08/11/2019 09:35, Andrew Rybchenko: > The problem: > ~~~~~~~~~~~~ > PMD wants to know before port start if application wants to > to use flow MARK/FLAG in the future. It is required because: > > 1. HW may be configured in a different way to reserve resources > for MARK/FLAG delivery > > 2. Datapath implementation choice may depend on it (e.g. vPMD > is faster, but does not support MARK)
Thank you for the clear problem statement. I agree with it. This is a real design issue. > Discussed solutions: > ~~~~~~~~~~~~~~~~~~~~ > A. Explicit Rx offload suggested by the patch. > > B. Implicit by validation of a flow rule with MARK/FLAG actions used. > > C. Use dynamic field/flag (i.e. application registers dynamic field > and/or flag and PMD uses lookup to solve the problem) plus part > of (B) to discover if a feature is supported. The dynamic field should be registered via a new API function named '<feature>_init'. It means the application must explicit request the feature. I agree this is the way to go. > All solutions require changes in applications which use these > features. There is a deprecation notice in place which advertises > DEV_RX_OFFLOAD_FLOW_MARK addition, but may be it is OK to substitute > it with solution (B) or (C). Solution (C) requires changes since > it should be combined with (B) in order to understand if > the feature is supported. I don't understand. Application request and PMD support are two different things. PMD support must be via rte_flow validation on a case by case anyway. > Advantages and drawbacks of solutions: > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > 1. The main drawback of (A) is a "duplication" since we already > have a way to request flow MARK using rte_flow API. > I don't fully agree that it is a duplication, but I agree > that it sounds like duplication and complicates a bit flow > MARK usage by applications. (B) complicates it as well. > > 2. One more drawback of the solution (A) is the necessity of > similar solution for META and it eats one more offload bit. > Yes, that's true and I think it is not a problem. > It would make it easier for applications to find out if > either MARK or META is supported. > > 3. The main advantage of the solution (A) is simplicity. > It is simple for application to understand if it supported. > It is simple in PMD to understand that it is required. > It is simple to disable it - just reconfigure. > Also it is easier to document it - just mention that > the offload should be supported and enabled. > > 4. The main advantage of the solution (B) is no "duplication". > I agree that it is valid argument. Solving the problem > without extra entities is always nice, but unfortunately > it is too complex in this case. > > 5. The main drawback of the solution (B) is the complexity. > It is necessary to choose a flow rule which should be used > as a criteria. It could be hardware dependent. > Complex logic is require in PMD if it wants to address the > problem and control MARK delivery based on validated flow > rules. It adds dependency between start/stop processing and > flow rules validation code. > It is pretty complicated to document it. > > 6. Useless enabling of the offload in the case of solution (A) > if really used flow rules do not support MARK looks like > drawback as well, but easily mitigated by a combination > with solution (B) and only required if the application wants > to dive in the level of optimization and complexity and > makes sense if application knows required flow rules in > advance. So, it is not a problem in this case. > > 7. Solution (C) has drawbacks of the solution (B) for > applications to understand if these features are supported, > but no drawbacks in PMD, since explicit criteria is used to > enable/disable (dynamic field/flag lookup). > > 8. Solution (C) is nice since it avoids "duplication". > > 9. The main drawback of the solution (C) is asymmetry. > As it was discussed in the case of RX_TIMESTAMP > (if I remember it correctly): > - PMD advertises RX_TIMESTAMP offload capability > - application enables the offload > - PMD registers dynamic field for timestamp > Solution (C): > - PMD advertises nothing > - application uses solution (B) to understand if > these features are supported > - application registers dynamic field/flag > - PMD does lookup and solve the problem > The asymmetry could be partially mitigated if RX_TIMESTAMP > solution is changed to require an application to register > dynamic fields and PMD to do lookup if the offload is > enabled. So, the only difference will be in no offload > in the case of flow MARK/FLAG and usage of complex logic > to understand if it is supported or no. > May be it would be really good since it will allow to > have dynamic fields registered before mempool population. > > 10. Common drawback of solutions (B) and (C) is no granularity. > Solution (A) may be per queue while (B) and (C) cannot be > per queue. Moreover (C) looks global - for all devices. > It could be really painful. > > (C) is nice, but I still vote for simplicity and > granularity of (A). I vote for clear separation of application needs and PMD support, by using the method C (dynamic fields). I agree timestamp must use the same path. I agree it's complicate because we don't know in advance whether a flow rule will be accepted, but that's the reality, config is complex.
