Re: [PATCH net-next v8 00/15] Introducing P4TC

Jamal Hadi Salim
2023-11-23T13:22:24Z

On Thu, Nov 23, 2023 at 1:36 AM Jiri Pirko <jiri@resnulli.us> wrote:
>
> Wed, Nov 22, 2023 at 08:35:21PM CET, jhs@mojatatu.com wrote:
> >On Wed, Nov 22, 2023 at 1:31 PM Jiri Pirko <jiri@resnulli.us> wrote:
> >>
> >> Wed, Nov 22, 2023 at 04:14:02PM CET, jhs@mojatatu.com wrote:
> >> >On Wed, Nov 22, 2023 at 4:25 AM Jiri Pirko <jiri@resnulli.us> wrote:
> >> >>
> >> >> Tue, Nov 21, 2023 at 04:21:44PM CET, jhs@mojatatu.com wrote:
> >> >> >On Tue, Nov 21, 2023 at 9:19 AM Jiri Pirko <jiri@resnulli.us> wrote:
> >> >> >>
> >> >> >> Tue, Nov 21, 2023 at 02:47:40PM CET, jhs@mojatatu.com wrote:
> >> >> >> >On Tue, Nov 21, 2023 at 8:06 AM Jiri Pirko <jiri@resnulli.us> wrote:
> >> >> >> >>
> >> >> >> >> Mon, Nov 20, 2023 at 11:56:50PM CET, jhs@mojatatu.com wrote:
> >> >> >> >> >On Mon, Nov 20, 2023 at 4:49 PM Daniel Borkmann <daniel@iogearbox.net> wrote:
> >> >> >> >> >>
> >> >> >> >> >> On 11/20/23 8:56 PM, Jamal Hadi Salim wrote:
> >> >> >> >> >> > On Mon, Nov 20, 2023 at 1:10 PM Jiri Pirko <jiri@resnulli.us> wrote:
> >> >> >> >> >> >> Mon, Nov 20, 2023 at 03:23:59PM CET, jhs@mojatatu.com wrote:
> >> >> >> >>
> >> >> >> >> [...]
> >> >> >> >>
> >> >> >> >> >
> >> >> >> >> >> tc BPF and XDP already have widely used infrastructure and can be developed
> >> >> >> >> >> against libbpf or other user space libraries for a user space control plane.
> >> >> >> >> >> With 'control plane' you refer here to the tc / netlink shim you've built,
> >> >> >> >> >> but looking at the tc command line examples, this doesn't really provide a
> >> >> >> >> >> good user experience (you call it p4 but people load bpf obj files). If the
> >> >> >> >> >> expectation is that an operator should run tc commands, then neither it's
> >> >> >> >> >> a nice experience for p4 nor for BPF folks. From a BPF PoV, we moved over
> >> >> >> >> >> to bpf_mprog and plan to also extend this for XDP to have a common look and
> >> >> >> >> >> feel wrt networking for developers. Why can't this be reused?
> >> >> >> >> >
> >> >> >> >> >The filter loading which loads the program is considered pipeline
> >> >> >> >> >instantiation - consider it as "provisioning" more than "control"
> >> >> >> >> >which runs at runtime. "control" is purely netlink based. The iproute2
> >> >> >> >> >code we use links libbpf for example for the filter. If we can achieve
> >> >> >> >> >the same with bpf_mprog then sure - we just dont want to loose
> >> >> >> >> >functionality though. off top of my head, some sample space:
> >> >> >> >> >- we could have multiple pipelines with different priorities (which tc
> >> >> >> >> >provides to us) - and each pipeline may have its own logic with many
> >> >> >> >> >tables etc (and the choice to iterate the next one is essentially
> >> >> >> >> >encoded in the tc action codes)
> >> >> >> >> >- we use tc block to map groups of ports (which i dont think bpf has
> >> >> >> >> >internal access of)
> >> >> >> >> >
> >> >> >> >> >In regards to usability: no i dont expect someone doing things at
> >> >> >> >> >scale to use command line tc. The APIs are via netlink. But the tc cli
> >> >> >> >> >is must for the rest of the masses per our traditions. Also i really
> >> >> >> >>
> >> >> >> >> I don't follow. You repeatedly mention "the must of the traditional tc
> >> >> >> >> cli", but what of the existing traditional cli you use for p4tc?
> >> >> >> >> If I look at the examples, pretty much everything looks new to me.
> >> >> >> >> Example:
> >> >> >> >>
> >> >> >> >> tc p4ctrl create myprog/table/mytable dstAddr 10.0.1.2/32 \
> >> >> >> >> action send_to_port param port eno1
> >> >> >> >>
> >> >> >> >> This is just TC/RTnetlink used as a channel to pass new things over. If
> >> >> >> >> that is the case, what's traditional here?
> >> >> >> >>
> >> >> >> >
> >> >> >> >
> >> >> >> >What is not traditional about it?
> >> >> >>
> >> >> >> Okay, so in that case, the following example communitating with
> >> >> >> userspace deamon using imaginary "p4ctrl" app is equally traditional:
> >> >> >> $ p4ctrl create myprog/table/mytable dstAddr 10.0.1.2/32 \
> >> >> >> action send_to_port param port eno1
> >> >> >
> >> >> >Huh? Thats just an application - classical tc which part of iproute2
> >> >> >that is sending to the kernel, no different than "tc flower.."
> >> >> >Where do you get the "userspace" daemon part? Yes, you can write a
> >> >> >daemon but it will use the same APIs as tc.
> >> >>
> >> >> Okay, so which part is the "tradition"?
> >> >>
> >> >
> >> >Provides tooling via tc cli that _everyone_ in the tc world is
> >> >familiar with - which uses the same syntax as other tc extensions do,
> >> >same expectations (eg events, request responses, familiar commands for
> >> >dumping, flushing etc). Basically someone familiar with tc will pick
> >> >this up and operate it very quickly and would have an easier time
> >> >debugging it.
> >> >There are caveats - as will be with all new classifiers - but those
> >> >are within reason.
> >>
> >> Okay, so syntax familiarity wise, what's the difference between
> >> following 2 approaches:
> >> $ tc p4ctrl create myprog/table/mytable dstAddr 10.0.1.2/32 \
> >> action send_to_port param port eno1
> >> $ p4ctrl create myprog/table/mytable dstAddr 10.0.1.2/32 \
> >> action send_to_port param port eno1
> >> ?
> >>
> >>
> >> >
> >> >> >>
> >> >> >> >
> >> >> >> >>
> >> >> >> >> >didnt even want to use ebpf at all for operator experience reasons -
> >> >> >> >> >it requires a compilation of the code and an extra loading compared to
> >> >> >> >> >what our original u32/pedit code offered.
> >> >> >> >> >
> >> >> >> >> >> I don't quite follow why not most of this could be implemented entirely in
> >> >> >> >> >> user space without the detour of this and you would provide a developer
> >> >> >> >> >> library which could then be integrated into a p4 runtime/frontend? This
> >> >> >> >> >> way users never interface with ebpf parts nor tc given they also shouldn't
> >> >> >> >> >> have to - it's an implementation detail. This is what John was also pointing
> >> >> >> >> >> out earlier.
> >> >> >> >> >>
> >> >> >> >> >
> >> >> >> >> >Netlink is the API. We will provide a library for object manipulation
> >> >> >> >> >which abstracts away the need to know netlink. Someone who for their
> >> >> >> >> >own reasons wants to use p4runtime or TDI could write on top of this.
> >> >> >> >> >I would not design a kernel interface to just meet p4runtime (we
> >> >> >> >> >already have TDI which came later which does things differently). So i
> >> >> >> >> >expect us to support both those two. And if i was to do something on
> >> >> >> >> >SDN that was more robust i would write my own that still uses these
> >> >> >> >> >netlink interfaces.
> >> >> >> >>
> >> >> >> >> Actually, what Daniel says about the p4 library used as a backend to p4
> >> >> >> >> frontend is pretty much aligned what I claimed on the p4 calls couple of
> >> >> >> >> times. If you have this p4 userspace tooling, it is easy for offloads to
> >> >> >> >> replace the backed by vendor-specific library which allows p4 offload
> >> >> >> >> suitable for all vendors (your plan of p4tc offload does not work well
> >> >> >> >> for our hw, as we repeatedly claimed).
> >> >> >> >>
> >> >> >> >
> >> >> >> >That's you - NVIDIA. You have chosen a path away from the kernel
> >> >> >> >towards DOCA. I understand NVIDIA's frustration with dealing with
> >> >> >> >upstream process (which has been cited to me as a good reason for
> >> >> >> >DOCA) but please dont impose these values and your politics on other
> >> >> >> >vendors(Intel, AMD for example) who are more than willing to invest
> >> >> >> >into making the kernel interfaces the path forward. Your choice.
> >> >> >>
> >> >> >> No, you are missing the point. This has nothing to do with DOCA.
> >> >> >
> >> >> >Right Jiri ;->
> >> >> >
> >> >> >> This
> >> >> >> has to do with the simple limitation of your offload assuming there are
> >> >> >> no runtime changes in the compiled pipeline. For Intel, maybe they
> >> >> >> aren't, and it's a good fit for them. All I say is, that it is not the
> >> >> >> good fit for everyone.
> >> >> >
> >> >> > a) it is not part of the P4 spec to dynamically make changes to the
> >> >> >datapath pipeline after it is create and we are discussing a P4
> >> >>
> >> >> Isn't this up to the implementation? I mean from the p4 perspective,
> >> >> everything is static. Hw might need to reshuffle the pipeline internally
> >> >> during rule insertion/remove in order to optimize the layout.
> >> >>
> >> >
> >> >But do note: the focus here is on P4 (hence the name P4TC).
> >> >
> >> >> >implementation not an extension that would add more value b) We are
> >> >> >more than happy to add extensions in the future to accomodate for
> >> >> >features but first _P4 spec_ must be met c) we had longer discussions
> >> >> >with Matty, Khalid and the Rice folks who wrote a paper on that topic
> >> >> >which you probably didnt attend and everything that needs to be done
> >> >> >can be from user space today for all those optimizations.
> >> >> >
> >> >> >Conclusion is: For what you need to do (which i dont believe is a
> >> >> >limitation in your hardware rather a design decision on your part) run
> >> >> >your user space daemon, do optimizations and update the datapath.
> >> >> >Everybody is happy.
> >> >>
> >> >> Should the userspace daemon listen on inserted rules to be offloade
> >> >> over netlink?
> >> >>
> >> >
> >> >I mean you could if you wanted to given this is just traditional
> >> >netlink which emits events (with some filtering when we integrate the
> >> >filter approach). But why?
> >>
> >> Nevermind.
> >>
> >>
> >> >
> >> >> >
> >> >> >>
> >> >> >> >Nobody is stopping you from offering your customers proprietary
> >> >> >> >solutions which include a specific ebpf approach alongside DOCA. We
> >> >> >> >believe that a singular interface regardless of the vendor is the
> >> >> >> >right way forward. IMHO, this siloing that unfortunately is also added
> >> >> >> >by eBPF being a double edged sword is not good for the community.
> >> >> >> >
> >> >> >> >> As I also said on the p4 call couple of times, I don't see the kernel
> >> >> >> >> as the correct place to do the p4 abstractions. Why don't you do it in
> >> >> >> >> userspace and give vendors possiblity to have p4 backends with compilers,
> >> >> >> >> runtime optimizations etc in userspace, talking to the HW in the
> >> >> >> >> vendor-suitable way too. Then the SW implementation could be easily eBPF
> >> >> >> >> and the main reason (I believe) why you need to have this is TC
> >> >> >> >> (offload) is then void.
> >> >> >> >>
> >> >> >> >> The "everyone wants to use TC/netlink" claim does not seem correct
> >> >> >> >> to me. Why not to have one Linux p4 solution that fits everyones needs?
> >> >> >> >
> >> >> >> >You mean more fitting to the DOCA world? no, because iam a kernel
> >> >> >>
> >> >> >> Again, this has 0 relation to DOCA.
> >> >> >>
> >> >> >>
> >> >> >> >first person and kernel interfaces are good for everyone.
> >> >> >>
> >> >> >> Yeah, not really. Not always the kernel is the right answer. Your/Intel
> >> >> >> plan to handle the offload by:
> >> >> >> 1) abuse devlink to flash p4 binary
> >> >> >> 2) parse the binary in kernel to match to the table ids of rules coming
> >> >> >> from p4tc ndo_setup_tc
> >> >> >> 3) abuse devlink to flash p4 binary for tc-flower
> >> >> >> 4) parse the binary in kernel to match to the table ids of rules coming
> >> >> >> from tc-flower ndo_setup_tc
> >> >> >> is really something that is making me a little bit nauseous.
> >> >> >>
> >> >> >> If you don't have a feasible plan to do the offload, p4tc does not make
> >> >> >> sense to me to be honest.
> >> >> >
> >> >> >You mean if there's no plan to match your (NVIDIA?) point of view.
> >> >> >For #1 - how's this different from DDP? Wasnt that your suggestion to
> >> >>
> >> >> I doubt that. Any flashing-blob-parsing-in-kernel is something I'm
> >> >> opposed to from day 1.
> >> >>
> >> >>
> >> >
> >> >Oh well - it is in the kernel and it works fine tbh.
> >> >
> >> >> >begin with? For #2 Nobody is proposing to do anything of the sort. The
> >> >> >ndo is passed IDs for the objects and associated contents. For #3+#4
> >> >>
> >> >> During offload, you need to parse the blob in driver to be able to match
> >> >> the ids with blob entities. That was presented by you/Intel in the past
> >> >> IIRC.
> >> >>
> >> >
> >> >You are correct - in case of offload the netlink IDs will have to be
> >> >authenticated against what the hardware can accept, but the devlink
> >> >flash use i believe was from you as a compromise.
> >>
> >> Definitelly not. I'm against devlink abuse for this from day 1.
> >>
> >>
> >> >
> >> >>
> >> >> >tc flower thing has nothing to do with P4TC that was just some random
> >> >> >proposal someone made seeing if they could ride on top of P4TC.
> >> >>
> >> >> Yeah, it's not yet merged and already mentally used for abuse. I love
> >> >> that :)
> >> >>
> >> >> >
> >> >> >Besides this nobody really has to satisfy your point of view - like i
> >> >> >said earlier feel free to provide proprietary solutions. From a
> >> >> >consumer perspective I would not want to deal with 4 different
> >> >> >vendors with 4 different proprietary approaches. The kernel is the
> >> >> >unifying part. You seemed happier with tc flower just not with the
> >> >>
> >> >> Yeah, that is my point, why the unifying part can't be a userspace
> >> >> daemon/library with multiple backends (p4tc, bpf, vendorX, vendorY, ..)?
> >> >>
> >> >> I just don't see the kernel as a good fit for abstraction here,
> >> >> given the fact that the vendor compilers does not run in kernel.
> >> >> That is breaking your model.
> >> >>
> >> >
> >> >Jiri - we want to support P4, first. Like you said the P4 pipeline,
> >> >once installed is static.
> >> >P4 doesnt allow dynamic update of the pipeline. For example, once you
> >> >say "here are my 14 tables and their associated actions and here's how
> >> >the pipeline main control (on how to iterate the tables etc) is going
> >> >to be" and after you instantiate/activate that pipeline, you dont go
> >> >back 5 minutes later and say "sorry, please introduce table 15, which
> >> >i want you to walk to after you visit table 3 if metadata foo is 5" or
> >> >"shoot, let's change that table 5 to be exact instead of LPM". It's
> >> >not anywhere in the spec.
> >> >That doesnt mean it is not useful thing to have - but it is an
> >> >invention that has _nothing to do with the P4 spec_; so saying a P4
> >> >implementation must support it is a bit out of scope and there are
> >> >vendors with hardware who support P4 today that dont need any of this.
> >>
> >> I'm not talking about the spec. I'm talking about the offload
> >> implemetation, the offload compiler the offload runtime manager. You
> >> don't have those in kernel. That is the issue. The runtime manager is
> >> the one to decide and reshuffle the hw internals. Again, this has
> >> nothing to do with p4 frontend. This is offload implementation.
> >>
> >> And that is why I believe your p4 kernel implementation is unoffloadable.
> >> And if it is unoffloadable, do we really need it? IDK.
> >>
> >
> >Say what?
> >It's not offloadable in your hardware, you mean? Because i have beside
> >me here an intel e2000 which offloads just fine (and the AMD folks
> >seem fine too).
>
> Will Intel and AMD have compiler in kernel, so no blob transfer and
> parsing it in kernel wound not be needed? No.

By that definition anything that parses anything is a compiler.

>
> >If your view is that all these runtime optimization surmount to a
> >compiler in the kernel/driver that is your, well, your view. In my
> >view (and others have said this to you already) the P4C compiler is
> >responsible for resource optimizations. The hardware supports P4, you
> >give it constraints and it knows what to do. At runtime, anything a
> >driver needs to do for resource optimization (resorting, reshuffling
> >etc), that is not a P4 problem - sorry if you have issues in your
> >architecture approach.
>
> Sure, it is the offload implementation problem. And for them, you need
> to use userspace components. And that is the problem. This discussion
> leads nowhere, I don't know how differently I should describe this.

Jiri's - that's your view based on whatever design you have in your
mind. This has nothing to do with P4.
So let me repeat again:
1) A vendor's backend for P4 when it compiles ensures that resource
constraints are taken care of.
2) The same program can run in s/w.
3) It makes *ZERO* sense to mix vendor specific constraint
optimization(what you described as resorting, reshuffling etc) as part
of P4TC or P4. Absolutely nothing to do with either. Write a
background task, specific to you, if you feel you need to move things
around at runtime.

We agree on one thing at least: This discussion is going nowhere.

cheers,
jamal

> >
> >> >In my opinion that is a feature that could be added later out of
> >> >necessity (there is some good niche value in being able to add some
> >> >"dynamicism" to any pipeline) and influence the P4 standards on why it
> >> >is needed.
> >> >It should be doable today in a brute force way (this is just one
> >> >suggestion that came to me when Rice University/Nvidia presented[1]);
> >> >i am sure there are other approaches and the idea is by no means
> >> >proven.
> >> >
> >> >1) User space Creates/compiles/Adds/activate your program that has 14
> >> >tables at tc prio X chain Y
> >> >2) a) 5 minutes later user space decides it wants to change and add
> >> >table 3 after table 15, visited when metadata foo=5
> >> > b) your compiler in user space compiles a brand new program which
> >> >satisfies #2a (how this program was authored is out of scope of
> >> >discussion)
> >> > c) user space adds the new program at tc prio X+1 chain Y or another chain Z
> >> > d) user space delete tc prio X chain Y (and make sure your packets
> >> >entry point is whatever #c is)
> >>
> >> I never suggested anything like what you describe. I'm not sure why you
> >> think so.
> >
> >It's the same class of problems - the paper i pointed to (coauthored
> >by Matty and others) has runtime resource optimizations which are
> >tantamount to changing the nature of the pipeline. We may need to
> >profile in the kernel but all those optimizations can be derived in
> >user space using the approach I described.
> >
> >cheers,
> >jamal
> >
> >
> >> >[1] https://www.cs.rice.edu/~eugeneng/papers/SIGCOMM23-Pipeleon.pdf
> >> >
> >> >>
> >> >> >kernel process - which is ironically the same thing we are going
> >> >> >through here ;->
> >> >> >
> >> >> >cheers,
> >> >> >jamal
> >> >> >
> >> >> >>
> >> >> >> >
> >> >> >> >cheers,
> >> >> >> >jamal