RE: [PATCH bpf-next v1] Document BPF_MAP_TYPE_LPM_TRIE

[John Fastabend <john.fastabend@xxxxxxxxx>]

Donald Hunter wrote:
> Add documentation for BPF_MAP_TYPE_LPM_TRIE including kernel
> BPF helper usage, userspace usage and examples.
> 
> Signed-off-by: Donald Hunter <donald.hunter@xxxxxxxxx>
> ---
>  Documentation/bpf/map_lpm_trie.rst | 179 +++++++++++++++++++++++++++++
>  1 file changed, 179 insertions(+)
>  create mode 100644 Documentation/bpf/map_lpm_trie.rst
> 
> diff --git a/Documentation/bpf/map_lpm_trie.rst b/Documentation/bpf/map_lpm_trie.rst
> new file mode 100644
> index 000000000000..d57c967d11d0
> --- /dev/null
> +++ b/Documentation/bpf/map_lpm_trie.rst
> @@ -0,0 +1,179 @@
> +.. SPDX-License-Identifier: GPL-2.0-only
> +.. Copyright (C) 2022 Red Hat, Inc.
> +
> +=====================
> +BPF_MAP_TYPE_LPM_TRIE
> +=====================
> +
> +.. note::
> +   - ``BPF_MAP_TYPE_LPM_TRIE`` was introduced in kernel version 4.11
> +
> +``BPF_MAP_TYPE_LPM_TRIE`` provides a longest prefix match algorithm that
> +can be used to match IP addresses to a stored set of prefixes.
> +Internally, data is stored in an unbalanced trie of nodes that uses
> +``prefixlen,data`` pairs as its keys. The ``data`` is interpreted in
> +network byte order, i.e. big endian, so ``data[0]`` stores the most
> +significant byte.
> +
> +LPM tries may be created with a maximum prefix length that is a multiple
> +of 8, in the range from 8 to 2048. The key used for lookup and update
> +operations is a ``struct bpf_lpm_trie_key``, extended by
> +``max_prefixlen/8`` bytes.
> +
> +- For IPv4 addresses the data length is 4 bytes
> +- For IPv6 addresses the data length is 16 bytes
> +
> +The value type stored in the LPM trie can be any user defined type.
> +
> +.. note::
> +   When creating a map of type ``BPF_MAP_TYPE_LPM_TRIE`` you must set the
> +   ``BPF_F_NO_PREALLOC`` flag.
> +
> +Usage
> +=====
> +
> +Kernel BPF
> +----------
> +
> +.. c:function::
> +   void *bpf_map_lookup_elem(struct bpf_map *map, const void *key)
> +
> +The longest prefix entry for a given data value can be found using the
> +``bpf_map_lookup_elem()`` helper. This helper returns a pointer to the
> +value associated with the longest matching ``key``, or ``NULL`` if no
> +entry was found.
> +
> +The ``key`` should have ``prefixlen`` set to ``max_prefixlen`` when
> +performing longest prefix lookups. For example, when searching for the
> +longest prefix match for an IPv4 address, ``prefixlen`` should be set to
> +``32``.
> +
> +.. c:function::
> +   long bpf_map_update_elem(struct bpf_map *map, const void *key, const void *value, u64 flags)
> +
> +Prefix entries can be added or updated using the ``bpf_map_update_elem()``
> +helper. This helper replaces existing elements atomically.
> +
> +``bpf_map_update_elem()`` returns ``0`` on success, or negative error in
> +case of failure.
> +
> + .. note::
> +    The flags parameter must be one of BPF_ANY, BPF_NOEXIST or BPF_EXIST,
> +    but the value is ignored, giving BPF_ANY semantics.
> +
> +.. c:function::
> +   long bpf_map_delete_elem(struct bpf_map *map, const void *key)
> +
> +Prefix entries can be deleted using the ``bpf_map_delete_elem()``
> +helper. This helper will return 0 on success, or negative error in case
> +of failure.

The map ops lookup, update, delete and below userspace are pretty generic to
all map types. How about moving those into a generic file about maps? Maybe
./Documentation/bpf/mpas.rst? Then perhaps there is a way to link to
them from here.

> +
> +Userspace
> +---------
> +
> +Access from userspace uses libbpf APIs with the same names as above, with
> +the map identified by ``fd``.
> +
> +.. c:function::
> +   int bpf_map_get_next_key (int fd, const void *cur_key, void *next_key)
> +
> +A userspace program can iterate through the entries in an LPM trie using
> +libbpf's ``bpf_map_get_next_key()`` function. The first key can be
> +fetched by calling ``bpf_map_get_next_key()`` with ``cur_key`` set to
> +``NULL``. Subsequent calls will fetch the next key that follows the
> +current key. ``bpf_map_get_next_key()`` returns ``0`` on success,
> +``-ENOENT`` if cur_key is the last key in the hash, or negative error in
> +case of failure.
> +
> +``bpf_map_get_next_key()`` will iterate through the LPM trie elements
> +from leftmost leaf first. This means that iteration will return more
> +specific keys before less specific ones.

So I tihnk none of this is specific to LPM tries.

> +
> +Examples
> +========
> +
> +Please see ``tools/samples/bpf/xdp_router_ipv4_user.c`` and

I wouldn't link to samples. Can we link to a selftest? Maybe move the
xdp_router_ipv4_user into a selftest otherwise no one ensures it is
always working.

> +``xdp_router_ipv4.bpf.c`` for a functional example. The code snippets
> +below demonstrates API usage.
> +
> +Kernel BPF
> +----------

rest lgtm. Thanks for working on docs.