On 11/8/19 1:28 PM, Thomas Monjalon wrote: > 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: >> ~~~~~~~~~~~~~~~~~~~~
May be it is not 100% clear since below are alternatives. >> 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. If I understand your statement correctly, but (C) is not ideal since it looks global. If registered dynamic field of mbuf and is flag that the feature should be enabled, it is a flag to all ports/devices. >> 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. I mean that application wants to understand if the feature is supported. Then, it wants to enable it. In the case of (B), if I understand the solution correctly, there is no explicit way to enable, PMD just detects it because of discovery is done (that's what I mean by "implicit" and it is a drawback from my point of view, but still could be considered). (C) solves the problem of (B). >> 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. Do you think that global nature of the (C) is acceptable?
