Spectre v2 is really complicated to deal with.  Just read this 
recent thread in the LKML
<https://lkml.org/lkml/2018/1/20/158>
I'm impressed how well written those messages are.

Intel Skylake CPUs are particularly problematic.  Those are most of the 
"6th generation Core" processors and some of the "7th generation Core".
<https://en.wikipedia.org/wiki/Skylake_(microarchitecture)>

The indirect branch predictor is a big problem.  The retpoline deals with 
most cases.  On Skylake, this predictor is used in another case: for 
return instructions that cause underflow in the call/return predictor 
stack.

That means that EVERY return instruction is suspect, and the cost of being 
suspect is high.  OK, if you can prove that the call/return predictor 
stack has not underflowed, you can just do a return.  But how?

The best fix is not on the table: it would be great if Intel could patch 
microcode so that the return predictor did not fall back to the indirect 
branch predictor.  It sounds easy, but I infer that it is not technically 
possible.
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