[PATCH bpf-next v5 1/1] bpf, docs: document BPF_MAP_TYPE_ARRAY

[Donald Hunter <donald.hunter@gmail.com>]

From: Dave Tucker <dave@dtucker.co.uk>

Add documentation for the BPF_MAP_TYPE_ARRAY including kernel version
introduced, usage and examples. Also documents BPF_MAP_TYPE_PERCPU_ARRAY
since this is similar.

Signed-off-by: Dave Tucker <dave@dtucker.co.uk>
Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
---
 Documentation/bpf/map_array.rst | 217 ++++++++++++++++++++++++++++++++
 1 file changed, 217 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..5eec4c99fda5
--- /dev/null
+++ b/Documentation/bpf/map_array.rst
@@ -0,0 +1,217 @@
+.. 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. 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 5.5, 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
+=====
+
+.. 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. Access from
+userspace uses the libbpf API of the same name.
+
+.. c:function::
+   long bpf_map_update_elem(struct bpf_map *map, const void *key, const void *value, u64 flags)
+
+Array elements can also be added using the ``bpf_map_update_elem()`` helper or
+libbpf API.
+
+``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 hash 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.
+
+Examples
+========
+
+Please see the ``tools/testing/selftests/bpf`` directory for functional
+examples. The sample code below demonstrates API usage.
+
+Kernel
+------
+
+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 shows how to access an array element.
+
+.. code-block:: c
+
+    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
+~~~~~~~~~~~~~~~~~~
+
+This example shows array creation, initialisation and lookup from userspace.
+
+.. 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;
+	    long value;
+	    __u32 index = 42;
+	    __u32 i;
+
+	    fd = bpf_map_create(BPF_MAP_TYPE_ARRAY, "example_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++) {
+		    value = i;
+		    ret = bpf_map_update_elem(fd, &i, &value, 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
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+This example shows per CPU array usage.
+
+.. 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_map_create(BPF_MAP_TYPE_PERCPU_ARRAY, "example_percpu",
+				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 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.
-- 
2.35.1