docs/bpf: Document BPF_MAP_TYPE_LPM_TRIE map

Add documentation for BPF_MAP_TYPE_LPM_TRIE including kernel
BPF helper usage, userspace usage and examples.
Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20221101114542.24481-2-donald.hunter@gmail.com

Documentation/bpf/map_lpm_trie.rst

+.. 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.
+
+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 trie, 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.
+
+Examples
+========
+
+Please see ``tools/testing/selftests/bpf/test_lpm_map.c`` for examples
+of LPM trie usage from userspace. The code snippets below demonstrate
+API usage.
+
+Kernel BPF
+----------
+
+The following BPF code snippet shows how to declare a new LPM trie for IPv4
+address prefixes:
+
+.. code-block:: c
+
+    #include <linux/bpf.h>
+    #include <bpf/bpf_helpers.h>
+
+    struct ipv4_lpm_key {
+            __u32 prefixlen;
+            __u32 data;
+    };
+
+    struct {
+            __uint(type, BPF_MAP_TYPE_LPM_TRIE);
+            __type(key, struct ipv4_lpm_key);
+            __type(value, __u32);
+            __uint(map_flags, BPF_F_NO_PREALLOC);
+            __uint(max_entries, 255);
+    } ipv4_lpm_map SEC(".maps");
+
+The following BPF code snippet shows how to lookup by IPv4 address:
+
+.. code-block:: c
+
+    void *lookup(__u32 ipaddr)
+    {
+            struct ipv4_lpm_key key = {
+                    .prefixlen = 32,
+                    .data = ipaddr
+            };
+
+            return bpf_map_lookup_elem(&ipv4_lpm_map, &key);
+    }
+
+Userspace
+---------
+
+The following snippet shows how to insert an IPv4 prefix entry into an
+LPM trie:
+
+.. code-block:: c
+
+    int add_prefix_entry(int lpm_fd, __u32 addr, __u32 prefixlen, struct value *value)
+    {
+            struct ipv4_lpm_key ipv4_key = {
+                    .prefixlen = prefixlen,
+                    .data = addr
+            };
+            return bpf_map_update_elem(lpm_fd, &ipv4_key, value, BPF_ANY);
+    }
+
+The following snippet shows a userspace program walking through the entries
+of an LPM trie:
+
+
+.. code-block:: c
+
+    #include <bpf/libbpf.h>
+    #include <bpf/bpf.h>
+
+    void iterate_lpm_trie(int map_fd)
+    {
+            struct ipv4_lpm_key *cur_key = NULL;
+            struct ipv4_lpm_key next_key;
+            struct value value;
+            int err;
+
+            for (;;) {
+                    err = bpf_map_get_next_key(map_fd, cur_key, &next_key);
+                    if (err)
+                            break;
+
+                    bpf_map_lookup_elem(map_fd, &next_key, &value);
+
+                    /* Use key and value here */
+
+                    cur_key = &next_key;
+            }
+    }