[P4-dev] Doubts Regarding use of Register

Swaroop Thool swaroopthool1991 at outlook.com
Thu Aug 13 11:48:49 EDT 2015

Hi Antonin,

Thank for your reply and sorry for my unclear question and annoying you as I am new in p4.

I have:

register r1 {  // THERE ARE 3 SUCH ARRAYS R1,R2,R3
    width : 16;
    instance_count : 500;   //single array of size 500 

I am considering array r1 of register of size 500. And there are total 3 such arrays say, r1,r2,r3.
If I store 3 different values corresponding to 1st input (like 1st IP address) at location r1[10], r2[20], r3[30] using metadata fields (manipulation/transfer) where, my metadata is like temporary structure having fields as location and value as:

header_type temp_metadata_t {  //THERE ARE 3 INSTANCES OF THIS FOR EACH R1, R2, R3
      location : 16;
      value : 10; }

Again storing 3 different values corresponding to 2nd input (like 2nd IP address) at location r1[50], r2[60], r3[70] and so on for different number of inputs. Now I want to know what is the value at location "r1[10], r2[20], r3[30] " which should be corresponding to same input as stated above(here its 1st input). As I am using temp_metadata_t for value and location manipulation which puts my values in r1,r2,r3 and gets refreshed with new one as I proceed further. I dont have record to get values from location r1[10], r2[20], r3[30] belonging to 1st input. 
Can I create table with run-time entries for this 3 values combination and use something like pointer? I dont know how to do this.

Also, how to view table entries? I tried the below to start CLI  
: ~/p4factory/cli$ ./pd_cli.py -p simple_router -s of-tests/pd_thrift:../../submodules/oft-infra -c localhost:22222

but gets error:
./pd_cli.py: line 15: import: command not found
./pd_cli.py: line 16: import: command not found
./pd_cli.py: line 17: import: command not found
./pd_cli.py: line 18: import: command not found
./pd_cli.py: line 19: import: command not found
./pd_cli.py: line 20: import: command not found
./pd_cli.py: line 21: import: command not found
./pd_cli.py: line 24: import: command not found
./pd_cli.py: line 27: syntax error near unexpected token `('
./pd_cli.py: line 27: `class PdCli(cmd.Cmd):'


Date: Thu, 13 Aug 2015 06:59:53 -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,

Please see inline

On Wed, Aug 12, 2015 at 9:10 AM, Swaroop Thool <swaroopthool1991 at outlook.com> wrote:

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?    

I am sorry but it is very difficult for me to understand what you are trying to achieve here. I don't understand how you are filling your register arrays (like a queue?), what you are putting there, how you want to use it later...? If you can give some pseudo-code, I may be able to help you better.
2)Is there any print statement in P4? 

That would not mean much, given that a) P4 is not a purely imperative language b) it is targeted at many kinds of target switches, including hardware ones.
a) Where would you be able to print exactly? Only in actions, control flow, some parts of the parse states?
b) Where would your message go?
Also, user strings are not supported in P4, which kind of makes it difficult.
In a nutshell, I don't think this belongs into standard P4. If you are writing P4 programs specifically for a software switch, you may be able to extend the P4 language with print statements that will be interpreted by your backend. But that would require modifying the frontend of the compiler (parser, hlir).

We are working on a P4 debugger which will interact with the p4lang software switch. This debugger would let you step through a P4 program and print information about fields / metadata as you go. It will not be ready to be released for a while though.
3)Are there any comparison statement among values (e.g  max, min) ?

I think it is in the works.




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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