[P4-dev] Behavioral model implementation of Hash extern with CRC32

Antonin Bas antonin at barefootnetworks.com
Tue Jul 17 13:16:06 EDT 2018

On Mon, Jul 16, 2018 at 4:13 PM, <cristina.dominicini at aluno.ufes.br> wrote:

> Hi Antonin,
> Thank you for your feedback.
> For the checksum extern, I understand there is a limitation in the current
> simple switch implementation, which only implements csum16.

I don't believe it is a limitation of the simple_switch implementation, I'm
pretty sure it is a compiler limitation here.

> As it is not possible to get the CRC32 output directly using the checksum
> extern, I assumed that it is possible to get this information using the
> hash extern in the way explained in my last e-mail. However, I get an
> output from the hash extern that is different from the expected result. I
> can see two possibilities:
> (1) The way I am using the hash extern is not correct. This was my first
> guess and the motivation to send this e-mail. Do you think the usage I
> described seems ok?
> (2) The bmv2 implementation of the hash extern is not following the
> expected behavior. This may not be noticed by applications that only use
> the output for hashing purposes, and do not need the exact value from the
> CRC32 output. From your response, I understood that my issue might be
> related to the bmv2 implementation. So, I will follow your advice and open
> an issue on behavioral-model github.

Your usage is correct, but it is much easier to debug the issue with the
actual P4 and generated JSON file.

> Thank you,
> Cristina
> Em 16.07.2018 15:16, Antonin Bas escreveu:
> Hi Cristina,
> The first issue seems to be a limitation of the p4c compiler (
> https://github.com/p4lang/p4c/blob/master/backends/bmv2/sim
> ple_switch/simpleSwitch.cpp#L801) and so I would recommend opening an
> issue on the p4c Github, unless there is already one for this.
> For the second issue, I would recommend opening an issue on the
> behavioral-model Github with your P4 program + the generated JSON. If this
> is an issue with bmv2 I will fix it. If this is an issue with the compiler,
> I can open the appropriate issue on the p4c Github.
> Thanks,
> Antonin
> On Mon, Jul 16, 2018 at 7:19 AM, <cristina.dominicini at aluno.ufes.br>
> wrote:
>> Hi All,
>> For my application, I need to calculate the output value of CRC32 in a P4
>> program using a arbitrary input data. I am running the simple switch of
>> BMv2 behavioral model using v1model.
>> Firstly, I checked if the behavioral model implementation was producing
>> the same results described here: http://crcmod.sourceforge.net/
>> crcmod.predefined.html#predefined-crc-algorithms
>> To that end, I executed the C++ code provided below. For input
>> "123456789", I got 3421780262 (int) or 0xcbf43926 (hex).
>> https://github.com/p4lang/behavioral-model/blob/master/src/b
>> m_sim/calculations.cpp#L285
>> So, the behavioral model implementation of CRC32 was working as expected.
>> Then, I tried to use the checksum extern with CRC32 in my P4 program, but I
>> discovery the only supported algorithm is csum16 : "error:
>> HashAlgorithm.crc32: the only supported algorithm is csum16"
>> My next try was to use the hash extern:
>> extern void hash<O, T, D, M>(out O result, in HashAlgorithm algo, in T
>> base, in D data, in M max);
>> My test P4 code:
>> ==========================================
>>         bit<32> nbase=0;
>>         bit<64> nmax=4294967296*2;
>>         bit<32> nselect;
>>         bit<32> operand_x = 123456789;
>>         hash(nselect, HashAlgorithm.crc32, nbase, {operand_x},nmax);
>> ==========================================
>> As the result is (nbase + (hash_value % nmax)), I used base as 0 and nmax
>> as a value greater than the maximum possible value for nselect (32 bits).
>> In this way, I expect to get the CRC32 output from input 123456789 without
>> any modification. Is this assumption right?
>> But, when I run this P4 code, I get the result 3488836380 (int). I have
>> already tried different format conversions (Big Endian, Little Endian,
>> Unsigned Int, ASCII, Hex, ...), but I was not able to understand this
>> output.
>> I would be grateful if somebody could give a help on this issue. My
>> questions are:
>> - Am I using the hash extern correctly?
>> - Does the hash function execute any additional operation apart from the
>> normal CRC calculation?
>> - Is there another way to compute the CRC32 output in P4?
>> Thank you,
>> Cristina Klippel Dominicini
>> _______________________________________________
>> P4-dev mailing list
>> P4-dev at lists.p4.org
>> http://lists.p4.org/mailman/listinfo/p4-dev_lists.p4.org
> --
> Antonin

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