On 11/8/19 4:17 PM, Thomas Monjalon wrote: > 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.
OK, I see. Of course, flow rule validation is required at runtime. I was rather concentrated on the stated problem solutions. >>> 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? Basically it means that we go just (B)+(C) in the case of META. I have no strong opinion but thought that it could be better to align the solution. Of course, we can wait with it. As I understand META is an experimental feature.
