From 64488ca57ab8645fa1b69fa130ec0aa4d93cdc9e Mon Sep 17 00:00:00 2001 From: Donald Hunter Date: Tue, 8 Nov 2022 09:33:14 +0000 Subject: 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 Signed-off-by: Andrii Nakryiko Acked-by: Yonghong Song Link: https://lore.kernel.org/bpf/20221108093314.44851-1-donald.hunter@gmail.com --- Documentation/bpf/map_queue_stack.rst | 122 ++++++++++++++++++++++++++++++++++ 1 file changed, 122 insertions(+) create mode 100644 Documentation/bpf/map_queue_stack.rst (limited to '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..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/ -- cgit v1.2.3