* [PATCH v3 bpf-next 0/2] bpf, docs: Document BPF_MAP_TYPE_ARRAY
@ 2021-12-07 13:31 Dave Tucker
2021-12-07 13:31 ` [PATCH v3 bpf-next 1/2] bpf, docs: add kernel version to map_cgroup_storage Dave Tucker
2021-12-07 13:31 ` [PATCH v3 bpf-next 2/2] bpf, docs: document BPF_MAP_TYPE_ARRAY Dave Tucker
0 siblings, 2 replies; 6+ messages in thread
From: Dave Tucker @ 2021-12-07 13:31 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
v2->v3:
- wrap text to 80 chars and add newline at end of file
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 | 182 +++++++++++++++++++++++
Documentation/bpf/map_cgroup_storage.rst | 2 +
2 files changed, 184 insertions(+)
create mode 100644 Documentation/bpf/map_array.rst
--
2.33.1
^ permalink raw reply [flat|nested] 6+ messages in thread
* [PATCH v3 bpf-next 1/2] bpf, docs: add kernel version to map_cgroup_storage
2021-12-07 13:31 [PATCH v3 bpf-next 0/2] bpf, docs: Document BPF_MAP_TYPE_ARRAY Dave Tucker
@ 2021-12-07 13:31 ` Dave Tucker
2021-12-07 13:31 ` [PATCH v3 bpf-next 2/2] bpf, docs: document BPF_MAP_TYPE_ARRAY Dave Tucker
1 sibling, 0 replies; 6+ messages in thread
From: Dave Tucker @ 2021-12-07 13:31 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 v3 bpf-next 2/2] bpf, docs: document BPF_MAP_TYPE_ARRAY
2021-12-07 13:31 [PATCH v3 bpf-next 0/2] bpf, docs: Document BPF_MAP_TYPE_ARRAY Dave Tucker
2021-12-07 13:31 ` [PATCH v3 bpf-next 1/2] bpf, docs: add kernel version to map_cgroup_storage Dave Tucker
@ 2021-12-07 13:31 ` Dave Tucker
2021-12-07 14:53 ` Jonathan Corbet
1 sibling, 1 reply; 6+ messages in thread
From: Dave Tucker @ 2021-12-07 13:31 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 | 182 ++++++++++++++++++++++++++++++++
1 file changed, 182 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..7ed5f7654ee8
--- /dev/null
+++ b/Documentation/bpf/map_array.rst
@@ -0,0 +1,182 @@
+.. 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.
+
--
2.33.1
^ permalink raw reply related [flat|nested] 6+ messages in thread
* Re: [PATCH v3 bpf-next 2/2] bpf, docs: document BPF_MAP_TYPE_ARRAY
2021-12-07 13:31 ` [PATCH v3 bpf-next 2/2] bpf, docs: document BPF_MAP_TYPE_ARRAY Dave Tucker
@ 2021-12-07 14:53 ` Jonathan Corbet
2021-12-07 15:29 ` Dave Tucker
0 siblings, 1 reply; 6+ messages in thread
From: Jonathan Corbet @ 2021-12-07 14:53 UTC (permalink / raw)
To: Dave Tucker, bpf
Cc: ast, daniel, andrii, kafai, songliubraving, john.fastabend,
kpsingh, linux-doc, Dave Tucker
Dave Tucker <dave@dtucker.co.uk> writes:
> 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 | 182 ++++++++++++++++++++++++++++++++
> 1 file changed, 182 insertions(+)
> create mode 100644 Documentation/bpf/map_array.rst
When you add a new file, you need to add it to index.rst as well to
bring it into the docs build.
> diff --git a/Documentation/bpf/map_array.rst b/Documentation/bpf/map_array.rst
> new file mode 100644
> index 000000000000..7ed5f7654ee8
> --- /dev/null
> +++ b/Documentation/bpf/map_array.rst
> @@ -0,0 +1,182 @@
> +.. 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.
It's really better not to use ``literal markup`` for function names.
Just write function() and the right thing will happen, including
cross-reference links to the kerneldoc for that function if it exists.
Thanks,
jon
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH v3 bpf-next 2/2] bpf, docs: document BPF_MAP_TYPE_ARRAY
2021-12-07 14:53 ` Jonathan Corbet
@ 2021-12-07 15:29 ` Dave Tucker
2021-12-07 15:36 ` Jonathan Corbet
0 siblings, 1 reply; 6+ messages in thread
From: Dave Tucker @ 2021-12-07 15:29 UTC (permalink / raw)
To: Jonathan Corbet, bpf
Cc: ast, daniel, andrii, kafai, songliubraving, john.fastabend,
kpsingh, linux-doc
On Tue, 7 Dec 2021, at 14:53, Jonathan Corbet wrote:
> Dave Tucker <dave@dtucker.co.uk> writes:
>
>> 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 | 182 ++++++++++++++++++++++++++++++++
>> 1 file changed, 182 insertions(+)
>> create mode 100644 Documentation/bpf/map_array.rst
>
> When you add a new file, you need to add it to index.rst as well to
> bring it into the docs build.
I believe I responded to this comment in an earlier version of this patch set.
The glob pattern in Documentaion/bpf/maps.rst includes this in the docs build.
>> diff --git a/Documentation/bpf/map_array.rst b/Documentation/bpf/map_array.rst
>> new file mode 100644
>> index 000000000000..7ed5f7654ee8
>> --- /dev/null
>> +++ b/Documentation/bpf/map_array.rst
>> @@ -0,0 +1,182 @@
>> +.. 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.
>
> It's really better not to use ``literal markup`` for function names.
> Just write function() and the right thing will happen, including
> cross-reference links to the kerneldoc for that function if it exists.
I've just tested this out, and for some reason it's not working as expected.
It's not formatted as ``literal markup`` nor is it linked to kerneldoc.
Perhaps because the issue is because these are part of tools/lib/bpf/bpf_helpers.h?
I also noticed I introduced a warning due to missing a newline after a code block :(
I'll fix that and push a v4, but will wait for confirmation on the use of `` for the helper
functions.
- Dave
> Thanks,
>
> jon
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH v3 bpf-next 2/2] bpf, docs: document BPF_MAP_TYPE_ARRAY
2021-12-07 15:29 ` Dave Tucker
@ 2021-12-07 15:36 ` Jonathan Corbet
0 siblings, 0 replies; 6+ messages in thread
From: Jonathan Corbet @ 2021-12-07 15:36 UTC (permalink / raw)
To: Dave Tucker, bpf
Cc: ast, daniel, andrii, kafai, songliubraving, john.fastabend,
kpsingh, linux-doc
"Dave Tucker" <dave@dtucker.co.uk> writes:
> On Tue, 7 Dec 2021, at 14:53, Jonathan Corbet wrote:
>> Dave Tucker <dave@dtucker.co.uk> writes:
>>
>>> 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 | 182 ++++++++++++++++++++++++++++++++
>>> 1 file changed, 182 insertions(+)
>>> create mode 100644 Documentation/bpf/map_array.rst
>>
>> When you add a new file, you need to add it to index.rst as well to
>> bring it into the docs build.
>
> I believe I responded to this comment in an earlier version of this patch set.
> The glob pattern in Documentaion/bpf/maps.rst includes this in the docs build.
Sorry...I even looked for :glob:, believe it or not, but not in
linux-next, sorry. Caffeine is taking effect now...
>> It's really better not to use ``literal markup`` for function names.
>> Just write function() and the right thing will happen, including
>> cross-reference links to the kerneldoc for that function if it exists.
>
> I've just tested this out, and for some reason it's not working as expected.
> It's not formatted as ``literal markup`` nor is it linked to kerneldoc.
> Perhaps because the issue is because these are part of tools/lib/bpf/bpf_helpers.h?
It can only link to kerneldoc that's actually pulled into the built
documentation. If there isn't a ".. kernel-doc::" directive for that
file (and it appears there is not) then Sphinx doesn't know about it.
Thanks,
jon
^ permalink raw reply [flat|nested] 6+ messages in thread
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2021-12-07 13:31 ` [PATCH v3 bpf-next 2/2] bpf, docs: document BPF_MAP_TYPE_ARRAY Dave Tucker
2021-12-07 14:53 ` Jonathan Corbet
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2021-12-07 15:36 ` Jonathan Corbet
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