15/05/2020 15:44, 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.
Yes there is a cost. We described it quite clearly last year. The default rule is now to add new flags and fields as dynamic. In case the rule was not clear, I will send a patch to insert some notes in the code and the doc. If you disagree with this new rule, you will have to give very good arguments.
