You are very wrong about that ;) https://lkml.org/

Clearly you and the person you replied to are talking about very different things.

I opened a request on Intel's Community Site requesting microcode changelog documentation. Hopefully I'll get an answer.

https://communities.intel.com/message/518872#518872

Please do click the "I have the same question" button on that thread. Hopefully if it gets enough heat Intel will answer our questions.

As best I can tell at the moment these changes are necessary in order to enable the IBRS and IBPB changes that are currently being discussed on LKML. AFAIK, IBRS and IBPB mitigate against Spectre V2 performance regressions that are introduced with the retpoline changes.