08/11/2019 11:42, Andrew Rybchenko: > 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?
That's a good question. Maybe the feature request should be per port. In this case, we are back to solution A with a flag per port? Note that A and C will not guarantee that the offload will be possible. We need B (flow rule validation) anyway. It seems A, B, C are not alternatives but all required as pieces of a puzzle...
