* [PATCH v2 bpf-next 0/2] bpf, docs: Document BPF_MAP_TYPE_ARRAY
@ 2021-11-23 16:24 Dave Tucker
2021-11-23 16:24 ` [PATCH v2 bpf-next 1/2] bpf, docs: add kernel version to map_cgroup_storage Dave Tucker
0 siblings, 1 reply; 6+ messages in thread
From: Dave Tucker @ 2021-11-23 16:24 UTC (permalink / raw)
To: bpf
Cc: corbet, ast, daniel, andrii, kafai, songliubraving,
john.fastabend, kpsingh, linux-doc, Dave Tucker
This series is the beginning of my attempt to improve the BPF map and
program type documentation. It expands the template from
map_cgroup_storage to include the kernel version it was introduced.
I then used this template to document BPF_MAP_TYPE_ARRAY and
BPF_MAP_TYPE_PERCPU_ARRAY
v1->v2:
- point to selftests for functional examples
- update examples to follow kernel style
- add docs for BPF_F_MMAPABLE
Dave Tucker (2):
bpf, docs: add kernel version to map_cgroup_storage
bpf, docs: document BPF_MAP_TYPE_ARRAY
Documentation/bpf/map_array.rst | 172 +++++++++++++++++++++++
Documentation/bpf/map_cgroup_storage.rst | 2 +
2 files changed, 174 insertions(+)
create mode 100644 Documentation/bpf/map_array.rst
--
2.33.1
^ permalink raw reply [flat|nested] 6+ messages in thread
* [PATCH v2 bpf-next 1/2] bpf, docs: add kernel version to map_cgroup_storage
2021-11-23 16:24 [PATCH v2 bpf-next 0/2] bpf, docs: Document BPF_MAP_TYPE_ARRAY Dave Tucker
@ 2021-11-23 16:24 ` Dave Tucker
2021-11-23 16:24 ` [PATCH v2 bpf-next 2/2] bpf, docs: document BPF_MAP_TYPE_ARRAY Dave Tucker
2021-11-27 1:53 ` [PATCH v2 bpf-next 1/2] bpf, docs: add kernel version to map_cgroup_storage Song Liu
0 siblings, 2 replies; 6+ messages in thread
From: Dave Tucker @ 2021-11-23 16:24 UTC (permalink / raw)
To: bpf
Cc: corbet, ast, daniel, andrii, kafai, songliubraving,
john.fastabend, kpsingh, linux-doc, Dave Tucker
This adds the version at which this map became available to use in the
documentation
Signed-off-by: Dave Tucker <dave@dtucker.co.uk>
---
Documentation/bpf/map_cgroup_storage.rst | 2 ++
1 file changed, 2 insertions(+)
diff --git a/Documentation/bpf/map_cgroup_storage.rst b/Documentation/bpf/map_cgroup_storage.rst
index cab9543017bf..b626cb068846 100644
--- a/Documentation/bpf/map_cgroup_storage.rst
+++ b/Documentation/bpf/map_cgroup_storage.rst
@@ -5,6 +5,8 @@
BPF_MAP_TYPE_CGROUP_STORAGE
===========================
+.. note:: Introduced in Kernel version 4.19
+
The ``BPF_MAP_TYPE_CGROUP_STORAGE`` map type represents a local fix-sized
storage. It is only available with ``CONFIG_CGROUP_BPF``, and to programs that
attach to cgroups; the programs are made available by the same Kconfig. The
--
2.33.1
^ permalink raw reply related [flat|nested] 6+ messages in thread
* [PATCH v2 bpf-next 2/2] bpf, docs: document BPF_MAP_TYPE_ARRAY
2021-11-23 16:24 ` [PATCH v2 bpf-next 1/2] bpf, docs: add kernel version to map_cgroup_storage Dave Tucker
@ 2021-11-23 16:24 ` Dave Tucker
2021-11-27 2:05 ` Song Liu
2021-12-02 20:08 ` Andrii Nakryiko
2021-11-27 1:53 ` [PATCH v2 bpf-next 1/2] bpf, docs: add kernel version to map_cgroup_storage Song Liu
1 sibling, 2 replies; 6+ messages in thread
From: Dave Tucker @ 2021-11-23 16:24 UTC (permalink / raw)
To: bpf
Cc: corbet, ast, daniel, andrii, kafai, songliubraving,
john.fastabend, kpsingh, linux-doc, Dave Tucker
This commit adds documentation for the BPF_MAP_TYPE_ARRAY including
kernel version introduced, usage and examples.
It also documents BPF_MAP_TYPE_PERCPU_ARRAY since this is similar.
Signed-off-by: Dave Tucker <dave@dtucker.co.uk>
---
Documentation/bpf/map_array.rst | 172 ++++++++++++++++++++++++++++++++
1 file changed, 172 insertions(+)
create mode 100644 Documentation/bpf/map_array.rst
diff --git a/Documentation/bpf/map_array.rst b/Documentation/bpf/map_array.rst
new file mode 100644
index 000000000000..8ba05ba5d4ee
--- /dev/null
+++ b/Documentation/bpf/map_array.rst
@@ -0,0 +1,172 @@
+.. SPDX-License-Identifier: GPL-2.0-only
+.. Copyright (C) 2021 Red Hat, Inc.
+
+================================================
+BPF_MAP_TYPE_ARRAY and BPF_MAP_TYPE_PERCPU_ARRAY
+================================================
+
+.. note:: ``BPF_MAP_TYPE_ARRAY`` was introduced in Kernel version 3.19 and ``BPF_MAP_TYPE_PERCPU_ARRAY`` in version 4.6
+
+``BPF_MAP_TYPE_ARRAY`` and ``BPF_MAP_TYPE_PERCPU_ARRAY`` provide generic array storage.
+The key type is an unsigned 32-bit integer (4 bytes) and the map is of constant size.
+All array elements are pre-allocated and zero initialized when created.
+``BPF_MAP_TYPE_PERCPU_ARRAY`` uses a different memory region for each CPU whereas
+``BPF_MAP_TYPE_ARRAY`` uses the same memory region.
+The maximum size of an array, defined in max_entries, is limited to 2^32.
+The value stored can be of any size, however, small values will be rounded up to 8 bytes.
+
+Since Kernel 5.4, memory mapping may be enabled for ``BPF_MAP_TYPE_ARRAY`` by setting the flag ``BPF_F_MMAPABLE``.
+The map definition is page-aligned and starts on the first page.
+Sufficient page-sized and page-aligned blocks of memory are allocated to store all array values, starting on the second page,
+which in some cases will result in over-allocation of memory. The benefit of using this is increased performance and
+ease of use since userspace programs would not be required to use helper functions to access and mutate data.
+
+Usage
+=====
+
+Array elements can be retrieved using the ``bpf_map_lookup_elem()`` helper.
+This helper returns a pointer into the array element, so to avoid data races with userspace reading the value,
+the user must use primitives like ``__sync_fetch_and_add()`` when updating the value in-place.
+Access from userspace uses the libbpf API of the same name.
+
+Array elements can also be added using the ``bpf_map_update_elem()`` helper or libbpf API.
+
+Since the array is of constant size, ``bpf_map_delete_elem()`` is not supported.
+To clear an array element, you may use ``bpf_map_update_eleme()`` to insert a zero value to that index.
+
+Values stored in ``BPF_MAP_TYPE_ARRAY`` can be accessed by multiple programs across different CPUs.
+To restrict storage to a single CPU, you may use a ``BPF_MAP_TYPE_PERCPU_ARRAY``.
+Since Kernel version 5.1, the BPF infrastructure provides ``struct bpf_spin_lock`` to synchronize access.
+
+``bpf_map_get_next_key()`` can be used to iterate over array values.
+
+Examples
+========
+
+Please see the `tools/testing/selftests/bpf`_ directory for functional examples.
+This sample code simply demonstrates the API.
+
+.. section links
+.. _tools/testing/selftests/bpf:
+ https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/testing/selftests/bpf
+
+Kernel
+------
+
+.. code-block:: c
+
+ struct {
+ __uint(type, BPF_MAP_TYPE_ARRAY);
+ __type(key, u32);
+ __type(value, long);
+ __uint(max_entries, 256);
+ } my_map SEC(".maps");
+
+ int bpf_prog(struct __sk_buff *skb)
+ {
+ int index = load_byte(skb, ETH_HLEN + offsetof(struct iphdr, protocol));
+ long *value;
+
+ if (skb->pkt_type != PACKET_OUTGOING)
+ return 0;
+
+ value = bpf_map_lookup_elem(&my_map, &index);
+ if (value)
+ __sync_fetch_and_add(value, skb->len);
+
+ return 0;
+ }
+
+Userspace
+---------
+
+BPF_MAP_TYPE_ARRAY
+~~~~~~~~~~~~~~~~~~
+
+.. code-block:: c
+
+ #include <assert.h>
+ #include <bpf/libbpf.h>
+ #include <bpf/bpf.h>
+
+ int main(int argc, char **argv)
+ {
+ int fd;
+ int ret = 0;
+ __u32 i, j;
+ __u32 index = 42;
+ long v, value;
+
+ fd = bpf_create_map(BPF_MAP_TYPE_ARRAY, sizeof(__u32), sizeof(long), 256, 0);
+ if (fd < 0)
+ return fd;
+
+ /* fill the map with values from 0-255 */
+ for (i = 0; i < 256 ; i++) {
+ ret = bpf_map_update_elem(fd, &i, &v, BPF_ANY);
+ if (ret < 0)
+ return ret;
+ }
+
+ ret = bpf_map_lookup_elem(fd, &index, &value);
+ if (ret < 0)
+ return ret;
+
+ assert(value == 42);
+
+ return ret;
+ }
+
+
+BPF_MAP_TYPE_PERCPU_ARRAY
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. code-block:: c
+
+ #include <assert.h>
+ #include <bpf/libbpf.h>
+ #include <bpf/bpf.h>
+
+ int main(int argc, char **argv)
+ {
+ int ncpus = libbpf_num_possible_cpus();
+ if (ncpus < 0)
+ return ncpus;
+
+ int fd;
+ int ret = 0;
+ __u32 i, j;
+ __u32 index = 42;
+ long v[ncpus], value[ncpus];
+
+
+ fd = bpf_create_map(BPF_MAP_TYPE_PERCPU_ARRAY, sizeof(__u32), sizeof(long), 256, 0);
+ if (fd < 0)
+ return -1;
+
+ /* fill the map with values from 0-255 for each cpu */
+ for (i = 0; i < 256 ; i++) {
+ for (j = 0; j < ncpus; j++)
+ v[j] = i;
+ ret = bpf_map_update_elem(fd, &i, &v, BPF_ANY);
+ if (ret < 0)
+ return ret;
+ }
+
+ ret = bpf_map_lookup_elem(fd, &index, &value);
+ if (ret < 0)
+ return ret;
+
+ for (j = 0; j < ncpus; j++)
+ assert(value[j] == 42);
+
+ return ret;
+ }
+
+Semantics
+=========
+
+As illustrated in the example above, when using a ``BPF_MAP_TYPE_PERCPU_ARRAY`` in userspace, the
+values are an array with ``ncpus`` elements.
+
+When calling ``bpf_map_update_elem()`` the flags ``BPF_NOEXIST`` can not be used for these maps.
\ No newline at end of file
--
2.33.1
^ permalink raw reply related [flat|nested] 6+ messages in thread
* Re: [PATCH v2 bpf-next 1/2] bpf, docs: add kernel version to map_cgroup_storage
2021-11-23 16:24 ` [PATCH v2 bpf-next 1/2] bpf, docs: add kernel version to map_cgroup_storage Dave Tucker
2021-11-23 16:24 ` [PATCH v2 bpf-next 2/2] bpf, docs: document BPF_MAP_TYPE_ARRAY Dave Tucker
@ 2021-11-27 1:53 ` Song Liu
1 sibling, 0 replies; 6+ messages in thread
From: Song Liu @ 2021-11-27 1:53 UTC (permalink / raw)
To: Dave Tucker
Cc: bpf, Jonathan Corbet, Alexei Starovoitov, Daniel Borkmann,
Andrii Nakryiko, Martin KaFai Lau, Song Liu, John Fastabend,
KP Singh, Linux Doc Mailing List
On Tue, Nov 23, 2021 at 8:24 AM Dave Tucker <dave@dtucker.co.uk> wrote:
>
> This adds the version at which this map became available to use in the
> documentation
>
> Signed-off-by: Dave Tucker <dave@dtucker.co.uk>
Acked-by: Song Liu <songliubraving@fb.com>
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH v2 bpf-next 2/2] bpf, docs: document BPF_MAP_TYPE_ARRAY
2021-11-23 16:24 ` [PATCH v2 bpf-next 2/2] bpf, docs: document BPF_MAP_TYPE_ARRAY Dave Tucker
@ 2021-11-27 2:05 ` Song Liu
2021-12-02 20:08 ` Andrii Nakryiko
1 sibling, 0 replies; 6+ messages in thread
From: Song Liu @ 2021-11-27 2:05 UTC (permalink / raw)
To: Dave Tucker
Cc: bpf, Jonathan Corbet, Alexei Starovoitov, Daniel Borkmann,
Andrii Nakryiko, Martin KaFai Lau, Song Liu, John Fastabend,
KP Singh, Linux Doc Mailing List
On Tue, Nov 23, 2021 at 8:24 AM Dave Tucker <dave@dtucker.co.uk> wrote:
>
> This commit adds documentation for the BPF_MAP_TYPE_ARRAY including
> kernel version introduced, usage and examples.
> It also documents BPF_MAP_TYPE_PERCPU_ARRAY since this is similar.
>
> Signed-off-by: Dave Tucker <dave@dtucker.co.uk>
It will be great to add an example with BPF_F_MMAPABLE, but I guess we can
do that in a follow up patch. So
Acked-by: Song Liu <songliubraving@fb.com>
With one nitpick.
[...]
> +
> +When calling ``bpf_map_update_elem()`` the flags ``BPF_NOEXIST`` can not be used for these maps.
> \ No newline at end of file
nit: Maybe add a newline at the end?
> --
> 2.33.1
>
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH v2 bpf-next 2/2] bpf, docs: document BPF_MAP_TYPE_ARRAY
2021-11-23 16:24 ` [PATCH v2 bpf-next 2/2] bpf, docs: document BPF_MAP_TYPE_ARRAY Dave Tucker
2021-11-27 2:05 ` Song Liu
@ 2021-12-02 20:08 ` Andrii Nakryiko
1 sibling, 0 replies; 6+ messages in thread
From: Andrii Nakryiko @ 2021-12-02 20:08 UTC (permalink / raw)
To: Dave Tucker
Cc: bpf, Jonathan Corbet, Alexei Starovoitov, Daniel Borkmann,
Andrii Nakryiko, Martin Lau, Song Liu, john fastabend, KP Singh,
Linux Doc Mailing List
On Tue, Nov 23, 2021 at 8:24 AM Dave Tucker <dave@dtucker.co.uk> wrote:
>
> This commit adds documentation for the BPF_MAP_TYPE_ARRAY including
> kernel version introduced, usage and examples.
> It also documents BPF_MAP_TYPE_PERCPU_ARRAY since this is similar.
>
> Signed-off-by: Dave Tucker <dave@dtucker.co.uk>
> ---
It looks good, thanks for the update. But it seems like all the
Documentation/bpf/*.rst are 80-char wrapped, can you please wrap the
lines to fit within 80 characters for consistency? It also helps
immensely when reading source text files and not the HTML-rendered
version.
> Documentation/bpf/map_array.rst | 172 ++++++++++++++++++++++++++++++++
> 1 file changed, 172 insertions(+)
> create mode 100644 Documentation/bpf/map_array.rst
>
[...]
^ permalink raw reply [flat|nested] 6+ messages in thread
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