From f720b84811b2133116e402090369d1be06de7ad4 Mon Sep 17 00:00:00 2001 From: Donald Hunter Date: Tue, 8 Nov 2022 10:22:15 +0000 Subject: 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 Signed-off-by: Donald Hunter Signed-off-by: Andrii Nakryiko Acked-by: Yonghong Song Link: https://lore.kernel.org/bpf/20221108102215.47297-1-donald.hunter@gmail.com --- Documentation/bpf/map_of_maps.rst | 126 ++++++++++++++++++++++++++++++++++++++ 1 file changed, 126 insertions(+) create mode 100644 Documentation/bpf/map_of_maps.rst (limited to 'Documentation/bpf') diff --git a/Documentation/bpf/map_of_maps.rst b/Documentation/bpf/map_of_maps.rst new file mode 100644 index 000000000000..07212b9227a9 --- /dev/null +++ b/Documentation/bpf/map_of_maps.rst @@ -0,0 +1,126 @@ +.. 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/ -- cgit v1.2.3