Errata patches for vmm have been released for OpenBSD 6.3 and 6.4. GDT and IDT limits were improperly restored during VMM context switches.
Binary updates for the amd64 and i386 platforms are available via the syspatch utility. Source code patches can be found on the respective errata page: https://www.openbsd.org/errata63.html https://www.openbsd.org/errata64.html As these affect the kernel, a reboot will be needed after patching.