Just came across this.
I guess this is why it's good to read the Debian Mailing List even if you don't use Debian.
One sharp eyed OCaml developer noticed a glitch in short loops of less than 64 instructions affecting Hyperthreading on Kabylake and Skylake CPU's.
Install the relevant microcode or BIOS update.
Scabby lake. Sorry, can't help myself.
Is this bug caused by some other update? Or just a rare occurrence? Seems like the parts have been around long enough this would have been found sooner.
AMD is laughing right now...
Disabling Hyper threading is ridiculous, but necessary if you have issues. Patches to follow