[PATCH] D41723: Introduce the "retpoline" x86 mitigation technique for variant #2 of the speculative execution vulnerabilities disclosed today, specifically identified by CVE-2017-5715, "Branch Target Injection", and is one of the two halves to Spectre..

Mon Jan 15 18:45:15 PST 2018

jyknight added a comment.

Per kernel [https://marc.info/?l=linux-kernel&m=151580566622935&w=2] and gcc [https://gcc.gnu.org/ml/gcc-patches/2018-01/msg01059.html], it seems AMD needs there to be an lfence in the speculation trap (and the pause is not useful for them, but does no harm). There seems to be some speculation (but no confirmation yet?) that pause *is* necessary vs lfence on intel. So in order to work generically, they seem to be suggesting using both instructions:

  loop:
    pause
    lfence
    jmp loop

Some more links
https://gcc.gnu.org/ml/gcc-patches/2018-01/msg01209.html
and final patch:
https://github.com/gcc-mirror/gcc/commit/a31e654fa107be968b802786d747e962c2fcdb2b

Repository:
  rL LLVM

https://reviews.llvm.org/D41723