[dpdk-dev] [EXT] Re: [PATCH 1/3] mbuf: add Tx offloads for packet marking

[Nithin Dabilpuram]

On Fri, May 15, 2020 at 03:12:59PM +0200, Thomas Monjalon wrote:
> 15/05/2020 12:08, Nithin Dabilpuram:
> > On Thu, May 14, 2020 at 10:29:31PM +0200, Olivier Matz wrote:
> > > I don't see any better approach than having a mbuf flag. However, I'm
> > > still not fully convinced that a dynamic flag won't do the job. Taking
> > > 3 additional flags (among 18 remaing) for this feature also means that
> > > we have 3 flags less for dynamic flags for all applications, even for
> > > applications that will not use this feature.
> > > 
> > > Would it be a problem to use a dynamic flag in this case?
> > Since packet marking feature itself is already part of spec,
> > if we move the flags to PMD specific dynamic flag, then it creates a confusion.
> > 
> > It is not the case of a custom feature supported by a specific PMD.
> > I believe when other PMD's implement packet marking, the same flags will
> > suffice.
> 
> A dynamic flag is not necessarily PMD-specific.
> It is just avoiding consuming bits if the feature is not used by the application.
> We must move more existing flags and fields to be dynamic.
> 
> In general, all new flags and fields in mbuf should be dynamic.
> And a work must be done to move existing stuff to free more space
> for more dynamic features.

My bad, I thought dynamic flags can only be used for PMD specific thing.

There is however a cost of using dynamic flag which I think should be avoided
for DPDK spec defined offloads, though it's fine for PMD specific things.

Dynamic offload flags causes application and PMD to use non constant offset 
or shift which are looked up at init, instead of having a constant shift or
offset. This indirection costs some cycles due to extra loads in fast path.


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