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

[Thomas Monjalon <thomas@monjalon.net>]

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?

Note that A and C will not guarantee that the offload will be possible.
We need B (flow rule validation) anyway.

It seems A, B, C are not alternatives but all required
as pieces of a puzzle...