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

[Andrew Rybchenko <arybchenko@solarflare.com>]

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?