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Date: Thu, 6 Oct 2022 09:19:36 +0100 From: Maryam Tahhan <mtahhan@...hat.com> To: sdf@...gle.com, Toke Høiland-Jørgensen <toke@...hat.com> Cc: Jakub Kicinski <kuba@...nel.org>, 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>, 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 On 05/10/2022 19:47, sdf@...gle.com wrote: > On 10/05, Toke H�iland-J�rgensen wrote: >> 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? > > Replied in the other thread, but to reiterate here: then btf_id in the > metadata has to stay and we either pre-generate those bpf_get_xdp_hint() > at libbpf or at kfunc load time level as you mention. > > But the program essentially has to handle all possible hints' btf ids > thrown > at it by the system. Not sure about the performance in this case :-/ > Maybe that's something that can be hidden behind "I might receive forwarded > packets and I know how to handle all metadata format" flag? By default, > we'll pre-generate parsing only for that specific device? I did a simple POC of Jespers xdp-hints with AF-XDP and CNDP (Cloud Native Data Plane). In the cases where my app had access to the HW I didn't need to handle all possible hints... I knew what Drivers were on the system and they were the hints I needed to deal with. So at program init time I registered the relevant BTF_IDs (and some callback functions to handle them) from the NICs that were available to me in a simple tailq (tbh there were so few I could've probably used a static array). When processing the hints then I only needed to invoke the appropriate callback function based on the received BTF_ID. I didn't have a massive chains of if...else if... else statements. In the case where we have redirection to a virtual NIC and we don't necessarily know the underlying hints that are exposed to the app, could we not still use the xdp_hints (as proposed by Jesper) themselves to indicate the relevant drivers to the application? or even indicate them via a map or something?
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