[PATCH v2 bpf-next 2/2] bpf, docs: document BPF_MAP_TYPE_ARRAY

[Dave Tucker <dave@dtucker.co.uk>]

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@dtucker.co.uk>
---
 Documentation/bpf/map_array.rst | 172 ++++++++++++++++++++++++++++++++
 1 file changed, 172 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
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+.. 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. 
\ No newline at end of file
-- 
2.33.1