[dpdk-dev] [PATCH v2 3/3] ethdev: improve flow mark Rx offload deprecation notice

[Thomas Monjalon]

25/11/2019 11:44, Jerin Jacob:
> On Sun, Nov 24, 2019 at 3:12 AM Thomas Monjalon <thomas at monjalon.net> wrote:
> >
> > 23/11/2019 10:42, Jerin Jacob:
> > > On Sat, Nov 23, 2019 at 3:58 AM Thomas Monjalon <thomas at monjalon.net> wrote:
> > > > 22/11/2019 12:53, Andrew Rybchenko:
> > > > > On 11/22/19 2:15 PM, Thomas Monjalon wrote:
> > > > > > 22/11/2019 11:12, Andrew Rybchenko:
> > > > > >> On 11/22/19 1:01 AM, Thomas Monjalon wrote:
> > > > > >>> 19/11/2019 13:12, Andrew Rybchenko:
> > > > > >>>> The deprecation notice is required since it adds more requirements
> > > > > >>>> when RTE flow mark and flag actions may be used and require
> > > > > >>>> changes in applications.
> > > > > >>> I am still not sure what is the best solution here.
> > > > > >>> I continued to think about it in this thread:
> > > > > >>>   http://mails.dpdk.org/archives/dev/2019-November/151960.html
> > > > > >>>
> > > > > >>> I think we cannot require any application change until 20.11
> > > > > >>> in order to keep API (and behaviour) compatibility.
> > > > > >> Expected, but still very disappointing.
> > > > > >>
> > > > > >> The feature is implemented by Pavan (@ Marvell), supported by me,
> > > > > >> used by Qi (@ Intel), looks better than alternatives from application
> > > > > >> developer point of view [1] and finally postponed for 1 year without really
> > > > > >> strong motivation.
> > > > > >
> > > > > > I see different valuable point of views. This is enough motivation.
> > > > >
> > > > > It looks like I miss it in previous discussion, I would be thankful if
> > > > > you give me links to read or hints how to find.
> > > >
> > > > http://mails.dpdk.org/archives/dev/2019-November/150793.html
> > > >
> > > > > Introducing new types of controls would make configuration more and
> > > > > more complex. I think that many different types of control would
> > > > > over-complicate it. May be it is unavoidable, but it should be clear
> > > > > why the problem cannot be solved using existing types of controls
> > > > > (e.g. offloads).
> > > >
> > > > The offload control is used as an effective configuration for now.
> > > > The features which are configured with DEV_RX_OFFLOAD_*
> > > > do not need any other API to be used.
> > > > Extending DEV_RX_OFFLOAD_* bits for enabling features which
> > > > must be configured via other API anyway, is possible.
> > > > The real problem is that features in DEV_RX_OFFLOAD_* are supposed
> > > > to be disabled by default. If we add some opt-in features here,
> > > > we cannot enable them by default for API compatibility and do the
> > > > right thing by default.
> > > >
> > > > Choosing DEV_RX_OFFLOAD_* bits or rte_eth_dev_opt* functions is a detail.
> > > > The real decision is to change the API for using all these features.
> > > > Can we keep all features available by default (opt-out)?
> > >
> > > IMO, *rte_eth_dev_opt* has following problems
> > >
> > > 1) It is not multi-process friendly. If we are changing the Rx/Tx
> > > function pointer, based on
> > > the selected offload, then, using *rte_eth_dev_opt* scheme won't
> > > really work(if the new API
> > > called after the secondary process launch)
> >
> > Yes it must be used before launching the secondary process.
> > It is the same as DEV_RX_OFFLOAD_* config.
> 
> Yes. rte_eth_dev_opt_* has another dimension to enable and disable as API.
> So, we need to document, opt-in -> start() -> opt-out case won't work
> in multi process
> case.
> 
> >
> > > 2) If we are taking rte_eth_dev_opt path then by default feature has
> > > to be enabled to
> > > not break the functional ABI. That scheme won't scale if as when we
> > > keep adding the new features.
> > > It is always easy for the application to define "what it wants" vs
> > > "what it does not want"
> >
> > Yes, opt-in may look more natural than opt-out.
> > But opt-in makes the default more complex, and changes the API.
> >
> > > 3) Defining the device state after the reconfigure operation.
> > >
> > > IMO, if any operation is connected to fastpath it is better to use
> > > DEV_RX_OFFLOAD_ like
> > > this feature where enable or disable PMDs from updating
> > > ``rte_mbuf::hash::fdir`` so that if possible
> > > we can use different Rx function pointer if possible(Hence it can work
> > > with the multi-process case case)
> >
> > I reply to 2 and 3 together.
> >
> > We decided that offloads must be disabled by default.
> > This is what we have in 19.11:
> >         - Some offloads are enabled before start with DEV_?X_OFFLOAD_*
> >         - Some offloads are enabled with functions at any time
> >
> > For the second type of offloads, you want to know, before start,
> > whether it will be used or not.
> > If adding the second type of offloads (like rte_flow ones)
> > to DEV_?X_OFFLOAD_*, it means it must be enabled 2 times:
> >         - before start with offload bits
> >         - later with more precise functions
> >
> > I would like to avoid changing the default behaviour,
> > which is to enable an offload only one time.
> > That's why I think this second category of offloads should
> > offer opt-out (global disabling), so it will continue
> > to work by default if they are configured.
> >
> > I hope you understand the difference between the two categories.
> 
> I understand the difference. The only point of "difference in opinion" is
> the default behavior of the feature/offload. If it is in RX_OFFLOAD scheme then
> by default it is disabled. opt_* scheme makes this new feature/offload
> enabled default to avoid changing the default behavior.

OK, this is where we disagree.
I am for keeping what we agreed this year: all offloads are disabled by default.
But I am against the need for double enablement.
The offloads which are enabled with a specific function should not need
to be also enabled (opt-in) before start.

> It is good to avoid functional ABI change. But bad as,
> 1) New API starts bloating the ethdev API.

In general, I want to clean-up the ethdev API during next year.

> 2) It is diffcult for application guys to figure out what are features need to
> be disabled to performance as he/she does not know, for the given release,
> the enabled features.

Yes this is a good point.

> Item (1) is a trade-off between elegance vs ABI compatibility. No
> strong opinion on this.
> 
> To fix the item (2), Can we get have an API in ethdev to get enabled
> features so that
> the application can probe and disable if required?

We can think about something like that.
Note that there is also a need to better advertise all capabilities.

> For example, rte_eth_dev_set_ptypes() comes in same category, By default,
> ptype parsing is enabled. I think, we can have a general interface to
> "probe" the by default enabled features
> and disable it if required. Not scattered API for each feature.

This is an issue. The packet type parsing should be disabled by default.

> The above scheme fixe my concerns.
> 
> Thoughts?