Re: [PATCH bpf-next v2] docs/bpf: Document BPF map types QUEUE and STACK

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On 11/7/22 7:05 AM, Donald Hunter 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>
---
v1 -> v2:
- Mention "libbpf's low-level API", as reported by Andrii Nakryiko
- Replace 0 with NULL in code snippet, as reported by Andrii Nakryiko
---
  Documentation/bpf/map_queue_stack.rst | 120 ++++++++++++++++++++++++++
  1 file changed, 120 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..6325648bf0c7
--- /dev/null
+++ b/Documentation/bpf/map_queue_stack.rst
@@ -0,0 +1,120 @@
+.. 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.

Besides BPF_ANY, BPF_EXIST is allowed as well?

+
+.. 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.
+
[...]



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