Re: [PATCH v5 net-next 06/29] bpf: add lookup/update/delete/iterate methods to BPF maps

From: Alexei Starovoitov
Date: Mon Aug 25 2014 - 18:07:20 EST

On Mon, Aug 25, 2014 at 2:33 PM, Cong Wang <cwang@xxxxxxxxxxxxxxxx> wrote:
> On Sun, Aug 24, 2014 at 1:21 PM, Alexei Starovoitov <ast@xxxxxxxxxxxx> wrote:
>> 'maps' is a generic storage of different types for sharing data between kernel
>> and userspace.
>> The maps are accessed from user space via BPF syscall, which has commands:
>> - create a map with given type and attributes
>> fd = bpf_map_create(map_type, struct nlattr *attr, int len)
>> returns fd or negative error
>> - lookup key in a given map referenced by fd
>> err = bpf_map_lookup_elem(int fd, void *key, void *value)
>> returns zero and stores found elem into value or negative error
>> - create or update key/value pair in a given map
>> err = bpf_map_update_elem(int fd, void *key, void *value)
>> returns zero or negative error
>> - find and delete element by key in a given map
>> err = bpf_map_delete_elem(int fd, void *key)
>> - iterate map elements (based on input key return next_key)
>> err = bpf_map_get_next_key(int fd, void *key, void *next_key)
> I think you need to document the bpf() syscall instead of wrappers on it,
> from a developer's point of view. You will anyway need to document a new
> syscall with a man page as a general rule.

yep. I've mentioned before that man page is on todo list. I'm delaying
writing it, because it's the most difficult part and I don't want to keep
rewriting it when interface changes (like it did from global id to fd).
Once implementation lands, manpage will be the highest priority.

> In the changelog I mean something like:
> err = bpf(BPF_MAP_LOOKUP_ELEM, ...);

Are you saying instead of:
err = bpf_map_lookup_elem(int fd, void *key, void *value)
err = bpf(BPF_MAP_LOOKUP_ELEM, fd, key, value)
in commit log?
I think that style carries less information per line.

For man page I'll document the syscall in a traditional way, but
for commit log I like to have maximum info in the fewest lines.
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