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Brouer" , Alexei Starovoitov , Daniel Borkmann , Jiri Olsa , Donald Hunter Subject: [PATCH bpf-next v9 1/1] bpf, docs: document BPF_MAP_TYPE_ARRAY Date: Tue, 25 Oct 2022 20:12:06 +0100 Message-Id: <20221025191206.95584-2-donald.hunter@gmail.com> X-Mailer: git-send-email 2.35.1 In-Reply-To: <20221025191206.95584-1-donald.hunter@gmail.com> References: <20221025191206.95584-1-donald.hunter@gmail.com> MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org X-Patchwork-Delegate: bpf@iogearbox.net From: Dave Tucker Add documentation for the BPF_MAP_TYPE_ARRAY including kernel version introduced, usage and examples. Also document BPF_MAP_TYPE_PERCPU_ARRAY which is similar. Signed-off-by: Dave Tucker Co-developed-by: Donald Hunter Signed-off-by: Donald Hunter --- Documentation/bpf/map_array.rst | 250 ++++++++++++++++++++++++++++++++ 1 file changed, 250 insertions(+) create mode 100644 Documentation/bpf/map_array.rst diff --git a/Documentation/bpf/map_array.rst b/Documentation/bpf/map_array.rst new file mode 100644 index 000000000000..97bb80333254 --- /dev/null +++ b/Documentation/bpf/map_array.rst @@ -0,0 +1,250 @@ +.. SPDX-License-Identifier: GPL-2.0-only +.. Copyright (C) 2022 Red Hat, Inc. + +================================================ +BPF_MAP_TYPE_ARRAY and BPF_MAP_TYPE_PERCPU_ARRAY +================================================ + +.. note:: + - ``BPF_MAP_TYPE_ARRAY`` was introduced in kernel version 3.19 + - ``BPF_MAP_TYPE_PERCPU_ARRAY`` was introduced in version 4.6 + +``BPF_MAP_TYPE_ARRAY`` and ``BPF_MAP_TYPE_PERCPU_ARRAY`` provide generic array +storage. The key type is an unsigned 32-bit integer (4 bytes) and the map is +of constant size. The size of the array is defined in ``max_entries`` at +creation time. All array elements are pre-allocated and zero initialized when +created. ``BPF_MAP_TYPE_PERCPU_ARRAY`` uses a different memory region for each +CPU whereas ``BPF_MAP_TYPE_ARRAY`` uses the same memory region. The value +stored can be of any size, however, all array elements are aligned to 8 +bytes. + +Since kernel 5.5, memory mapping may be enabled for ``BPF_MAP_TYPE_ARRAY`` by +setting the flag ``BPF_F_MMAPABLE``. The map definition is page-aligned and +starts on the first page. Sufficient page-sized and page-aligned blocks of +memory are allocated to store all array values, starting on the second page, +which in some cases will result in over-allocation of memory. The benefit of +using this is increased performance and ease of use since userspace programs +would not be required to use helper functions to access and mutate data. + +Usage +===== + +Kernel BPF +---------- + +.. c:function:: + void *bpf_map_lookup_elem(struct bpf_map *map, const void *key) + +Array elements can be retrieved using the ``bpf_map_lookup_elem()`` helper. +This helper returns a pointer into the array element, so to avoid data races +with userspace reading the value, the user must use primitives like +``__sync_fetch_and_add()`` when updating the value in-place. + +.. c:function:: + long bpf_map_update_elem(struct bpf_map *map, const void *key, const void *value, u64 flags) + +Array elements can be updated using the ``bpf_map_update_elem()`` helper. + +``bpf_map_update_elem()`` returns 0 on success, or negative error in case of +failure. + +Since the array is of constant size, ``bpf_map_delete_elem()`` is not supported. +To clear an array element, you may use ``bpf_map_update_elem()`` to insert a +zero value to that index. + +Per CPU Array +~~~~~~~~~~~~~ + +Values stored in ``BPF_MAP_TYPE_ARRAY`` can be accessed by multiple programs +across different CPUs. To restrict storage to a single CPU, you may use a +``BPF_MAP_TYPE_PERCPU_ARRAY``. + +When using a ``BPF_MAP_TYPE_PERCPU_ARRAY`` the ``bpf_map_update_elem()`` and +``bpf_map_lookup_elem()`` helpers automatically access the slot for the current +CPU. + +.. c:function:: + void *bpf_map_lookup_percpu_elem(struct bpf_map *map, const void *key, u32 cpu) + +The ``bpf_map_lookup_percpu_elem()`` helper can be used to lookup the array +value for a specific CPU. Returns value on success , or ``NULL`` if no entry was +found or ``cpu`` is invalid. + +Concurrency +----------- + +Since kernel version 5.1, the BPF infrastructure provides ``struct bpf_spin_lock`` +to synchronize access. + +Userspace +--------- + +Access from userspace uses libbpf APIs with the same names as above, with +the map identified by its ``fd``. + +Examples +======== + +Please see the ``tools/testing/selftests/bpf`` directory for functional +examples. The code samples below demonstrate API usage. + +Kernel BPF +---------- + +This snippet shows how to declare an array in a BPF program. + +.. code-block:: c + + struct { + __uint(type, BPF_MAP_TYPE_ARRAY); + __type(key, u32); + __type(value, long); + __uint(max_entries, 256); + } my_map SEC(".maps"); + + +This example BPF program shows how to access an array element. + +.. code-block:: c + + int bpf_prog(struct __sk_buff *skb) + { + struct iphdr ip; + int index; + long *value; + + if (bpf_skb_load_bytes(skb, ETH_HLEN, &ip, sizeof(ip)) < 0) + return 0; + + index = ip.protocol; + value = bpf_map_lookup_elem(&my_map, &index); + if (value) + __sync_fetch_and_add(value, skb->len); + + return 0; + } + +Userspace +--------- + +BPF_MAP_TYPE_ARRAY +~~~~~~~~~~~~~~~~~~ + +This snippet shows how to create an array, using ``bpf_map_create_opts`` to +set flags. + +.. code-block:: c + + #include + #include + + int create_array() + { + int fd; + LIBBPF_OPTS(bpf_map_create_opts, opts, .map_flags = BPF_F_MMAPABLE); + + fd = bpf_map_create(BPF_MAP_TYPE_ARRAY, + "example_array", /* name */ + sizeof(__u32), /* key size */ + sizeof(long), /* value size */ + 256, /* max entries */ + &opts); /* create opts */ + return fd; + } + +This snippet shows how to initialize the elements of an array. + +.. code-block:: c + + int initialize_array(int fd) + { + __u32 i; + long value; + int ret; + + for (i = 0; i < 256; i++) { + value = i; + ret = bpf_map_update_elem(fd, &i, &value, BPF_ANY); + if (ret < 0) + return ret; + } + + return ret; + } + +This snippet shows how to retrieve an element value from an array. + +.. code-block:: c + + int lookup(int fd) + { + __u32 index = 42; + long value; + int ret; + + ret = bpf_map_lookup_elem(fd, &index, &value); + if (ret < 0) + return ret; + + /* use value here */ + assert(value == 42); + + return ret; + } + +BPF_MAP_TYPE_PERCPU_ARRAY +~~~~~~~~~~~~~~~~~~~~~~~~~ + +This snippet shows how to initialize the elements of a per CPU array. + +.. code-block:: c + + int initialize_array(int fd) + { + int ncpus = libbpf_num_possible_cpus(); + long values[ncpus]; + __u32 i, j; + int ret; + + for (i = 0; i < 256 ; i++) { + for (j = 0; j < ncpus; j++) + values[j] = i; + ret = bpf_map_update_elem(fd, &i, &values, BPF_ANY); + if (ret < 0) + return ret; + } + + return ret; + } + +This snippet shows how to access the per CPU elements of an array value. + +.. code-block:: c + + int lookup(int fd) + { + int ncpus = libbpf_num_possible_cpus(); + __u32 index = 42, j; + long values[ncpus]; + int ret; + + ret = bpf_map_lookup_elem(fd, &index, &values); + if (ret < 0) + return ret; + + for (j = 0; j < ncpus; j++) { + /* Use per CPU value here */ + assert(values[j] == 42); + } + + return ret; + } + +Semantics +========= + +As shown in the example above, when accessing a ``BPF_MAP_TYPE_PERCPU_ARRAY`` +in userspace, each value is an array with ``ncpus`` elements. + +When calling ``bpf_map_update_elem()`` the flag ``BPF_NOEXIST`` can not be used +for these maps.