Message-ID: <CAOuuhY-JT5mLKDBDScjyDA5grA2M8E9ECvyC6FJE3Ot-VLKh=w@mail.gmail.com>
Date:   Fri, 27 Jan 2023 15:06:32 -0800
From:   Tom Herbert <tom@...anda.io>
To:     sdf@...gle.com
Cc:     Jamal Hadi Salim <hadi@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>,
        Jakub Kicinski <kuba@...nel.org>,
        Jamal Hadi Salim <jhs@...atatu.com>, netdev@...r.kernel.org,
        kernel@...atatu.com, deb.chatterjee@...el.com,
        anjali.singhai@...el.com, namrata.limaye@...el.com,
        khalidm@...dia.com, pratyush@...anda.io, xiyou.wangcong@...il.com,
        davem@...emloft.net, edumazet@...gle.com, pabeni@...hat.com,
        vladbu@...dia.com, simon.horman@...igine.com, stefanc@...vell.com,
        seong.kim@....com, mattyk@...dia.com, dan.daly@...el.com,
        john.andy.fingerhut@...el.com
Subject: Re: [PATCH net-next RFC 00/20] Introducing P4TC

On Fri, Jan 27, 2023 at 2:26 PM <sdf@...gle.com> wrote:
>
> On 01/27, Jamal Hadi Salim wrote:
> > On Fri, Jan 27, 2023 at 1:26 PM Jiri Pirko <jiri@...nulli.us> wrote:
> > >
> > > Fri, Jan 27, 2023 at 12:30:22AM CET, kuba@...nel.org wrote:
> > > >On Tue, 24 Jan 2023 12:03:46 -0500 Jamal Hadi Salim wrote:
> > > >> There have been many discussions and meetings since about 2015 in
> > regards to
> > > >> P4 over TC and now that the market has chosen P4 as the datapath
> > specification
> > > >> lingua franca
> > > >
> > > >Which market?
> > > >
> > > >Barely anyone understands the existing TC offloads. We'd need strong,
> > > >and practical reasons to merge this. Speaking with my "have suffered
> > > >thru the TC offloads working for a vendor" hat on, not the "junior
> > > >maintainer" hat.
> > >
> > > You talk about offload, yet I don't see any offload code in this RFC.
> > > It's pure sw implementation.
> > >
> > > But speaking about offload, how exactly do you plan to offload this
> > > Jamal? AFAIK there is some HW-specific compiler magic needed to generate
> > > HW acceptable blob. How exactly do you plan to deliver it to the driver?
> > > If HW offload offload is the motivation for this RFC work and we cannot
> > > pass the TC in kernel objects to drivers, I fail to see why exactly do
> > > you need the SW implementation...
>
> > Our rule in TC is: _if you want to offload using TC you must have a
> > s/w equivalent_.
> > We enforced this rule multiple times (as you know).
> > P4TC has a sw equivalent to whatever the hardware would do. We are
> > pushing that
> > first. Regardless, it has value on its own merit:
> > I can run P4 equivalent in s/w in a scriptable (as in no compilation
> > in the same spirit as u32 and pedit),
> > by programming the kernel datapath without changing any kernel code.
>
> Not to derail too much, but maybe you can clarify the following for me:
> In my (in)experience, P4 is usually constrained by the vendor
> specific extensions. So how real is that goal where we can have a generic
> P4@TC with an option to offload? In my view, the reality (at least
> currently) is that there are NIC-specific P4 programs which won't have
> a chance of running generically at TC (unless we implement those vendor
> extensions).
>
> And regarding custom parser, someone has to ask that 'what about bpf
> question': let's say we have a P4 frontend at TC, can we use bpfilter-like
> usermode helper to transparently compile it to bpf (for SW path) instead
> inventing yet another packet parser? Wrestling with the verifier won't be
> easy here, but I trust it more than this new kParser.

Yes, wrestling with the verifier is tricky, however we do have a
solution to compile arbitrarily complex parsers into eBFP. We
presented this work at Netdev 0x15
https://netdevconf.info/0x15/session.html?Replacing-Flow-Dissector-with-PANDA-Parser.
Of course this has the obvious advantage that we don't have to change
the kernel (however, as we talk about in the presentation, this method
actually produces a faster more extensible parser than flow dissector,
so it's still on my radar to replace flow dissector itself with an
eBPF parser :-) )

The value of kParser is that it is not compiled code, but dynamically
scriptable. It's much easier to change on the fly and depends on a CLI
interface which works well with P4TC. The front end is the same as
what we are using for PANDA parser, that is the same parser frontend
(in C code or other) can be compiled into XDP/eBPF, kParser CLI, or
other targets (this is based on establishing a IR which we talked
about in https://myfoobar2022.sched.com/event/1BhCX/high-performance-programmable-parsers

Tom

>
>
> > To answer your question in regards to what the interfaces "P4
> > speaking" hardware or drivers
> > are going to be programmed, there are discussions going on right now:
> > There is a strong
> > leaning towards devlink for the hardware side loading.... The idea
> > from the driver side is to
> > reuse the tc ndos.
> > We have biweekly meetings which are open. We do have Nvidia folks, but
> > would be great if
> > we can have you there. Let me find the link and send it to you.
> > Do note however, our goal is to get s/w first as per tradition of
> > other offloads with TC .
>
> > cheers,
> > jamal

Powered by blists - more mailing lists