It was because of vbox Settings, in which checked (by default) was System - Motherboard - “Enable I/O APIC”…🤕
Although I don’t know it has something to do with the failure, I found VirtualBox documentation telling:
software support for I/O APICs has been unreliable with some OSes other than Windows.
It happened just after I finished answering to the OpenBSD installer and installation began. (See the first line below.)
Installing bsd 0% (...) dc_atapi_start: not ready, st = 40 fatal protection fault in supervisor mode trap type 4 code 0 rip ff(...) cs 8 rflags 10286 cr2 42f9c8 cpl 6 rsp ff(...) gsbase 0xff(...) kgsbase 0x0 panic: trap type 4, code=0, pc=ff(...) syncing disks...6 6 (...)
I had to uncheck “Enable I/O APIC” of System - Motherboard in VirtualBox Settings to successfully install OpenBSD as guest.
Anyway, it solved my problem 🙂
This post is based on my tweets.