Re: [dpdk-dev] [PATCH v6 01/18] mbuf: redefine packet_type in rte_mbuf

["Zhang, Helin" <helin.zhang@intel.com>]

> -----Original Message-----
> From: Panu Matilainen [mailto:pmatilai@redhat.com]
> Sent: Friday, June 12, 2015 3:24 PM
> To: Thomas Monjalon; Olivier MATZ; O'Driscoll, Tim; Zhang, Helin;
> nhorman@tuxdriver.com
> Cc: dev@dpdk.org
> Subject: Re: [dpdk-dev] [PATCH v6 01/18] mbuf: redefine packet_type in
> rte_mbuf
> 
> On 06/10/2015 07:14 PM, Thomas Monjalon wrote:
> > 2015-06-10 16:32, Olivier MATZ:
> >> On 06/02/2015 03:27 PM, O'Driscoll, Tim wrote:
> >>> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Olivier MATZ
> >>>> On 06/01/2015 09:33 AM, Helin Zhang wrote:
> >>>>> In order to unify the packet type, the field of 'packet_type' in
> >>>>> 'struct rte_mbuf' needs to be extended from 16 to 32 bits.
> >>>>> Accordingly, some fields in 'struct rte_mbuf' are re-organized to
> >>>>> support this change for Vector PMD. As 'struct rte_kni_mbuf' for
> >>>>> KNI should be right mapped to 'struct rte_mbuf', it should be
> >>>>> modified accordingly. In addition, Vector PMD of ixgbe is disabled
> >>>>> by default, as 'struct rte_mbuf' changed.
> >>>>> To avoid breaking ABI compatibility, all the changes would be
> >>>>> enabled by RTE_UNIFIED_PKT_TYPE, which is disabled by default.
> >>>>
> >>>> What are the plans for this compile-time option in the future?
> >>>>
> >>>> I wonder what are the benefits of having this option in terms of
> >>>> ABI compatibility: when it is disabled, it is ABI-compatible but
> >>>> the packet-type feature is not present, and when it is enabled we
> >>>> have the feature but it breaks the compatibility.
> >>>>
> >>>> In my opinion, the v5 is preferable: for this kind of features, I
> >>>> don't see how the ABI can be preserved, and I think packet-type
> >>>> won't be the only feature that will modify the mbuf structure. I
> >>>> think the process described here should be applied:
> >>>> http://dpdk.org/browse/dpdk/tree/doc/guides/rel_notes/abi.rst
> >>>>
> >>>> (starting from "Some ABI changes may be too significant to
> >>>> reasonably maintain multiple versions of").
> >>>
> >>> This is just like the change that Steve (Cunming) Liang submitted
> >>> for Interrupt Mode. We have the same problem in both cases: we want
> >>> to find a way to get the features included, but need to comply with
> >>> our ABI policy. So, in both cases, the proposal is to add a config
> >>> option to enable the change by default, so we maintain backward
> compatibility.
> >>> Users that want these changes, and are willing to accept the
> >>> associated ABI change, have to specifically enable them.
> >>>
> >>> We can note in the Deprecation Notices in the Release Notes for 2.1
> >>> that these config options will be removed in 2.2. The features will
> >>> then be enabled by default.
> >>>
> >>> This seems like a good compromise which allows us to get these
> >>> changes into 2.1 but avoids breaking the ABI policy.
> >>
> >> Sorry for the late answer.
> >>
> >> After some thoughts on this topic, I understand that having a
> >> compile-time option is perhaps a good compromise between keeping
> >> compatibility and having new features earlier.
> >>
> >> I'm just afraid about having one #ifdef in the code for each new
> >> feature that cannot keep the ABI compatibility.
> >> What do you think about having one option -- let's call it
> >> "CONFIG_RTE_NEXT_ABI" --, that is disabled by default, and that would
> >> surround any new feature that breaks the ABI?
> >>
> >> This would have several advantages:
> >> - only 2 cases (on or off), the combinatorial is smaller than
> >>     having one option per feature
> >> - all next features breaking the abi can be identified by a grep
> >> - the code inside the #ifdef can be enabled in a simple operation
> >>     by Thomas after each release.
> >>
> >> Thomas, any comment?
> >
> > As previously discussed (1to1) with Olivier, I think that's a good
> > proposal to introduce changes breaking deeply the ABI.
> >
> > Let's sum up the current policy:
> > 1/ For changes which have a limited impact on the ABI, the backward
> > compatibility must be kept during 1 release including the notice in
> doc/guides/rel_notes/abi.rst.
> > 2/ For important changes like mbuf rework, there was an agreement on
> > skipping the backward compatibility after having 3 acknowledgements and an
> 1-release long notice.
> > Then the ABI numbering must be incremented.
> >
> > This CONFIG_RTE_NEXT_ABI proposal would change the rules for the second
> case.
> > In order to be adopted, a patch for the file
> > doc/guides/rel_notes/abi.rst must be submitted and strongly acknowledged.
> >
> > The ABI numbering must be also clearly explained:
> > 1/ Should we have different libraries version number depending of
> CONFIG_RTE_NEXT_ABI?
> > It seems straightforward to use "ifeq" when LIBABIVER in the Makefiles
> 
> An incompatible ABI must be reflected by a soname change, otherwise the
> whole library versioning is irrelevant.
> 
> > 2/ Are we able to have some "if CONFIG_RTE_NEXT_ABI" statement in
> the .map files?
> > Maybe we should remove these files and generate them with some
> preprocessing.
> >
> > Neil, as the ABI policy author, what is your opinion?
> 
> I'm not Neil but my 5c...
> 
> Working around ABI compatibility policy via config options seems like a slippery
> slope. Going forward this will likely mean there are always two different ABIs for
> any given version, and the thought of keeping track of it all in a truly compatible
> manner makes my head hurt.
> 
> That said its easy to understand the desire to move faster than the ABI policy
> allows. In a project where so many structs are in the open it gets hard to do much
> anything at all without breaking the ABI.
> 
> The issue could be mitigated somewhat by reserving some space at the end of
> the structs eg when the ABI needs to be changed anyway, but it has obvious
> downsides as well. The other options I see tend to revolve around changing
> release policies one way or the other: releasing ABI compatible micro versions
> between minor versions and relaxing the ABI policy a bit, or just releasing new
> minor versions more often than the current cycle.
> 
> 	- Panu -

Does it mean releasing R2.01 right now with announcement of all ABI changes, which
based on R2.0 first, and then releasing R2.1 several weeks later with all the code changes?

- Helin