[bpf-next,v1] docs/bpf: Document BPF map types QUEUE and STACK

Commit Message

Donald Hunter Nov. 4, 2022, 5:21 p.m. UTC

Add documentation for BPF_MAP_TYPE_QUEUE and BPF_MAP_TYPE_STACK,
including usage and examples.

Signed-off-by: Donald Hunter <[e-mail]>
---
 Documentation/bpf/map_queue_stack.rst | 119 ++++++++++++++++++++++++++
 1 file changed, 119 insertions(+)
 create mode 100644 Documentation/bpf/map_queue_stack.rst

Comments

Andrii Nakryiko Nov. 4, 2022, 11 p.m. UTC | #1

On Fri, Nov 4, 2022 at 10:21 AM Donald Hunter <[e-mail]> wrote:
>
> Add documentation for BPF_MAP_TYPE_QUEUE and BPF_MAP_TYPE_STACK,
> including usage and examples.
>
> Signed-off-by: Donald Hunter <[e-mail]>
> ---
>  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
>

Donald Hunter Nov. 7, 2022, 9:17 a.m. UTC | #2

Andrii Nakryiko <[e-mail]> writes:

> On Fri, Nov 4, 2022 at 10:21 AM Donald Hunter <[e-mail]> wrote:
>>
>> +
>> +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).

I'll incorporate this and also see if I can reword to highlight the
preferred declarative approach.

>> +
>> +.. 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

Good catch, thanks!

Patch

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:
+
+.. 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 */
+    }
+
+
+References
+==========
+
+https://lwn.net/ml/netdev/153986858555.9127.14517764371945179514.stgit@kernel/