Re: [xdp-hints] Re: [PATCH RFCv2 bpf-next 00/18] XDP-hints: XDP gaining access to HW offload hints via BTF

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Stanislav Fomichev <sdf@google.com> writes:

> On Tue, Oct 4, 2022 at 5:59 PM Jakub Kicinski <kuba@kernel.org> wrote:
>>
>> On Tue, 4 Oct 2022 17:25:51 -0700 Martin KaFai Lau wrote:
>> > A intentionally wild question, what does it take for the driver to return the
>> > hints.  Is the rx_desc and rx_queue enough?  When the xdp prog is calling a
>> > kfunc/bpf-helper, like 'hwtstamp = bpf_xdp_get_hwtstamp()', can the driver
>> > replace it with some inline bpf code (like how the inline code is generated for
>> > the map_lookup helper).  The xdp prog can then store the hwstamp in the meta
>> > area in any layout it wants.
>>
>> Since you mentioned it... FWIW that was always my preference rather than
>> the BTF magic :)  The jited image would have to be per-driver like we
>> do for BPF offload but that's easy to do from the technical
>> perspective (I doubt many deployments bind the same prog to multiple
>> HW devices)..
>
> +1, sounds like a good alternative (got your reply while typing)
> I'm not too versed in the rx_desc/rx_queue area, but seems like worst
> case that bpf_xdp_get_hwtstamp can probably receive a xdp_md ctx and
> parse it out from the pre-populated metadata?
>
> Btw, do we also need to think about the redirect case? What happens
> when I redirect one frame from a device A with one metadata format to
> a device B with another?

Yes, we absolutely do! In fact, to me this (redirects) is the main
reason why we need the ID in the packet in the first place: when running
on (say) a veth, an XDP program needs to be able to deal with packets
from multiple physical NICs.

As far as API is concerned, my hope was that we could solve this with a
CO-RE like approach where the program author just writes something like:

hw_tstamp = bpf_get_xdp_hint("hw_tstamp", u64);

and bpf_get_xdp_hint() is really a macro (or a special kind of
relocation?) and libbpf would do the following on load:

- query the kernel BTF for all possible xdp_hint structs
- figure out which of them have an 'u64 hw_tstamp' member
- generate the necessary conditionals / jump table to disambiguate on
  the BTF_ID in the packet


Now, if this is better done by a kfunc I'm not terribly opposed to that
either, but I'm not sure it's actually better/easier to do in the kernel
than in libbpf at load time?

-Toke