Re: [PATCH v3] docs/bpf: Document BPF map types QUEUE and STACK

[Bagas Sanjaya <bagasdotme@gmail.com>]

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On Tue, Nov 08, 2022 at 09:33:14AM +0000, Donald Hunter wrote:
> diff --git a/Documentation/bpf/map_queue_stack.rst b/Documentation/bpf/map_queue_stack.rst
> new file mode 100644
> index 000000000000..f20e31a647b9
> --- /dev/null
> +++ b/Documentation/bpf/map_queue_stack.rst
> @@ -0,0 +1,122 @@
> +.. SPDX-License-Identifier: GPL-2.0-only
> +.. Copyright (C) 2022 Red Hat, Inc.
> +
> +=========================================
> +BPF_MAP_TYPE_QUEUE and BPF_MAP_TYPE_STACK
> +=========================================
> +
> +.. note::
> +   - ``BPF_MAP_TYPE_QUEUE`` and ``BPF_MAP_TYPE_STACK`` were introduced
> +     in kernel version 4.20
> +
> +``BPF_MAP_TYPE_QUEUE`` provides FIFO storage and ``BPF_MAP_TYPE_STACK``
> +provides LIFO storage for BPF programs. These maps support peek, pop and
> +push operations that are exposed to BPF programs through the respective
> +helpers. These operations are exposed to userspace applications using
> +the existing ``bpf`` syscall in the following way:
> +
> +- ``BPF_MAP_LOOKUP_ELEM`` -> peek
> +- ``BPF_MAP_LOOKUP_AND_DELETE_ELEM`` -> pop
> +- ``BPF_MAP_UPDATE_ELEM`` -> push
> +
> +``BPF_MAP_TYPE_QUEUE`` and ``BPF_MAP_TYPE_STACK`` do not support
> +``BPF_F_NO_PREALLOC``.
> +
> +Usage
> +=====
> +
> +Kernel BPF
> +----------
> +
> +.. c:function::
> +   long bpf_map_push_elem(struct bpf_map *map, const void *value, u64 flags)
> +
> +An element ``value`` can be added to a queue or stack using the
> +``bpf_map_push_elem`` helper. The ``flags`` parameter must be set to
> +``BPF_ANY`` or ``BPF_EXIST``. If ``flags`` is set to ``BPF_EXIST`` then,
> +when the queue or stack is full, the oldest element will be removed to
> +make room for ``value`` to be added. Returns ``0`` on success, or
> +negative error in case of failure.
> +
> +.. c:function::
> +   long bpf_map_peek_elem(struct bpf_map *map, void *value)
> +
> +This helper fetches an element ``value`` from a queue or stack without
> +removing it. Returns ``0`` on success, or negative error in case of
> +failure.
> +
> +.. c:function::
> +   long bpf_map_pop_elem(struct bpf_map *map, void *value)
> +
> +This helper removes an element into ``value`` from a queue or
> +stack. Returns ``0`` on success, or negative error in case of failure.
> +
> +
> +Userspace
> +---------
> +
> +.. c:function::
> +   int bpf_map_update_elem (int fd, const void *key, const void *value, __u64 flags)
> +
> +A userspace program can push ``value`` onto a queue or stack using libbpf's
> +``bpf_map_update_elem`` function. The ``key`` parameter must be set to
> +``NULL`` and ``flags`` must be set to ``BPF_ANY`` or ``BPF_EXIST``, with the
> +same semantics as the ``bpf_map_push_elem`` kernel helper. Returns ``0`` on
> +success, or negative error in case of failure.
> +
> +.. c:function::
> +   int bpf_map_lookup_elem (int fd, const void *key, void *value)
> +
> +A userspace program can peek at the ``value`` at the head of a queue or stack
> +using the libbpf ``bpf_map_lookup_elem`` function. The ``key`` parameter must be
> +set to ``NULL``.  Returns ``0`` on success, or negative error in case of
> +failure.
> +
> +.. c:function::
> +   int bpf_map_lookup_and_delete_elem (int fd, const void *key, void *value)
> +
> +A userspace program can pop a ``value`` from the head of a queue or stack using
> +the libbpf ``bpf_map_lookup_and_delete_elem`` function. The ``key`` parameter
> +must be set to ``NULL``. Returns ``0`` on success, or negative error in case of
> +failure.
> +
> +Examples
> +========
> +
> +Kernel BPF
> +----------
> +
> +This snippet shows how to declare a queue in a BPF program:
> +
> +.. code-block:: c
> +
> +    struct {
> +            __uint(type, BPF_MAP_TYPE_QUEUE);
> +            __type(value, __u32);
> +            __uint(max_entries, 10);
> +    } queue SEC(".maps");
> +
> +
> +Userspace
> +---------
> +
> +This snippet shows how to use libbpf's low-level API to create a queue from
> +userspace:
> +
> +.. code-block:: c
> +
> +    int create_queue()
> +    {
> +            return bpf_map_create(BPF_MAP_TYPE_QUEUE,
> +                                  "sample_queue", /* name */
> +                                  0,              /* key size, must be zero */
> +                                  sizeof(__u32),  /* value size */
> +                                  10,             /* max entries */
> +                                  NULL);          /* create options */
> +    }
> +
> +
> +References
> +==========
> +
> +https://lwn.net/ml/netdev/153986858555.9127.14517764371945179514.stgit@kernel/

You forgot to add the documentation to BPF toctree:

---- >8 ----

diff --git a/Documentation/bpf/index.rst b/Documentation/bpf/index.rst
index 1b50de1983ee2c..113872fa0193d7 100644
--- a/Documentation/bpf/index.rst
+++ b/Documentation/bpf/index.rst
@@ -22,6 +22,7 @@ that goes into great technical depth about the BPF Architecture.
    kfuncs
    programs
    maps
+   map_queue_stack
    bpf_prog_run
    classic_vs_extended.rst
    bpf_licensing

Thanks.

-- 
An old man doll... just what I always wanted! - Clara

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