docs/bpf: Document BPF map types QUEUE and STACK

Add documentation for BPF_MAP_TYPE_QUEUE and BPF_MAP_TYPE_STACK,
including usage and examples.
Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20221108093314.44851-1-donald.hunter@gmail.com

Documentation/bpf/map_queue_stack.rst

+.. 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/