Такие дела.
SMT (Simultanious Multi Threading) implementations typically share TLBs and L1 caches between threads. This can make cache timing attacks a lot easier and we strongly suspect that this will make several spectre-class bugs exploitable...
Since many modern machines no longer provide the ability to disable Hyper-threading in the BIOS setup, provide a way to disable the use of additional processor threads in our scheduler.
And since we suspect there are serious risks, we disable them by default. This can be controlled through a new hw.smt sysctl. For now this only works on Intel CPUs when running OpenBSD/amd64. But we're planning to extend this feature to CPUs from other vendors and other hardware architectures...
https://www.mail-archive.com/source-changes@openbsd.org/msg99141.html
