Addressing Meltdown and Spectre in the kernel

In RHEL, we turn on all the mitigations by default, and we stuck with the microcode approach rather than deal with getting retpolines right on day one. My recommendation was that after the craziness subsided we got the mainline GCC (and LLVM) communities to hammer that out. We fully expected this to eventually get figured out and then potentially go back and rework/replace the initial mitigations.