bpf, docs: Document BPF_MAP_TYPE_ARRAY

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.
Co-developed-by: NDonald Hunter <donald.hunter@gmail.com>
Signed-off-by: NDave Tucker <dave@dtucker.co.uk>
Signed-off-by: NDonald Hunter <donald.hunter@gmail.com>
Signed-off-by: NAndrii Nakryiko <andrii@kernel.org>
Reviewed-by: NMaryam Tahhan <mtahhan@redhat.com>
Reviewed-by: NBagas Sanjaya <bagasdotme@gmail.com>
Link: https://lore.kernel.org/bpf/20221109174604.31673-2-donald.hunter@gmail.com

Documentation/bpf/map_array.rst

+.. 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 <bpf/libbpf.h>
+    #include <bpf/bpf.h>
+
+    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.