[P4-discuss] Doubts after my first try on p4lang

Yousong Zhou yszhou4tech at gmail.com
Fri Jun 23 20:27:26 EDT 2017


Thanks, Andy.  Your reply is very informative.  p4-guide comes very
convenient as the language and community around it in its current
state (probably also an early stage) seems to be a bit fragmented to a
newcomer like myself.  I guess I will later find some time to give
bcc, then p4c and ebpf a try.

Cheers,
                yousong


On 23 June 2017 at 10:24, Andy Fingerhut <andy.fingerhut at gmail.com> wrote:
> Yousong:
>
> P4_14 is the name of the first version of the P4 language, originally
> published in or near 2014.
>
> P4_16 is the name of the second version of the P4 language, originally
> published as a draft late in 2016, with version 1.0.0 published in May 2017.
>
> The P4_14 language spec had a few corrections and clarifications published
> only recently, to bring it closer to how the language actually was
> implemented by multiple people.  However, the expectation is that P4_16 will
> represent the majority of future implementations of the language.
>
> If you encounter a P4 program, the quickest way to tell whether you are
> dealing with a P4_14 or P4_16 is that if you see bit<5> and bit<10> in
> header definitions, or assignments inside of actions that look like C
> assignments, then it is a P4_16 program.  If you see headers with
> "field_name : 5" and "field_name : 10", and actions with
> "modify_field(dest_field, value);" in them, then it is a P4_14 program.
>
> I have a p4-guide repository here [1] that may help give something of an
> overview of how the repositories relate to each other, and which are
> 'deprecated' or not (although their top level README should also state if
> they are deprecated).
>
> Going forward, the p4lang/p4c repository contains a compiler for both P4_14
> and P4_16 programs.  The p4lang/behavioral-model repository contains the
> source code for 'bmv2', which stands for behavioral model version 2, which
> can emulate any P4_14 or P4_16 program after it has been compiled with
> p4lang/p4c (or one of the older compilers that can only handle P4_14 source
> code).  They are still in a state where some P4_16 features are not fully
> implemented yet.
>
> This may not address all of your questions, but it should be a good start.
>
> [1] https://github.com/jafingerhut/p4-guide
>
> Andy
>
>
> On Thu, Jun 22, 2017 at 6:53 PM, Yousong Zhou <yszhou4tech at gmail.com> wrote:
>>
>> Hi, I have a few doubts and encountered several issues that I'd like
>> to share with the list in the hope that it may help the project.
>>
>> I think there is a typo in the description of "Figure 10" of p4-16
>> spec.  It says "Parameters are bound by the data plane", but it says
>> also in a few other places that directionless parameters are to be
>> bound the control plane though I still have no clue who the control
>> plane is supposed to be and how that is actually done, but this is
>> another issue...
>>
>> The other thing that doubts me is about the versioning scheme of
>> p4lang specs.  Currently I see we have p4-16 as the current release
>> and p4-14 as the previous release.  But
>>
>>  - p4-14 has version 1.0.4 > version 1.0.0 of p4-16
>>  - p4-14 has date May 24, 2017 > May 22, 2017 of p4-16
>>
>> How is this so and I am curious about what the suffixes -14 and -16
>> are for?  Maybe preparing a changelog can also help in the new release
>> of a spec
>>
>> After reading "Example: a very simple switch" of p4-16, I thought then
>> I should now have a fairly good working understanding of p4lang, but
>> later when trying p4lang/tutorials I got struck with the impression
>> that the lang must have changed significantly somehow....  The content
>> and structure of .p4 files there are different and it took me a while
>> to find core.p4 in p4lang/p4c which I do not know its relationship to
>> bmv2 and p4c-bm...
>>
>> I have a few minor patches [1] while preparing and trying the
>> copy-to-cpu example in tutorials.  But I am not sure whether those 3
>> repos (bmv2, p4c-bm, tutorials) are still relevant or are they also to
>> be deprecated
>>
>>  [1]
>> https://github.com/yousong/build-scripts/commit/91282702eafe1aa21a41d056ab00fda204c20d59#diff-ee344f88969e36b33e3d0479ee719d22R29
>>
>> Regards,
>>                 yousong
>>
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>
>




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