[P4-dev] Does P4 imply a specific programming model ?

Vladimir Gurevich vladimir.gurevich at barefootnetworks.com
Mon Oct 16 00:43:22 EDT 2017


Hell Chuck,

Welcome to P4. Please, find (most of) the answers inline.

On Sun, Oct 15, 2017 at 8:55 PM, Chuck Ashley <chuck.ashley4 at gmail.com>
wrote:

>
> I am new to this group and very excited about P4 ideas and the change it
> may drive to hardware.
> I have a question about the programming model assumptions P4 makes, if any.
> Does P4 assume/dictate any specific target device architecture?
>

If by the word "architecture" you mean the same thing as P4_16 spec (a set
of P4-programmable and non-programmable components that form a specific
platofrm as well as their interfaces), then starting with P4_16 it
officially does not dictate any specific architecture, but even P4_14 is
flexible enough to accommodate different  architectures.

As a shameless plug I'd recommend you to take a look at the P4_16 video
tutorial <https://www.youtube.com/watch?v=GslseT4hY1w> and pay specific
attention starting at about mark 28:24
<https://www.youtube.com/watch?v=GslseT4hY1w&t=1704s>
.

> Is there a "standard" P4 library structure that P4 implies / provides as a
> public reference implementation (not only for software simulators but also
> for real hardware targets)?
>

I am not sure what you mean by the "standard P4 library structure", but if
you mean "architecture", then there are a couple, although HW target
support is not yet complete:

   - The so-called "V1 Model", discussed in the presentation. This is a
   simple architecture that was intended to provide an easy transition from
   P4_14 with all its legacy naming and such. It supports only simple programs
   - The Portable Switch Architecture (PSA) that is intended to be a fully
   functional and portable P4_16 architecture for switches. It is currently
   going through standardization process and you can see it for yourself on
   Github at https://github.com/p4lang/p4-spec/tree/master/p4-16/psa.

Are P4 programs written in feature split or in pipeline stage split?
>
In my view, feature split is analogous to a run to completion model that is
> probably suitable to run on NICs NPUs, while per-stage split is analogous
> to a traditional pipeline programming model and probably matches how P4
> will describe high capacity switches that are pipelined because that is how
> they are architected.
> While the feature programming model for switches may be novel, it also may
> be very brittle when applied to pipeline architecture and limit how
> creative customers can become.
>

Suffice it to say that P4 is designed to be target-independent language and
has no concept of stages per se.

At the same time it is difficult for me to comment on your assertions about
something being "brittle" or "limiting" without knowing exactly what you
mean. P4 was, indeed, designed to allow efficient implementation on modern
high-speed pipelines. One of the specific consequences is the absence of
the loops in the control flow functions and that was a deliberate choice,
but any flow that can be represented as a direct acyclic graph is
expressible.

So far, the main limit to creativity I've observed was the reluctance to
sit down and write a program, but this is not an exclusive property of P4 :)

Happy hacking,
Vladimir

-- 
*Vladimir Gurevich*

*Barefoot Networks*
*Technical Lead, Customer Engineering*
Email: vag at barefootnetworks.com
Phone: (408) 833-4505
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