John Fastabend <john.fastabend@xxxxxxxxx> writes: > Donald Hunter wrote: >> + >> +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. For the map types I have looked at so far, there has been a bit of variation in semantics. E.g. array has u32 key does not support delete and only supports update with BPF_EXIST/ANY, lpm_trie has custom keys and specific lookup and iteration semantics. Hash has the most generic semantics for get, lookup, update and delete. I am happy to add some generic documentation about the ops in ./Documentation/bpf/maps.rst but I think it will still be necessary to have specific documentation for each map type where the op behaviour deviates. I'd prefer to keep the map specific pages self-contained enough that a reader gets a clear view of behaviour for both kernel BPF and userspace all in one place. >> + >> +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. I will look for a suitable selftest that I can link to. >> +``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.
