OpenBSD has disabled Intel’s hyper-threading performance-boosting feature by default. The feature is Intel’s implementation of Simultaneous Multi-Threading (SMT) that allows the operating system to use a virtual core for each physical core present in processors in order to improve performance. OpenBSD maintainer Mark Kettenis said such processor implementations could lead to Spectre-style timing attacks. In cryptography, side-channel timing attacks allows attackers to compromise a system by analyzing the time taken to execute cryptographic algorithms.
Source: https://thehackernews.com/2018/06/openbsd-hyper-threading.html