08/11/2019 13:00, Andrew Rybchenko: > On 11/8/19 2:03 PM, Thomas Monjalon wrote: > > 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? > > Offloads are natively per-queue as well, so (A) keeps the choice > between per-port vs per-queue to PMDs as usual. > > > Note that A and C will not guarantee that the offload will be possible. > > Yes, definitely. > > > We need B (flow rule validation) anyway. > > Strictly speaking (B) (checking flow rules before device > startup) is required if an application can predict flow > rules and wants to ensure that MARK offload will be usable. > Otherwise, it may be skipped.
No no, I mean flow rule validation MUST be used anyway during the runtime before applying a rule. I agree it is hard to predict. I speak only about real rules. > > It seems A, B, C are not alternatives but all required > > as pieces of a puzzle... > > Unfortunately true in the most complex case. > Right now it will be A with B if required as explained above. > C will come a bit later when the field migrates to dynamic. > > May be it is even better if application registers dynamic > fields before an attempt to enable offload to be sure that > it will not fail because of impossibility to register > dynamic field (lack of space). I'm not sure, but it is not > not that important. Yes of course, lack of mbuf space is another reason for disabling the feature. > If we finally go way A, should we add offloads for META back? > I guess separate Rx and Tx are required. I would prefer to add it as dynamic flags. Why rushing on a very temporary solution while it is not a new issue?
