On Mon, Nov 22, 2021 at 9:19 AM Dave Tucker <dave@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx> > --- > Documentation/bpf/map_array.rst | 150 ++++++++++++++++++++++++++++++++ > 1 file changed, 150 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..f9eb5473a240 > --- /dev/null > +++ b/Documentation/bpf/map_array.rst > @@ -0,0 +1,150 @@ > +.. 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. > + > +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. > + It would be good to also mention BPF_F_MMAPABLE flag and ability to mmap() contents of BPF_MAP_TYPE_ARRAY created with such a flag. We need to double-check, but there might be also a restriction to have value_size be a multiple of page size in such case, we need to consult the code. > +```bpf_map_get_next_key()`` can be used to iterate over array values. > + > +Examples > +======== > + > +Please see the `bpf/samples`_ directory for functional examples. Let's point to tools/testing/selftests/bpf for functional examples. It's much more complete and more actively maintained and tested. > +This sample code simply demonstrates the API. > + > +.. section links > +.. _bpf/samples: > + https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/samples/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) > + { something is off with this curly brace > + > + int fd = bpf_create_map(BPF_MAP_TYPE_ARRAY, sizeof(__u32), sizeof(long), 256, 0); > + if (fd < 0) > + return -1; return not indented and the example itself doesn't follow kernel style guide with C89-style variable block separate from the rest of the code. Would be good to stick to that in kernel documentation. > + > + // fill the map with values from 0-255 > + for(__u32 i=0; i < 256 ; i++) { __u32 inside the for isn't C89-compatible either. Also C++-style comment above isn't allowed. > + long v = i; > + bpf_map_update_elem(fd, &i, &v, BPF_ANY); makes sense to do error checking for update and lookup > + } > + > + __u32 index = 42; > + long value; > + bpf_map_lookup_elem(fd, &index, &value); > + assert(value == 42); > + return 0; > + } > + > + [...]
