Re: [dpdk-dev] [dpdk-announce] important design choices - statistics - ABI

[Thomas Monjalon <thomas.monjalon@6wind.com>]

2015-06-19 09:02, Neil Horman:
> On Fri, Jun 19, 2015 at 02:32:33PM +0200, Thomas Monjalon wrote:
> > 2015-06-19 06:26, Neil Horman:
> > > On Thu, Jun 18, 2015 at 04:55:45PM +0000, O'Driscoll, Tim wrote:
> > > > For the 2.1 release, I think we should agree to make patches that change
> > > > the ABI controllable via a compile-time option. I like Olivier's proposal
> > > > on using a single option (CONFIG_RTE_NEXT_ABI) to control all of these
> > > > changes instead of a separate option per patch set (see
> > > > http://dpdk.org/ml/archives/dev/2015-June/019147.html), so I think we
> > > > should rework the affected patch sets to use that approach for 2.1.
> > > 
> > > This is a bad idea.  Making ABI dependent on compile time options isn't a
> > > maintainable solution.  It breaks the notion of how LIBABIVER is supposed to
> > > work (that is to say you make it impossible to really tell what ABI version you
> > > are building).
> > 
> > The idea was to make LIBABIVER increment dependent of CONFIG_RTE_NEXT_ABI.
> > So one ABI version number refers always to the same ABI.
> > 
> > > If you have two compile time options that modify the ABI, you
> > > have to burn through 4 possible LIBABIVER version values to accomodate all
> > > possible combinations, and then you need to remember that when you make them
> > > statically applicable.
> > 
> > The idea is to have only 1 compile-time option: CONFIG_RTE_NEXT_ABI.
> > 
> > Your intent when introducing ABI policy was to allow smooth porting of
> > applications from a DPDK version to another. Right?
> > The adopted solution was to provide backward compatibility during 1 release.
> > But there are cases where it's not possible. So the policy was to notice
> > the future change and wait one release cycle to break the ABI (failing
> > compatibility goals).
> > The compile-time option may provide an alternative DPDK packaging when the
> > ABI backward compatibility cannot be provided (case of mbuf changes).
> > In such case, it's still possible to upgrade DPDK by providing 2 versions of
> > DPDK libs. So the existing apps continue to link with the previous ABI and
> > have the possibility of migrating to the new one.
> > Another advantage of this approach is that we don't have to wait 1 release
> > to integrate the changes.
> > The last advantage is to benefit early of these changes with static libraries.
> 
> Hm, ok, thats a bit more reasonable, but it still seems shaky to me.
> Implementing an ABI preview option like this implies the notion that, after a
> release, you have to remove all the ifdefs that you inserted to create the new
> ABI.  That seems like an easy task, but it becomes a pain when the ABI delta is
> large, and is predicated on the centralization of work effort (that is to say
> you need to identify someone to submit the 'remove the NEXT_ABI config ifdefs
> from the build' patch every release.

It won't be so huge if we reserve the NEXT_ABI solution to changes which cannot
have easy backward compatibility with the compat macros you introduced.
I feel I can do the job of removing the ifdefs NEXT_ABI after each release.
At the same time, the deprecated API, using the compat macros, will be removed.

> What might be better would be a dpdk-next branch (or even a dpdk-next tree, of
> the sort that Thomas Herbert proposed a few weeks ago).

This tree was created after Thomas' request:
	http://dpdk.org/browse/next/dpdk-next/

> Patches that aren't ABI stable can be put on the next-branch/tree in thier
> final format.  You can delcare the branch unstable (thereby reserving your
> right to rebase it).  People can use that to preview the next ABI version
> (complete with the update LIBABIVER bump), and when you release dpdk-X,
> the new ABI for dpdk-X+1 is achieved by simply merging.

Having this tree living would be a nice improvement but it won't provide any
stable (and enough validated) releases to rely on.