Debian Warns Of Hyper Threading Issue With Intel Sky/Kaby Lake CPUs

The Debian project is warning Intel Skylake and Kaby Lake users to disable Hyper Threading (HT) on their CPUs due to a possible issue affecting those with out-of-date microcode.

Intel Skylake and Kabylake (6th and 7th gen CPUs) could "dangerously misbehave" when Hyper Threading is enabled. Users are advised to get an updated BIOS/UEFI while for some Skylake CPUs the updated Intel microcode packages available on Linux have a fix.

More details via this announcement. Intel's notice is labeled as KBL095 or KBW095 for the workstation hardware. From the spec update:

Under complex micro-architectural conditions, short loops of less than 64 instructions that use AH, BH, CH or DH registers as well as their corresponding wider register (e.g. RAX, EAX or AX for AH) may cause unpredictable system behavior. This can only happen when both logical processors on the same physical processor are active