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Date: Wed, 05 Oct 2022 12:06:15 +0200 From: Toke Høiland-Jørgensen <toke@...hat.com> To: Stanislav Fomichev <sdf@...gle.com>, Jakub Kicinski <kuba@...nel.org> Cc: Martin KaFai Lau <martin.lau@...ux.dev>, Jesper Dangaard Brouer <jbrouer@...hat.com>, brouer@...hat.com, bpf@...r.kernel.org, netdev@...r.kernel.org, xdp-hints@...-project.net, larysa.zaremba@...el.com, memxor@...il.com, Lorenzo Bianconi <lorenzo@...nel.org>, mtahhan@...hat.com, Alexei Starovoitov <alexei.starovoitov@...il.com>, Daniel Borkmann <borkmann@...earbox.net>, Andrii Nakryiko <andrii.nakryiko@...il.com>, dave@...cker.co.uk, Magnus Karlsson <magnus.karlsson@...el.com>, bjorn@...nel.org Subject: Re: [xdp-hints] Re: [PATCH RFCv2 bpf-next 00/18] XDP-hints: XDP gaining access to HW offload hints via BTF Stanislav Fomichev <sdf@...gle.com> writes: > On Tue, Oct 4, 2022 at 5:59 PM Jakub Kicinski <kuba@...nel.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
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