netdev - Re: [PATCH net-next RFC 00/20] Introducing P4TC

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Message-ID: <CAKH8qBtU-1A1iKnvTXV=5v8Dim1FBmtvL6wOqgdspSFRCwNohA@mail.gmail.com>
Date:   Fri, 27 Jan 2023 16:47:54 -0800
From:   Stanislav Fomichev <sdf@...gle.com>
To:     Jamal Hadi Salim <jhs@...atatu.com>
Cc:     Jamal Hadi Salim <hadi@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        kernel@...atatu.com, deb.chatterjee@...el.com,
        anjali.singhai@...el.com, namrata.limaye@...el.com,
        khalidm@...dia.com, tom@...anda.io, 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 3:27 PM Jamal Hadi Salim <jhs@...atatu.com> wrote:
>
> On Fri, Jan 27, 2023 at 5:26 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).
>
> We are going to implement all the PSA/PNA externs. Most of these
> programs tend to
> be set or ALU operations on headers or metadata which we can handle.
> Do you have
> any examples of NIC-vendor-specific features that cant be generalized?

I don't think I can share more without giving away something that I
shouldn't give away :-)
But IIUC, and I might be missing something, it's totally within the
standard for vendors to differentiate and provide non-standard
'extern' extensions.
I'm mostly wondering what are your thoughts on this. If I have a p4
program depending on one of these externs, we can't sw-emulate it
unless we also implement the extension. Are we gonna ask NICs that
have those custom extensions to provide a SW implementation as well?
Or are we going to prohibit vendors to differentiate that way?

> > 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.
> >
>
> We dont compile anything, the parser (and rest of infra) is scriptable.

As I've replied to Tom, that seems like a technicality. BPF programs
can also be scriptable with some maps/tables. Or it can be made to
look like "scriptable" by recompiling it on every configuration change
and updating it on the fly. Or am I missing something?

Can we have a P4TC frontend and whenever configuration is updated, we
upcall into userspace to compile this whatever p4 representation into
whatever bpf bytecode that we then run. No new/custom/scriptable
parsers needed.

> cheers,
> jamal