On Fri, Nov 4, 2022 at 10:21 AM Donald Hunter <donald.hunter@xxxxxxxxx> wrote:
>
> Add documentation for BPF_MAP_TYPE_QUEUE and BPF_MAP_TYPE_STACK,
> including usage and examples.
>
> Signed-off-by: Donald Hunter <donald.hunter@xxxxxxxxx>
> ---
> Documentation/bpf/map_queue_stack.rst | 119 ++++++++++++++++++++++++++
> 1 file changed, 119 insertions(+)
> create mode 100644 Documentation/bpf/map_queue_stack.rst
>
> diff --git a/Documentation/bpf/map_queue_stack.rst b/Documentation/bpf/map_queue_stack.rst
> new file mode 100644
> index 000000000000..a27e7f573869
> --- /dev/null
> +++ b/Documentation/bpf/map_queue_stack.rst
> @@ -0,0 +1,119 @@
> +.. 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. 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``. 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 to create a queue from userspace:
I'd prefer "how to use libbpf's low-level API to create a queue".
Because ideally people use the declarative way shown above, which is
also "use libbpf to create", but is simpler and preserves all the BTF
type information (if map supports it).
> +
> +.. 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 */
> + 0); /* create options */
NULL, it's a pointer
> + }
> +
> +
> +References
> +==========
> +
> +https://lwn.net/ml/netdev/153986858555.9127.14517764371945179514.stgit@kernel/
> --
> 2.35.1
>
