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Date: Mon, 10 Oct 2022 23:29:07 -0700 From: Martin KaFai Lau <martin.lau@...ux.dev> To: Jesper Dangaard Brouer <jbrouer@...hat.com>, Magnus Karlsson <magnus.karlsson@...il.com> Cc: brouer@...hat.com, Stanislav Fomichev <sdf@...gle.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, Jakub Kicinski <kuba@...nel.org> Subject: Re: [PATCH RFCv2 bpf-next 00/18] XDP-hints: XDP gaining access to HW offload hints via BTF On 10/6/22 8:29 AM, Jesper Dangaard Brouer wrote: > > On 06/10/2022 11.14, Magnus Karlsson wrote: >> On Wed, Oct 5, 2022 at 9:27 PM Martin KaFai Lau <martin.lau@...ux.dev> wrote: >>> >>> On 10/4/22 7:15 PM, Stanislav Fomichev wrote: >>>> On Tue, Oct 4, 2022 at 6:24 PM Jakub Kicinski <kuba@...nel.org> wrote: >>>>> >>>>> On Tue, 4 Oct 2022 18:02:56 -0700 Stanislav Fomichev wrote: >>>>>> +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? >>>>> >>>>> I'd think so, worst case the driver can put xdp_md into a struct >>>>> and container_of() to get to its own stack with whatever fields >>>>> it needs. >>>> >>>> Ack, seems like something worth exploring then. >>>> >>>> The only issue I see with that is that we'd probably have to extend >>>> the loading api to pass target xdp device so we can pre-generate >>>> per-device bytecode for those kfuncs? >>> >>> There is an existing attr->prog_ifindex for dev offload purpose. May be we can >>> re-purpose/re-use some of the offload API. How this kfunc can be presented also >>> needs some thoughts, could be a new ndo_xxx.... not sure. >>>> And this potentially will block attaching the same program >>> > to different drivers/devices? >>>> Or, Martin, did you maybe have something better in mind? >>> >>> If the kfunc/helper is inline, then it will have to be per device. Unless the >>> bpf prog chooses not to inline which could be an option but I am also not sure >>> how often the user wants to 'attach' a loaded xdp prog to a different device. >>> To some extend, the CO-RE hints-loading-code will have to be per device also, >>> no? >>> >>> Why I asked the kfunc/helper approach is because, from the set, it seems the >>> hints has already been available at the driver. The specific knowledge that the >>> xdp prog missing is how to get the hints from the rx_desc/rx_queue. The >>> straight forward way to me is to make them (rx_desc/rx_queue) available to xdp >>> prog and have kfunc/helper to extract the hints from them only if the xdp prog >>> needs it. The xdp prog can selectively get what hints it needs and then >>> optionally store them into the meta area in any layout. >> >> This sounds like a really good idea to me, well worth exploring. To >> only have to pay, performance wise, for the metadata you actually use >> is very important. I did some experiments [1] on the previous patch >> set of Jesper's and there is substantial overhead added for each >> metadata enabled (and fetched from the NIC). This is especially >> important for AF_XDP in zero-copy mode where most packets are directed >> to user-space (if not, you should be using the regular driver that is >> optimized for passing packets to the stack or redirecting to other >> devices). In this case, the user knows exactly what metadata it wants >> and where in the metadata area it should be located in order to offer >> the best performance for the application in question. But as you say, >> your suggestion could potentially offer a good performance upside to >> the regular XDP path too. Yeah, since we are on this flexible hint layout, after reading the replies in other threads, now I am also not sure why we need a xdp_hints_common and probably I am missing something also. It seems to be most useful in __xdp_build_skb_from_frame. However, the xdp prog can also fill in the xdp_hints_common by itself only when needed instead of having the driver always filling it in. > > Okay, lets revisit this again. And let me explain why I believe this > isn't going to fly. > > I was also my initial though, lets just give XDP BPF-prog direct access > to the NIC rx_descriptor, or another BPF-prog populate XDP-hints prior > to calling XDP-prog. Going down this path (previously) I learned three > things: > > (1) Understanding/decoding rx_descriptor requires access to the > programmers datasheet, because it is very compacted and the mean of the > bits depend on other bits and plus current configuration status of the HW. > > (2) HW have bugs and for certain chip revisions driver will skip some > offload hints. Thus, chip revisions need to be exported to BPF-progs > and handled appropriately. > > (3) Sometimes the info is actually not available in the rx_descriptor. > Often for HW timestamps, the timestamp need to be read from a HW > register. How do we expose this to the BPF-prog? hmm.... may be I am missing those hw specific details here. How would the driver handle the above cases and fill in the xdp_hints in the meta? Can the same code be called by the xdp prog? > >> [1] >> https://lore.kernel.org/bpf/CAJ8uoz1XVqVCpkKo18qbkh6jq_Lejk24OwEWCB9cWhokYLEBDQ@mail.gmail.com/ > > > Notice that this patchset doesn't block this idea, as it is orthogonal. > After we have established a way to express xdp_hints layouts via BTF, > then we can still add a pre-XDP BPF-prog that populates the XDP-hints, > and squeeze out more performance by skipping some of the offloads that > your-specific-XDP-prog are not interested in. > > --Jesper >
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