[PATCH RFC 0/2] x86: don't enable deprecated ISA extensions by default

[Jan Beulich]

IMO it has been a mistake from the beginning that all ISA extensions are
enabled by default. This way possible flaws e.g. in the compiler simply
go unnoticed. But that ship has sailed. However, for extensions where
support was removed from the compiler, we should at least consider whether
to disable them by default. That's what this series proposes (but see a
timing related RFC remark in patch 2).

Beyond that there are further discontinued extensions, which however gcc
still supports: HLE and VP2INTERSECT on the Intel side, TBM, XOP, FMA4,
and LWP for AMD. Question is what (if anything) to do there, which may
depend on what gcc's plans are.

1: default to not accepting MPX insns
2: default to not accepting Xeon Phi insns

Jan