(__u32), 256, NULL);
120 if (fd < 0)
121 return fd;
122
123 return bpf_map_update_elem(outer_fd, &index, &fd, BPF_ANY);
124 }
125
126 References
127 ==========
128
129 - https://lore.kernel.org/netdev/20170322170035.923581-3-kafai@fb.com/
130 - https://lore.kernel.org/netdev/20170322170035.923581-4-kafai@fb.com/
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