Re: [dpdk-dev] [PATCH v2 1/5] ethdev: add capability to keep flow rules on restart

[Dmitry Kozlyuk <dkozlyuk@oss.nvidia.com>]

> > [...]
> > Add a device capability bit for PMDs that can keep at least some
> > of the flow rules across restart. Without this capability behavior
> > is still unspecified, which is now explicitly stated.
> > Declare that the application can test for persitence of flow rules
> > of a particular kind by attempting to create a rule of that kind
> > when the device is stopped and checking for the specific error.
> 
> stopped -> configured but not yet started

Correct, fixed in v3.

> > diff --git a/doc/guides/prog_guide/rte_flow.rst
> b/doc/guides/prog_guide/rte_flow.rst
> > index 2b42d5ec8c..b0ced4209b 100644
> > --- a/doc/guides/prog_guide/rte_flow.rst
> > +++ b/doc/guides/prog_guide/rte_flow.rst
> > @@ -87,6 +87,33 @@ To avoid resource leaks on the PMD side, handles must
> be explicitly
> >  destroyed by the application before releasing associated resources such
> as
> >  queues and ports.
> >
> > +By default it is unspecified if the flow rules persist after the device
> stop.
> 
> or can be created before the first device start

Correct, fixed in v3.

> 
> > +If ``RTE_ETH_DEV_CAPA_FLOW_RULE_KEEP`` is not advertised,
> > +then rules must be explicitly flushed before stopping the device
> > +if the application needs to ensure they are removed.
> > +If it is advertised, this means the PMD can keep at least some rules
> > +across the device stop and start with possible reconfiguration in
> between.
> > +However, it may be only supported for some kinds of rules.
> > +The kind is a combination of the following rule properties:
> > +
> > +- the sequence of item types;
> > +- the sequence of action types;
> > +- the value of the transfer attribute.
> > +
> > +To test if a particular kind of rules is kept, the application must try
> > +to create a valid rule of that kind when the device is stopped
> > +(after it has been configured or started previously).
> > +If it succeeds, all rules of the same kind are kept at the device stop.
> > +If it fails with an error of type ``RTE_FLOW_ERROR_TYPE_STATE``,
> > +rules of this kind are flushed when the device is stopped.
> > +Rules of a kept kind that are created when the device is stopped,
> including
> > +the rules created for the test, will be kept after the device is
> started.
> 
> It must be defined what application should expect for
> not tested rule kinds.
> 
> For me about check sounds extremely complicated and hardly
> doable. Yes, some applications know kinds of rule it would
> like to create, but some, like OvS, do not. Please, correct
> me if I'm wrong. OvS knows which types of actions and even
> possible combinations of actions (harder, but still possible)
> it would like to install. But all possible combinations of
> items together with all possible combinations of actions
> could be very-very big.
> 
> May be I still misunderstand the above idea.

This is a very valid concern.
After an offline consideration me and Ori concluded
that an item/action type + transfer bit ("a feature") is enough.
That is, if some feature cannot be kept, no rules using it can be kept.