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

[Yonghong Song <yhs@meta.com>]

On 11/8/22 4:57 AM, Bagas Sanjaya wrote:
> 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

It will be automatically indexed in maps.rst:

...
Map Types
=========

.. toctree::
    :maxdepth: 1
    :glob:

    map_*

Usage Notes
===========
...

The visualized example:
   https://docs.kernel.org/bpf/maps.html

> 
> Thanks.
>