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Implementing Externs in PSA Switch

Externs in PSA Switch are different than those in Simple Switch. Simple Switch uses externs baked into the BMV2 model where as PSA Switch uses externs specific to its own architecture. To understand what this translates to in code, look at the counter used in Simple Switch and the one in PSA Switch. The Simple Switch counter lives inside of src/bm_sim where as the counter for PSA Switch lives inside of the target specific directory. This doc is meant to provide guidance for developers on how to develop new externs in PSA Switch.

Background Knowledge

For those who are familiar with the internals of BMV2 feel free to skip this section. For those who are new/less familiar with how BMV2 operates, consider reading P4Objects.cpp as a starting point as this is where externs will be created. From here you can also see what the expected JSON for externs should be. Next, read the changes in the following commit. It details all the changes made to support the counter extern in PSA. It will also be useful to understand on a high level how P4 Runtime interacts with PSA Switch. Briefly skimming runtime_CLI.py will do.

High-Level Process

The following process is meant to serve as a general guide on how to develop new externs in PSA Switch.

  1. Provide support for emitting the extern in p4c. Currently, only counters declared in P4 are translated to JSON in the p4c for PSA Switch. This commit is an example for how to emit an extern object to JSON, and this commit is an example for how to emit extern methods to JSON. If the examples aren't clear, one can always refer back to the JSON spec defined here. As for internals of how to the compiler works, check out its docs here.
  2. Create the extern type in PSA Switch. There are several important things to note.
    1. The extern type must sub-class bm::ExternType.
    2. The extern should live within the bm::psa namespace.
    3. You must register the newly created extern type to the extern factory used by BMV2 to create externs from JSON using BM_REGISTER_EXTERN_W_NAME.
    4. You must also register any extern methods used by P4 using BM_REGISTER_EXTERN_W_NAME_METHOD.
    5. Create an import_yourExternNameHere dummy method and call it within psa_switch.cpp.
  3. Provide P4 Runtime support if necessary. This means overriding parts of the PSA Switch/P4 Runtime interface as necessary so that P4 Runtime can configure/monitor the newly developed extern.

Refer to this as a complete example.

Common Issues

Here is a list of all the bugs encountered during the development of counters and might serve as a helpful resource for others.

  • The extern can't be created.
    • Most likely a mis-match between the supplied and expected JSON format.
    • Did you remember to register the extern into the extern factory?
  • The extern method is not being called inside of an action.
    • Did you remember to register the extern method? These must be done by the programmer.
  • P4 Runtime can't interact with my extern.
    • Did you remember to override necessary methods in the P4 Runtime interface?
  • I created a new extern, added it to the Makefile, but it isn't being linked.
    • Dont forget step 2.5 from above.
  • Everything in BMV2 looks good, but it seems that P4c is failing.
    • Supporting additional externs might have allowed previous tests marked as XFAIL to pass which will cause P4c to fail. Double check the test cases that have failed and see if this was the issue.