2015-06-10 16:32, Olivier MATZ: > On 06/02/2015 03:27 PM, O'Driscoll, Tim wrote: > > From: dev [mailto:dev-bounces at 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 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?
