Re: [PATCH bpf-next v8 1/1] bpf, docs: document BPF_MAP_TYPE_ARRAY

Jesper D. Brouer
2022-10-25T13:57:55Z

On 21/10/2022 16.22, Donald Hunter wrote:
> From: Dave Tucker <dave@dtucker.co.uk>
>
> 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.
>
> Signed-off-by: Dave Tucker <dave@dtucker.co.uk>
> Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
> ---
> Documentation/bpf/map_array.rst | 243 ++++++++++++++++++++++++++++++++
> 1 file changed, 243 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..3acc5a294428
> --- /dev/null
> +++ b/Documentation/bpf/map_array.rst
> @@ -0,0 +1,243 @@
> +.. 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
> +=====

Can we make it more clear, that below refers to usage from BPF programs.
E.g. changing title "Usage" to something else, or create a sub-section.
Below we have subsections "Kernel BPF" and "Userspace", do set aside
kernel-side and userspace API users.

Sorry for bringing this up so late (v8), but I think it is important
that the documentation makes it easy for the reader to quickly grasp
which section is BPF-prog code and which is userspace libbpf APIs.
IMHO this should then be consistent across out docs.

> +
> +.. 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 >

When reading last sentence, the read will of-cause realize this was BPF
kernel-side code, as it reference userspace API (have same name).

--Jesper

> +
> +.. 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 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 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.