AMD Adding STIBP "Always-On Preferred Mode" To Linux

Initially during the Linux 4.20 kernel merge window with the STIBP addition for cross-hyperthread Spectre V2 mitigation it was turned on by default for all processes. But that turned out to have a sizable performance hit so the behavior was changed to only turn it on for processes under SECCOMP or when requested via the PRCTL interface. However, AMD is landing a patch that for select CPUs will have an always-on mode as evidently that's preferred for some AMD processors...