[dpdk-dev] [PATCH 1/2] ethdev: add flow action type update as an offload
Thomas Monjalon
thomas at monjalon.net
Fri Nov 8 11:28:00 CET 2019
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.
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