Re: [RFC bpf-next 0/4] Add XDP rx hw hints support performing XDP_REDIRECT

Toke Høiland-Jørgensen
2024-09-30T10:58:10Z

Lorenzo Bianconi <lorenzo@kernel.org> writes:

>> > We could combine such a registration API with your header format, so
>> > that the registration just becomes a way of allocating one of the keys
>> > from 0-63 (and the registry just becomes a global copy of the header).
>> > This would basically amount to moving the "service config file" into the
>> > kernel, since that seems to be the only common denominator we can rely
>> > on between BPF applications (as all attempts to write a common daemon
>> > for BPF management have shown).
>>
>> That sounds reasonable. And I guess we'd have set() check the global
>> registry to enforce that the key has been registered beforehand?
>>
>> >
>> > -Toke
>>
>> Thanks for all the feedback!
>
> I like this 'fast' KV approach but I guess we should really evaluate its
> impact on performances (especially for xdp) since, based on the kfunc calls
> order in the ebpf program, we can have one or multiple memmove/memcpy for
> each packet, right?

Yes, with Arthur's scheme, performance will be ordering dependent. Using
a global registry for offsets would sidestep this, but have the
synchronisation issues we discussed up-thread. So on balance, I think
the memmove() suggestion will probably lead to the least pain.

For the HW metadata we could sidestep this by always having a fixed
struct for it (but using the same set/get() API with reserved keys). The
only drawback of doing that is that we statically reserve a bit of
space, but I'm not sure that is such a big issue in practice (at least
not until this becomes to popular that the space starts to be contended;
but surely 256 bytes ought to be enough for everybody, right? :)).

> Moreover, I still think the metadata area in the xdp_frame/xdp_buff is not
> so suitable for nic hw metadata since:
> - it grows backward
> - it is probably in a different cacheline with respect to xdp_frame
> - nic hw metadata will not start at fixed and immutable address, but it depends
> on the running ebpf program
>
> What about having something like:
> - fixed hw nic metadata: just after xdp_frame struct (or if you want at the end
> of the metadata area :)). Here he can reuse the same KV approach if it is fast
> - user defined metadata: in the metadata area of the xdp_frame/xdp_buff

AFAIU, none of this will live in the (current) XDP metadata area. It
will all live just after the xdp_frame struct (so sharing the space with
the metadata area in the sense that adding more metadata kv fields will
decrease the amount of space that is usable by the current XDP metadata
APIs).

-Toke