[P4-dev] Doubts Regarding use of Register

Swaroop Thool swaroopthool1991 at outlook.com
Wed Aug 12 12:10:37 EDT 2015

Hi Antonin,

Thank you for your reply. 

1) Furthermore, few basic operations implemented in many languages making it difficult to implement in P4.

Is there way in P4 where I can keep record of values for particular flow/input.
For example, I have stored 3 values of 1st input(like 1st IP address) in 3 different register arrays(one in each) through metadata field transfer. And again stored 3 values of 2nd input(like another IP address) in those 3 different register arrays but at different locations. Again for next 3rd input and so on. 
Issue here is that ALL those values are in 3 register arrays and now I want to pick only that 3 values of 1st input. As metadata values are overwritten with latest values which cant rescue me. How to collect this 3 values of each input when register array almost becomes full or when inputs are stopped? If this is not the way, can I create run-time instances of metadatas for each new input which will hold values of that particular input/flow? But again here how can I refer to 1st flow metadatas after few flows?    

2)Is there any print statement in P4? 

3)Are there any comparison statement among values (e.g  max, min) ?


Date: Mon, 10 Aug 2015 08:31:32 -0700
Subject: Re: [P4-dev] Doubts Regarding use of Register
From: antonin at barefootnetworks.com
To: swaroopthool1991 at outlook.com
CC: p4-dev at p4.org

Hi Swaroop,

Sorry I sent you the wrong link, here is the correct one: https://github.com/p4lang/p4-hlir/blob/master/p4_hlir/frontend/primitives.json#L61

As for your questions:

1) myfield is independent of the register, it should be a (any) header field / metadata field. Using the register name here will result in an error.

2) Please submit an issue here: https://github.com/p4lang/p4c-behavioral/issues for the Python error you are seeing. You can just avoid using 'static' in your P4 program for now. I am pretty sure this attribute is ignored by the behavioral-model anyway.



On Mon, Aug 10, 2015 at 8:07 AM, Swaroop Thool <swaroopthool1991 at outlook.com> wrote:

Hi Antonin,

Thanks for the reply. 
When I used the lines mentioned by you:

register_read(myfield, r, runTimeValueofOtherMetadata); 
add_to_field(myfield, 0x01);
register_write(r, runTimeValueofOtherMetadata, myfield); 

Which "myfield" actually here related to register r ? Can I use different field reference(e.g. some metadata field ref) instead of it?

I was not getting compile error at start.It compiled for few lines however, later I got error stated as follows:

/home/swaroop/p4-other/p4factory/submodules/p4c-behavioral/p4c_bm/shell.py p4src/sample.p4 --gen-dir=/home/swaroop/p4-other/p4factory/targets/sample/build//bm/ --thrift --public-inc-path=/home/swaroop/p4-other/p4factory/targets/sample/build//inc/p4_sim --p4-prefix=sample
WARNING: No error rule is defined for exclusive state 'pragma'
WARNING: Token 'DIVIDE' defined, but not used
WARNING: Token 'PPHASH' defined, but not used
WARNING: Token 'MOD' defined, but not used
WARNING: There are 3 unused tokens
parsing successful
Semantic warning: action 'no_op' is not reachable and will be removed
semantic checking successful
Header type standard_metadata_t not byte-aligned, adding padding
validating:  True
validating:  True
validating:  True
validating:  True
Generating files in directory /home/swaroop/p4-other/p4factory/targets/sample/build/bm
total phv length (in bytes): 124
Traceback (most recent call last):
  File "/home/swaroop/p4-other/p4factory/submodules/p4c-behavioral/p4c_bm/shell.py", line 130, in <module>
  File "/home/swaroop/p4-other/p4factory/submodules/p4c-behavioral/p4c_bm/shell.py", line 124, in main
    dump_yaml = args.dump_yaml)
  File "/home/swaroop/p4-other/p4factory/submodules/p4c-behavioral/p4c_bm/smart.py", line 1141, in render_dict_create
    render_dict_populate_registers(render_dict, hlir)
  File "/home/swaroop/p4-other/p4factory/submodules/p4c-behavioral/p4c_bm/smart.py", line 1076, in render_dict_populate_registers
    r_info["binding"] = ("static", table)
UnboundLocalError: local variable 'table' referenced before assignment
make: *** [/home/swaroop/p4-other/p4factory/targets/sample/build//inc/p4_sim/rmt.h] Error 1

P.S: Repository by the Link is not accessible 


Date: Wed, 5 Aug 2015 14:29:07 -0700
Subject: Re: [P4-dev] Doubts Regarding use of Register
From: antonin at barefootnetworks.com
To: swaroopthool1991 at outlook.com
CC: p4-dev at p4.org

Hi Swaroop,

Thanks for your interest. I will give more details inline. However the basic answer to all your questions is that P4 registers are almost not supported at all (yet) in the behavioral model software switch. Some stateful operations are supported, but differe from what is in the spec.

On Wed, Aug 5, 2015 at 12:08 PM, Swaroop Thool <swaroopthool1991 at outlook.com> wrote:

Hi all ,

Thanks for the answers. Few more questions:

If I write-

       register r {

            width : 9;

            static : table_which_invokes_register;

            instance_count : 500;

            attributes : saturating; }

1) If I create register array of 500(cells) as above, is it like 1-D array whose location can be read/write? How to access that because I am getting syntax error when I used modify_field(). What is the proper syntax if I want to put some value at run-time generated location of array, say 50?

You can do this, just not with modify_field(). You need to use register_read() and register_write(), which are described here: https://github.com/barefootnetworks/p4-hlir/blob/master/p4_hlir/frontend/primitives.json#L61
Something like this should work in an action:
register_read(my_field, r, 50);
add_to_field(my_field, 0xaba);
register_write(r, 50, my_field);
As you can see you can read / write a register, and once the value is in a field, you can do arithmetic on it. However you cannot operate directly on the register (for now).
2) Further extension to above,How can I store value 1 at location say, 50 in register array of 500
 created above and later increment value 1 to 2 when same location is 
encountered and maintain those values?

I guess the above answers your question?

3)What is layout declaration(instead of width declaration) means which require header_type_name?

It is a convenient way of accessing register values using field names, no more. Let's say I have the following code:

header_type my_header_type {
  fields {
    fA: 16;
    fB: 32;
    fC: 16;

register r2 {
  layout: my_header_type;
  instance_count: 100;

Then each cell in your register would be 64-bit wide. You would be able to write things like this: r2[50].fB
So it is just a way to give "meaning" to the bits in a register cell.
However this is NOT supported in the p4lang software switch.

4) If I create metadata for a packet to copy some value into its field, 
will value get updated with new one for another packet when same 
metadata is used for example : modify_field (metadata, somevalue) ?If yes, 
then how to maintain value of individual packets as I dont want to loose that values ? 

registers will let you maintain state, the value of metadata will be overwritten with each new packet
5)OR should I use add_field() in above. What is the difference between both? 

I don't see how this relates to 4)

Also since you are using it in your code, I need to point out that attributes (e.g. saturating) are not supported in the switch yet.






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