* [PATCH bpf-next v7 0/1] Document BPF_MAP_TYPE_ARRAY
@ 2022-10-07 16:27 Donald Hunter
2022-10-07 16:27 ` [PATCH bpf-next v7 1/1] bpf, docs: document BPF_MAP_TYPE_ARRAY Donald Hunter
0 siblings, 1 reply; 4+ messages in thread
From: Donald Hunter @ 2022-10-07 16:27 UTC (permalink / raw)
To: bpf, linux-doc; +Cc: dave, Donald Hunter
Add documentation for BPF_MAP_TYPE_ARRAY and BPF_MAP_TYPE_PERCPU_ARRAY
variant, including kernel version introduced, usage and examples.
v6->v7:
- Remove 2^32 reference and reword paragraph
reported by Jiri Olsa and Daniel Borkmann
v5->v6:
- Rework sample code into individual snippets
- Grammar mods suggested by Bagas Sanjaja
v4->v5:
- Use formatting consistent with *_TYPE_HASH docs
- Dropped cgroup doc patch from this set
- Fix grammar and typos reported by Bagas Sanjaya
- Fix typo and version reported by Donald Hunter
- Update examples to be libbpf v1 compatible
v3->v4:
- fix doctest failure due to missing newline
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 (1):
bpf, docs: document BPF_MAP_TYPE_ARRAY
Documentation/bpf/map_array.rst | 232 ++++++++++++++++++++++++++++++++
1 file changed, 232 insertions(+)
create mode 100644 Documentation/bpf/map_array.rst
--
2.35.1
^ permalink raw reply [flat|nested] 4+ messages in thread
* [PATCH bpf-next v7 1/1] bpf, docs: document BPF_MAP_TYPE_ARRAY
2022-10-07 16:27 [PATCH bpf-next v7 0/1] Document BPF_MAP_TYPE_ARRAY Donald Hunter
@ 2022-10-07 16:27 ` Donald Hunter
2022-10-20 0:05 ` Alexei Starovoitov
0 siblings, 1 reply; 4+ messages in thread
From: Donald Hunter @ 2022-10-07 16:27 UTC (permalink / raw)
To: bpf, linux-doc; +Cc: dave, Donald Hunter
From: Dave Tucker <dave@dtucker.co.uk>
Add documentation for the BPF_MAP_TYPE_ARRAY including kernel version
introduced, usage and examples. Also document BPF_MAP_TYPE_PERCPU_ARRAY
which is similar.
Signed-off-by: Dave Tucker <dave@dtucker.co.uk>
Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
---
Documentation/bpf/map_array.rst | 232 ++++++++++++++++++++++++++++++++
1 file changed, 232 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..c3c56ffe5334
--- /dev/null
+++ b/Documentation/bpf/map_array.rst
@@ -0,0 +1,232 @@
+.. SPDX-License-Identifier: GPL-2.0-only
+.. Copyright (C) 2022 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
+ - ``BPF_MAP_TYPE_PERCPU_ARRAY`` was introduced 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. The size of the array is defined in ``max_entries`` at
+creation time. 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 value
+stored can be of any size, however, small values will be rounded up to 8
+bytes.
+
+Since kernel 5.5, 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
+=====
+
+.. c:function::
+ void *bpf_map_lookup_elem(struct bpf_map *map, const void *key)
+
+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.
+
+.. c:function::
+ long bpf_map_update_elem(struct bpf_map *map, const void *key, const void *value, u64 flags)
+
+Array elements can also be added using the ``bpf_map_update_elem()`` helper or
+libbpf API.
+
+``bpf_map_update_elem()`` returns 0 on success, or negative error in case of
+failure.
+
+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_elem()`` to insert a
+zero value to that index.
+
+Per CPU Array
+-------------
+
+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``.
+
+When using a ``BPF_MAP_TYPE_PERCPU_ARRAY`` the ``bpf_map_update_elem()`` and
+``bpf_map_lookup_elem()`` helpers automatically access the hash slot for the
+current CPU.
+
+.. c:function::
+ void *bpf_map_lookup_percpu_elem(struct bpf_map *map, const void *key, u32 cpu)
+
+The ``bpf_map_lookup_percpu_elem()`` helper can be used to lookup the array
+value for a specific CPU. Returns value on success , or ``NULL`` if no entry was
+found or ``cpu`` is invalid.
+
+Concurrency
+-----------
+
+Since kernel version 5.1, the BPF infrastructure provides ``struct bpf_spin_lock``
+to synchronize access.
+
+Examples
+========
+
+Please see the ``tools/testing/selftests/bpf`` directory for functional
+examples. The code samples below demonstrate API usage.
+
+Kernel BPF
+----------
+
+This snippet shows how to declare an array in a BPF program.
+
+.. code-block:: c
+
+ struct {
+ __uint(type, BPF_MAP_TYPE_ARRAY);
+ __type(key, u32);
+ __type(value, long);
+ __uint(max_entries, 256);
+ } my_map SEC(".maps");
+
+
+This example BPF program shows how to access an array element.
+
+.. code-block:: c
+
+ 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
+~~~~~~~~~~~~~~~~~~
+
+This snippet shows how to create an array, using ``bpf_map_create_opts`` to
+set flags.
+
+.. code-block:: c
+
+ #include <bpf/libbpf.h>
+ #include <bpf/bpf.h>
+
+ int create_array() {
+ int fd;
+ LIBBPF_OPTS(bpf_map_create_opts, opts, .map_flags = BPF_F_MMAPABLE);
+ fd = bpf_map_create(BPF_MAP_TYPE_ARRAY,
+ "example_array", /* name */
+ sizeof(__u32), /* key size */
+ sizeof(long), /* value size */
+ 256, /* max entries */
+ &opts); /* create opts */
+ return fd;
+ }
+
+This snippet shows how to initialize the elements of an array.
+
+.. code-block:: c
+
+ int initialize_array(int fd) {
+ __u32 i;
+ long value;
+ int ret;
+
+ for (i = 0; i < 256; i++) {
+ value = i;
+ ret = bpf_map_update_elem(fd, &i, &value, BPF_ANY);
+ if (ret < 0)
+ return ret;
+ }
+
+ return ret;
+ }
+
+This snippet shows how to retrieve an element value from an array.
+
+.. code-block:: c
+
+ int lookup(int fd) {
+ __u32 index = 42;
+ long value;
+ int ret = bpf_map_lookup_elem(fd, &index, &value);
+ if (ret < 0)
+ return ret;
+
+ /* use value here */
+ assert(value == 42);
+
+ return ret;
+ }
+
+BPF_MAP_TYPE_PERCPU_ARRAY
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+This snippet shows how to initialize the elements of a per CPU array.
+
+.. code-block:: c
+
+ int initialize_array(int fd) {
+ int ncpus = libbpf_num_possible_cpus();
+ long values[ncpus];
+ __u32 i, j;
+ int ret;
+
+ for (i = 0; i < 256 ; i++) {
+ for (j = 0; j < ncpus; j++)
+ values[j] = i;
+ ret = bpf_map_update_elem(fd, &i, &values, BPF_ANY);
+ if (ret < 0)
+ return ret;
+ }
+
+ return ret;
+ }
+
+This snippet shows how to access the per CPU elements of an array value.
+
+.. code-block:: c
+
+ int lookup(int fd) {
+ int ncpus = libbpf_num_possible_cpus();
+ __u32 index = 42, j;
+ long values[ncpus];
+ int ret = bpf_map_lookup_elem(fd, &index, &values);
+ if (ret < 0)
+ return ret;
+
+ for (j = 0; j < ncpus; j++) {
+ /* Use per CPU value here */
+ assert(values[j] == 42);
+ }
+
+ return ret;
+ }
+
+Semantics
+=========
+
+As shown in the example above, when accessing a ``BPF_MAP_TYPE_PERCPU_ARRAY``
+in userspace, each value is an array with ``ncpus`` elements.
+
+When calling ``bpf_map_update_elem()`` the flag ``BPF_NOEXIST`` can not be used
+for these maps.
--
2.35.1
^ permalink raw reply related [flat|nested] 4+ messages in thread
* Re: [PATCH bpf-next v7 1/1] bpf, docs: document BPF_MAP_TYPE_ARRAY
2022-10-07 16:27 ` [PATCH bpf-next v7 1/1] bpf, docs: document BPF_MAP_TYPE_ARRAY Donald Hunter
@ 2022-10-20 0:05 ` Alexei Starovoitov
2022-10-21 13:37 ` Donald Hunter
0 siblings, 1 reply; 4+ messages in thread
From: Alexei Starovoitov @ 2022-10-20 0:05 UTC (permalink / raw)
To: Donald Hunter; +Cc: bpf, open list:DOCUMENTATION, dave
On Fri, Oct 7, 2022 at 9:39 AM Donald Hunter <donald.hunter@gmail.com> wrote:
>
> From: Dave Tucker <dave@dtucker.co.uk>
>
> Add documentation for the BPF_MAP_TYPE_ARRAY including kernel version
> introduced, usage and examples. Also document BPF_MAP_TYPE_PERCPU_ARRAY
> which is similar.
>
> Signed-off-by: Dave Tucker <dave@dtucker.co.uk>
> Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
> ---
> Documentation/bpf/map_array.rst | 232 ++++++++++++++++++++++++++++++++
> 1 file changed, 232 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..c3c56ffe5334
> --- /dev/null
> +++ b/Documentation/bpf/map_array.rst
> @@ -0,0 +1,232 @@
> +.. SPDX-License-Identifier: GPL-2.0-only
> +.. Copyright (C) 2022 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
> + - ``BPF_MAP_TYPE_PERCPU_ARRAY`` was introduced 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. The size of the array is defined in ``max_entries`` at
> +creation time. 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 value
> +stored can be of any size, however, small values will be rounded up to 8
> +bytes.
Actually all values are rounded up to 8.
Maybe we should say that all array elements are aligned to 8
instead of values are rounded?
Because values_size=4 stays as 4 from bpf prog pov.
The progs cannot access bytes 5,6,7,8 though that memory is consumed.
> +
> +Since kernel 5.5, 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
> +=====
> +
> +.. c:function::
> + void *bpf_map_lookup_elem(struct bpf_map *map, const void *key)
> +
> +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.
> +
> +.. c:function::
> + long bpf_map_update_elem(struct bpf_map *map, const void *key, const void *value, u64 flags)
> +
> +Array elements can also be added using the ``bpf_map_update_elem()`` helper or
> +libbpf API.
> +
> +``bpf_map_update_elem()`` returns 0 on success, or negative error in case of
> +failure.
> +
> +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_elem()`` to insert a
> +zero value to that index.
> +
> +Per CPU Array
> +-------------
> +
> +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``.
> +
> +When using a ``BPF_MAP_TYPE_PERCPU_ARRAY`` the ``bpf_map_update_elem()`` and
> +``bpf_map_lookup_elem()`` helpers automatically access the hash slot for the
> +current CPU.
hash slot?
the copy paste went wrong? :)
> +
> +.. c:function::
> + void *bpf_map_lookup_percpu_elem(struct bpf_map *map, const void *key, u32 cpu)
> +
> +The ``bpf_map_lookup_percpu_elem()`` helper can be used to lookup the array
> +value for a specific CPU. Returns value on success , or ``NULL`` if no entry was
> +found or ``cpu`` is invalid.
> +
> +Concurrency
> +-----------
> +
> +Since kernel version 5.1, the BPF infrastructure provides ``struct bpf_spin_lock``
> +to synchronize access.
> +
> +Examples
> +========
> +
> +Please see the ``tools/testing/selftests/bpf`` directory for functional
> +examples. The code samples below demonstrate API usage.
> +
> +Kernel BPF
> +----------
> +
> +This snippet shows how to declare an array in a BPF program.
> +
> +.. code-block:: c
> +
> + struct {
> + __uint(type, BPF_MAP_TYPE_ARRAY);
> + __type(key, u32);
> + __type(value, long);
> + __uint(max_entries, 256);
> + } my_map SEC(".maps");
> +
> +
> +This example BPF program shows how to access an array element.
> +
> +.. code-block:: c
> +
> + int bpf_prog(struct __sk_buff *skb)
> + {
> + int index = load_byte(skb,
> + ETH_HLEN + offsetof(struct iphdr, protocol));
> + long *value;
Please avoid using deprecated instructions like load_byte in examples.
> +
> + 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
> +~~~~~~~~~~~~~~~~~~
> +
> +This snippet shows how to create an array, using ``bpf_map_create_opts`` to
> +set flags.
> +
> +.. code-block:: c
> +
> + #include <bpf/libbpf.h>
> + #include <bpf/bpf.h>
> +
> + int create_array() {
> + int fd;
> + LIBBPF_OPTS(bpf_map_create_opts, opts, .map_flags = BPF_F_MMAPABLE);
Add empty line pls.
> + fd = bpf_map_create(BPF_MAP_TYPE_ARRAY,
> + "example_array", /* name */
> + sizeof(__u32), /* key size */
> + sizeof(long), /* value size */
> + 256, /* max entries */
> + &opts); /* create opts */
> + return fd;
> + }
> +
> +This snippet shows how to initialize the elements of an array.
> +
> +.. code-block:: c
> +
> + int initialize_array(int fd) {
> + __u32 i;
> + long value;
> + int ret;
> +
> + for (i = 0; i < 256; i++) {
> + value = i;
> + ret = bpf_map_update_elem(fd, &i, &value, BPF_ANY);
> + if (ret < 0)
> + return ret;
> + }
> +
> + return ret;
> + }
> +
> +This snippet shows how to retrieve an element value from an array.
> +
> +.. code-block:: c
> +
> + int lookup(int fd) {
> + __u32 index = 42;
> + long value;
> + int ret = bpf_map_lookup_elem(fd, &index, &value);
Empty line pls.
Or better yet do 'int ret;'
and ret = bpf_map... on a separate line.
> + if (ret < 0)
> + return ret;
> +
> + /* use value here */
> + assert(value == 42);
> +
> + return ret;
> + }
> +
> +BPF_MAP_TYPE_PERCPU_ARRAY
> +~~~~~~~~~~~~~~~~~~~~~~~~~
> +
> +This snippet shows how to initialize the elements of a per CPU array.
> +
> +.. code-block:: c
> +
> + int initialize_array(int fd) {
> + int ncpus = libbpf_num_possible_cpus();
> + long values[ncpus];
> + __u32 i, j;
> + int ret;
> +
> + for (i = 0; i < 256 ; i++) {
> + for (j = 0; j < ncpus; j++)
> + values[j] = i;
> + ret = bpf_map_update_elem(fd, &i, &values, BPF_ANY);
> + if (ret < 0)
> + return ret;
> + }
> +
> + return ret;
> + }
> +
> +This snippet shows how to access the per CPU elements of an array value.
> +
> +.. code-block:: c
> +
> + int lookup(int fd) {
> + int ncpus = libbpf_num_possible_cpus();
> + __u32 index = 42, j;
> + long values[ncpus];
> + int ret = bpf_map_lookup_elem(fd, &index, &values);
same here.
> + if (ret < 0)
> + return ret;
> +
> + for (j = 0; j < ncpus; j++) {
> + /* Use per CPU value here */
> + assert(values[j] == 42);
> + }
> +
> + return ret;
> + }
> +
> +Semantics
> +=========
> +
> +As shown in the example above, when accessing a ``BPF_MAP_TYPE_PERCPU_ARRAY``
> +in userspace, each value is an array with ``ncpus`` elements.
> +
> +When calling ``bpf_map_update_elem()`` the flag ``BPF_NOEXIST`` can not be used
> +for these maps.
> --
> 2.35.1
>
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [PATCH bpf-next v7 1/1] bpf, docs: document BPF_MAP_TYPE_ARRAY
2022-10-20 0:05 ` Alexei Starovoitov
@ 2022-10-21 13:37 ` Donald Hunter
0 siblings, 0 replies; 4+ messages in thread
From: Donald Hunter @ 2022-10-21 13:37 UTC (permalink / raw)
To: Alexei Starovoitov; +Cc: bpf, open list:DOCUMENTATION, dave
Alexei Starovoitov <alexei.starovoitov@gmail.com> writes:
> On Fri, Oct 7, 2022 at 9:39 AM Donald Hunter <donald.hunter@gmail.com> wrote:
>
> Actually all values are rounded up to 8.
> Maybe we should say that all array elements are aligned to 8
> instead of values are rounded?
> Because values_size=4 stays as 4 from bpf prog pov.
> The progs cannot access bytes 5,6,7,8 though that memory is consumed.
Agreed, I will reword to mention alignment instead of rounding.
>> +When using a ``BPF_MAP_TYPE_PERCPU_ARRAY`` the ``bpf_map_update_elem()`` and
>> +``bpf_map_lookup_elem()`` helpers automatically access the hash slot for the
>> +current CPU.
>
> hash slot?
> the copy paste went wrong? :)
Good catch.
>> + int bpf_prog(struct __sk_buff *skb)
>> + {
>> + int index = load_byte(skb,
>> + ETH_HLEN + offsetof(struct iphdr, protocol));
>> + long *value;
>
> Please avoid using deprecated instructions like load_byte in examples.
Will rewrite to use bpf_skb_load_bytes.
>> + int create_array() {
>> + int fd;
>> + LIBBPF_OPTS(bpf_map_create_opts, opts, .map_flags = BPF_F_MMAPABLE);
>
> Add empty line pls.
Ack.
>> + int lookup(int fd) {
>> + __u32 index = 42;
>> + long value;
>> + int ret = bpf_map_lookup_elem(fd, &index, &value);
>
> Empty line pls.
> Or better yet do 'int ret;'
> and ret = bpf_map... on a separate line.
Ack.
>> + int lookup(int fd) {
>> + int ncpus = libbpf_num_possible_cpus();
>> + __u32 index = 42, j;
>> + long values[ncpus];
>> + int ret = bpf_map_lookup_elem(fd, &index, &values);
>
> same here.
Ack. Thansk for the review!
^ permalink raw reply [flat|nested] 4+ messages in thread
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2022-10-20 0:05 ` Alexei Starovoitov
2022-10-21 13:37 ` Donald Hunter
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