[P4] Fwd: Actions in P4-16

[Tejfel Máté]

Sziasztok!

   A mai megbeszélés alapján elkezdtem válaszolni (lásd lent). Kérlek 
esetleg egészítsétek ki/pontosítsátok. Főleg a 4. és 6. pontra lehetne 
kicsit bőbeszédűbb a válasz... Ha lehet péntek délután továbbküldeném 
Cristian felé...

   Köszi!

          M.


2017-11-09 13:36 keltezéssel, Tejfel Máté írta:
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> -------- Továbbított üzenet --------
> Tárgy: 	Actions in P4-16
> Dátum: 	Tue, 7 Nov 2017 17:16:50 +0000
> Feladó: 	Dumitrescu, Cristian <cristian.dumitrescu at intel.com>
> Címzett: 	Sándor Laki <lakis at inf.elte.hu>, Tejfel Máté 
> <matej at caesar.elte.hu>, 'Horpácsi Dániel' <daniel-h at elte.hu>
> CC: 	Gergely.Pongracz at Ericsson.com <Gergely.Pongracz at Ericsson.com>, 
> Kinsella, Ray <ray.kinsella at intel.com>, Ferriter, Cian 
> <cian.ferriter at intel.com>, Zhu, Heqing <heqing.zhu at intel.com>
>
>
>
> Hi guys,
>
> I have read the P4-16 spec and I have some questions on actions, it 
> would be great to have your input:
>
> 1.Action implementations. My takeaway is that actions are implemented 
> using the two mechanisms below, did I miss any other mechanism?
>
> a.Statements and expressions. Similar to writing a C function. 
> Departure from P4-14, where actions could only be built using a 
> (small) predefined set of primitive actions.
>
> i.Inputs: packet headers, packet meta-data, table entry data
>
> ii.Outputs: packet headers, packet meta-data, (table entry data?)
>
> b.Extern object instantiations and extern functions. This allows 
> pushing out of the language some P4-14 objects such as: meters, 
> registers, counters.
>
Basically this s right but probably not this is the best classification, 
because calling extern function will be also a statement, so you can 
create block like this:
     action actionA(){
         externFun();
         hdr.ethernet.srcAddr = hdr.ethernet.dstAddr;
     }
  As well as you can call an other action inside an action. For example:
     action actionB(macAddr_t dstAddr){
         actionA();
hdr.ethernet.dstAddr = dstAddr;
      }

> 2.For a given table, several actions can be defined; each table entry 
> (including the default entry) has exactly one action from the action 
> list of the table, right?
>
Right. For one entry you can have one action with a given parameter list.

> a.If entry X does operations {A, B, C, D, E} and entry Y does 
> operations {A, B, C, D, F}, we need to implement actionX (doing A, B, 
> C, D,E) and actionY (doing A, B, C, D, F) explicitly and map them to 
> entries X and Y, as opposed to saying: entry X executes actions A, B, 
> C, D, E and entry Y executes actions A, B, C, D, F, right?
>
Technically you can create a common part: commonXY (doing A,B,C,D) and 
call it from actionX and actionY, but basically right, you need two 
different action.
>
> 3.Action portability. If we use extern objects and functions, the 
> actions are not that portable anymore, right? They become target 
> specific and P4 program can only work for that target.
>
You have to define the possible externs in the architecture file. So the 
concept is, the program will be portable between targets supporting the 
same architecture file.
>
> 4.Can an action modify the table entry data or is table entry data 
> read-only?
>
Only the controller can modify table entry data. In an action you can 
generate a digest to the controller "asking it" to modify the data...
>
> a.Example: implement a packet counter per flow; pkt hits table entry, 
> a counter stored in the entry gets incremented. Is this possible in P4-16?
>
You can do something similar with the counter method, but in that case 
the counter will not stored in the entry...
>
> 5.Do you have examples of P4-16 actions that you could share? The more 
> examples the better.
>
For example you can find examples as exercises solutions of P4 tutorials 
(in 2017). See https://github.com/p4lang/tutorials
>
> 6.Can somebody give me an example for the “implementation” key word 
> from section 12.2.1.5 Additional properties and the associated concept 
> of action profile, please ?
>
We have no real example, but if we understand well in this case you can 
define the action list for a table as a list of external function calls.
Here you don not need to define precisely the external functions only 
the maximum number of possible functions. Probably you can see P4_14 
action_profile
construct for more detail.
>
> 7.When should we expect P4-16 support in the T4P4S compiler?
>
We like to create a T4P4S version supporting v1 model until end of 
January 2018.
>
> Thanks very much for your help!
>
> Regards,
>
> Cristian
>
>
>
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