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 | 150 ++++++++++++++++++++++++++++++++ 1 file changed, 150 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..f9eb5473a240 --- /dev/null +++ b/Documentation/bpf/map_array.rst @@ -0,0 +1,150 @@ +.. 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. + +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 `bpf/samples`_ directory for functional examples. +This sample code simply demonstrates the API. + +.. section links +.. _bpf/samples: + https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/samples/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 = bpf_create_map(BPF_MAP_TYPE_ARRAY, sizeof(__u32), sizeof(long), 256, 0); + if (fd < 0) + return -1; + + // fill the map with values from 0-255 + for(__u32 i=0; i < 256 ; i++) { + long v = i; + bpf_map_update_elem(fd, &i, &v, BPF_ANY); + } + + __u32 index = 42; + long value; + bpf_map_lookup_elem(fd, &index, &value); + assert(value == 42); + return 0; + } + + +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(); + + int 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(__u32 i=0; i < 256 ; i++) { + long v[ncpus]; + for (int j=0; j < ncpus; j++ ) { + v[j] = i; + } + bpf_map_update_elem(fd, &i, &v, BPF_ANY); + } + + sleep(60); + + __u32 index = 42; + long value[ncpus]; + bpf_map_lookup_elem(fd, &index, &value); + for (int j=0; j < ncpus; j++ ) { + assert(value[j] == 42); + } + return 0; + } + +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. \ No newline at end of file -- 2.33.1
