This commit adds documentation for the BPF_MAP_TYPE_ARRAY including kernel version introduced, usage and examples. It also documents BPF_MAP_TYPE_PERCPU_ARRAY since this is similar. Signed-off-by: Dave Tucker <dave@xxxxxxxxxxxxx> --- Documentation/bpf/map_array.rst | 183 ++++++++++++++++++++++++++++++++ 1 file changed, 183 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..eadc714591d2 --- /dev/null +++ b/Documentation/bpf/map_array.rst @@ -0,0 +1,183 @@ +.. SPDX-License-Identifier: GPL-2.0-only +.. Copyright (C) 2021 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 and + ``BPF_MAP_TYPE_PERCPU_ARRAY`` 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. 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 maximum +size of an array, defined in max_entries, is limited to 2^32. The value stored +can be of any size, however, small values will be rounded up to 8 bytes. + +Since Kernel 5.4, 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 +===== + +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. Access from +userspace uses the libbpf API of the same name. + +Array elements can also be added using the ``bpf_map_update_elem()`` helper or +libbpf API. + +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_eleme()`` to insert a +zero value to that index. + +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``. Since Kernel version 5.1, the BPF infrastructure +provides ``struct bpf_spin_lock`` to synchronize access. + +``bpf_map_get_next_key()`` can be used to iterate over array values. + +Examples +======== + +Please see the `tools/testing/selftests/bpf`_ directory for functional examples. +This sample code simply demonstrates the API. + +.. section links +.. _tools/testing/selftests/bpf: + https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/testing/selftests/bpf + +Kernel +------ + +.. code-block:: c + + struct { + __uint(type, BPF_MAP_TYPE_ARRAY); + __type(key, u32); + __type(value, long); + __uint(max_entries, 256); + } my_map SEC(".maps"); + + int bpf_prog(struct __sk_buff *skb) + { + int index = load_byte(skb, + ETH_HLEN + offsetof(struct iphdr, protocol)); + long *value; + + if (skb->pkt_type != PACKET_OUTGOING) + return 0; + + value = bpf_map_lookup_elem(&my_map, &index); + if (value) + __sync_fetch_and_add(value, skb->len); + + return 0; + } + +Userspace +--------- + +BPF_MAP_TYPE_ARRAY +~~~~~~~~~~~~~~~~~~ + +.. code-block:: c + + #include <assert.h> + #include <bpf/libbpf.h> + #include <bpf/bpf.h> + + int main(int argc, char **argv) + { + int fd; + int ret = 0; + __u32 i, j; + __u32 index = 42; + long v, value; + + fd = bpf_create_map(BPF_MAP_TYPE_ARRAY, sizeof(__u32), sizeof(long), + 256, 0); + if (fd < 0) + return fd; + + /* fill the map with values from 0-255 */ + for (i = 0; i < 256 ; i++) { + ret = bpf_map_update_elem(fd, &i, &v, BPF_ANY); + if (ret < 0) + return ret; + } + + ret = bpf_map_lookup_elem(fd, &index, &value); + if (ret < 0) + return ret; + + assert(value == 42); + + return ret; + } + +BPF_MAP_TYPE_PERCPU_ARRAY +~~~~~~~~~~~~~~~~~~~~~~~~~ + +.. code-block:: c + + #include <assert.h> + #include <bpf/libbpf.h> + #include <bpf/bpf.h> + + int main(int argc, char **argv) + { + int ncpus = libbpf_num_possible_cpus(); + if (ncpus < 0) + return ncpus; + + int fd; + int ret = 0; + __u32 i, j; + __u32 index = 42; + long v[ncpus], value[ncpus]; + + + fd = bpf_create_map(BPF_MAP_TYPE_PERCPU_ARRAY, sizeof(__u32), + sizeof(long), 256, 0); + if (fd < 0) + return -1; + + /* fill the map with values from 0-255 for each cpu */ + for (i = 0; i < 256 ; i++) { + for (j = 0; j < ncpus; j++) + v[j] = i; + ret = bpf_map_update_elem(fd, &i, &v, BPF_ANY); + if (ret < 0) + return ret; + } + + ret = bpf_map_lookup_elem(fd, &index, &value); + if (ret < 0) + return ret; + + for (j = 0; j < ncpus; j++) + assert(value[j] == 42); + + return ret; + } + +Semantics +========= + +As illustrated in the example above, when using a ``BPF_MAP_TYPE_PERCPU_ARRAY`` +in userspace, the values are an array with ``ncpus`` elements. + +When calling ``bpf_map_update_elem()`` the flags ``BPF_NOEXIST`` can not be used +for these maps. + -- 2.37.0
