Psa Extern Development Guide #779
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Thanks for creating the PR. It looks like a reasonable level of detail to me. I have not gone through the process myself before, so can't currently make any intelligent comments on whether it is correct. |
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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 | |||
| [Simple Switch](https://github.com/p4lang/behavioral-model/tree/master/src/bm_sim) | ||
| and the one in | ||
| [PSA Switch](https://github.com/p4lang/behavioral-model/tree/master/targets/psa_switch/externs). | ||
| The Simple Switch counter lives inside of `src/bm_sim` where as the |
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| # Implementing Externs in PSA Switch | |||
| Externs in PSA Switch are different than those in Simple Switch. | |||
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I think this would be more accurate as "The implementation of P4 externs in PSA Switch is different..."
| what the expected JSON for externs should be. Next, read the changes | ||
| in the following [commit](https://github.com/p4lang/behavioral-model/pull/767). | ||
| 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 |
| in the following [commit](https://github.com/p4lang/behavioral-model/pull/767). | ||
| 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. |
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That is incorrect, runtime_CLI.py does not use P4Runtime. runtime_CLI.py uses the Thrift RPC service defined in https://github.com/p4lang/behavioral-model/blob/master/thrift_src/standard.thrift. P4Runtime is a gRPC service. It is implemented over the PI C library (https://github.com/p4lang/PI) which is then itself implemented (for bmv2) in this repository (https://github.com/p4lang/behavioral-model/tree/master/PI).
Both implementations converge at bm::RuntimeInterface (https://github.com/p4lang/behavioral-model/blob/master/include/bm/bm_sim/runtime_interface.h).
runtime_CLI --- Thrift service ---> Thrift service implementation --- RuntimeInterface ---> bmv2 built-in objects + target-specific objects
[any P4Runtime client] --- P4Runtime service ---> P4Runtime implementation (DeviceMgr) --- PI API ---> PI bmv2 implementation ---> RuntimeInterface ---> bmv2 built-in objects + target-specific objects
"Target-specific objects" include your PSA extern implementations (you added an override implementation of read_counters and write_counters to PSASwitch).
| `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 |
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Same as above. "P4 Runtime" is misleading. If you mean "P4Runtime", then it is incorrect. If you mean "P4 runtime", then it is better, but maybe could use some clarification.
| * 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 |
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bm::RuntimeInterface would be less misleading
| The following process is meant to serve as a general guide on how | ||
| to develop new externs in PSA Switch. | ||
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| 1. Provide support for emitting the extern in p4c. Currently, only |
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Something that is worth mentioning because it explains the design we came up with: IIRC p4c is still generating "built-in" objects as part of the JSON (they are unused by PSASwitch) in order to make sure that runtime tools which consume the bmv2 JSON (such as the runtime_CLI) still work. This is temporary until such tools can be updated. Please correct me if I'm wrong.
Added a README to the docs detailing high level steps on how to develop externs for PSA Switch.