Consider enabling support in bmv2 backend for default_action in tables with action profiles or action selectors

[jafingerhut]

Since 2016, the p4c bmv2 backend has issued a warning if a table has a default_action property, and an implementation of an action profile or action selector. Then it produces a BMv2 JSON file that has no mention of the default_action at all. I believe this was done because at that time, BMv2 did not correctly implement those cases.

Since 2018, BMv2 should implement this correctly.

Links to commits/PRs with the 2016/2018 changes mentioned above can be found in the discussion thread following this comment: https://github.com/p4lang/behavioral-model/pull/1287/files#r1913623565

This issue is to track a possible enhancement to the p4c bmv2 backend that:
(a) no longer issues a warning for tables with a default_action property, if they have implementation that is action_profile or action_selector.
(b) for such tables, the BMv2 JSON data written for that table includes a description of the default_action of the table.

Of course, such a change should not be merged into p4c until after we have tested and verified that we believe BMv2 is working as expected for such tables.

[Vineet1101]

@jafingerhut I guess it can be solved. First we can remove warnings from backends/bmv2 files and then we can update the json generation. One thing that I would like to test is that does bmv2 handles the warning correctly

[jafingerhut]

@jafingerhut I guess it can be solved. First we can remove warnings from backends/bmv2 files and then we can update the json generation. One thing that I would like to test is that does bmv2 handles the warning correctly

It is definitely possible. I did some further investigation on the details and added my findings to this comment on a related issue in the behavioral-model repo: p4lang/behavioral-model#1290 (comment)

As far as I can tell, enabling this would require changes in all of these repositories, if you want the P4Runtime API to be able to configure the default_action of such tables at runtime:

• p4c
• behavioral-model
• p4runtime - the P4Runtime API specification, which currently says explicitly that it does not support configuring the default action on such tables
• probably also PI - the implementation of the P4Runtime server used by behavioral-model

When I saw that the work required was about 4 times more than I originally expected, I became less interested in pursuing such changes any time soon, personally. If someone else wants to take up that work, I certainly do not mind. Just go into it eyes open knowing that it is a much bigger task than I originally guessed.

[Vineet1101]

@jafingerhut what would be the impact of this change. If it has significant impact I would like to work on it

[jafingerhut]

It is difficult for me to judge if the impact is "significant". It is a nice small generalization to what you can do now. People have obviously been living with that restriction for 8-9 years now.