Re: [dpdk-dev] [PATCH 1/2] ethdev: add flow action type update as an offload

[Thomas Monjalon <thomas@monjalon.net>]

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.