Wow, I had assumed that retpoline was a heavy hammer that we could use to squash this class of bugs until new hardware designs arrive.
…In the long run, we believe that these patches are ad hoc and that new attack vectors will continue to emerge. Current systems are fundamentally insecure un- less speculation is disabled. However, we believe that it is possible to design future generations of CPUs that re- tain speculation but also close speculative leakage chan- nels, for example by keeping speculative data in separate CPU structures than committed data.
Of course some of those side channels are issues in sensitive code regardless of speculative execution... but there seems to be an interesting place where the two overlap. How do you fix that?