RE: [PATCH v7] ethdev: add special flags when creating async transfer table

[Ori Kam <orika@nvidia.com>]

Hi Ivan,

> -----Original Message-----
> From: Ivan Malov <ivan.malov@arknetworks.am>
> Sent: Wednesday, 1 February 2023 16:04
> 
> On Wed, 1 Feb 2023, Thomas Monjalon wrote:
> 
> > 01/02/2023 12:50, Ivan Malov:
> >> On Wed, 1 Feb 2023, Thomas Monjalon wrote:
> >>> 31/01/2023 06:30, Ivan Malov:
> >>>> I still hope community will comment on the possibility to
> >>>> provide a hint mechanism for always-the-same match items,
> >>>> with the perspective of becoming more versatile.
> >>>
> >>> Any hint could be imagined.
> >>> But please keep this in mind: a hint is *not* a matching criteria,
> >>> for the simple reason that a hint can be ignored by the PMD.
> >>> So you cannot use a hint to avoid specifying a match item,
> >>> but you could use a hint to specify that an item is the same
> >>> for all the rules of a table.
> >>
> >> Reading the same thought expressed in your words, the penny drops.
> >> So, a hint then. But even not being a match criterion itself, it
> >> is still confined to knowledge about a too special particularity.
> >> If one needs to add similar hints for other aspects of matching,
> >> they will have to add more and more bits to this namespace.
> >> So why at all detach the namespace of hints from such of
> >> the match items? A more generic solution might be needed.
> >
> > The hints are not necessarily related to the matching.
> > That's why it is more flexible to have separate definitions.
> 
> I hear ya.
> 
> >
> >> In another email of yours, [1], you suggest that documentation be
> >> improved. But it seems that addressing the "fixed match" issue
> >> described by Ori (in the quote) could be that "more generic"
> >> approach. For example, if one added "always_fixed_spec" bit
> >> to struct rte_flow_item, this bit could be taken into
> >> account by PMD in rte_flow_pattern_template_create().
> >> When it has spotted this bit for item ANY_VPORT,
> >> it will treat it the way this "specialise" hint
> >> does, collecting the same upfront knowledge.
> >>
> >> Yes, I do acknowledge that encountering such a bit in
> >> a regular/sync flow parsing is irrelevant, but this
> >> is just a general idea and not the final proposal.
> >
> > Yes it does not have sense outside of template table.
> 
> Yes, but something similar can be devised to attach
> some "always the same exact spec" meaning to given
> items in the pattern. Not necessarily via this bit.
> 
We thought about it but it made the api to complex but we can revisit it.
I'm open to suggestions.

> >
> >> Also, in mail [2], Ori talks about separate pipelines
> >> for ingress and egress. That sheds some light on this
> >> hint, thanks. On the one hand, yes, vendors do tend
> >> to have separate pipelines for this, this and this,
> >> but, on the other hand, assuming this particular
> >> separation of pipelines and making a customised
> >> hint for it might not be quite generic. It is
> >> that special particularity which I am talking
> >> about in the first paragraph of my response.
> >>
> >> So why not combine addressing "fixed match" items
> >> and solving the problem of this "direction" hint?
> >
> > I agree we should try to better address templates
> > with some fixed matching items, but we could still need
> > to have some hints for other kind of optimizations.
> 
> Agreed.
> 
+1
> >
> >> Again, I can't come up with an immediate example
> >> of how precisely this could be useful, but since
> >> DPDK strives to being as much generic/neutral as
> >> possible, then why not consider this?
> >
> > I agree the hints may be quite vendor-specifics,
> > but they are optional and does not hurt vendors not implementing them.
> > For applications supporting many drivers,
> > they can use some hints without losing portability.
> >
> > So I don't think such approach is against genericity or neutrality,
> > it is just bringing some flexibility for the best performances.
> > And in DPDK, the first priority is the performance.
> 
> I'm questioning this because I suspect that, had the original
> flow design had more flexibility / better decomposition,
> then perhaps it would have been unneeded to add these
> extra hints in the first instance. I don't mean to
> criticise them too much, though.
> 
One of the problems is that the API is too flexible this means that 
PMD must assume everything so it can't optimize, this is why we
need hints .

> >
> >> [1] https://mails.dpdk.org/archives/dev/2023-February/260667.html
> >> [2] https://mails.dpdk.org/archives/dev/2023-February/260668.html
> >>
> >>>> Other
> >>>> than that, your current patch might be OK, but, again,
> >>>> I think other reviewers' comments (if any) shall
> >>>> be addressed. But no strong objections from me.
> >>>>
> >>>> By the way, for this "specialise" field, in your opinion,
> >>>> which extra flags could emerge in future / would be nice
> >>>> to have? I mean, is there any concept of what can be
> >>>> added to this field's namespace and what can't be?
> >>>
> >>> I think there is no limit with hint flags to be added.
> >>> I repeat it again: hints can be ignored by the PMDs.
> >>
> >> Thank you.
> >
> > The template flow API is experimental and will probably remain as such
> > for a long time, so if you find a more elegant approach,
> > we will consider it.
> 
> Thanks for explaining this.
> 
> You know, now you mention it, are there non-debug app
> examples available that make use of this template API?
> Back in the day, I reviewed the template API, but
> since then, I've never come across any real-life
> examples. I'd appreciate you point something out.
> 
> > But given we don't know how to make it better today,
> > and there is no real problem with its definition,
> > I don't see a reason to postpone its integration as experimental.
> >
> > In my opinion, having hint is good.
> > The real discussion is on the flags.
> > If we find how to manage the same optimization without these flags,
> > we could drop them, but the hint flexibility should remain.
> >
> >
> >
> 
> Thank you.