OpenBSD as VirtualBox guest halts on reboot on Linux host (not on macOS host)

Question

I have an installation of OpenBSD-current running headlessly within VirtualBox 6.1.16 on a headless Ubuntu Linux (Focal) system. Whenever I reboot the virtual machine (issuing the reboot command as root in the guest system), the virtual machine halts, i.e. it stops running.

This is a nuisance when running the OpenBSD sysupgrade tool to upgrade to a new snapshot release, as it means having to log into the host system to restart the virtual machine twice (after the initial reboot, and then again the upgrade has completed and the system reboots again).

This is different from how the same identical virtual machine functions in VirtualBox 6.1.18 on macOS (using the GUI there). There, the virtual machine is kept running, allowing the system to properly reboot without having to interact with the VirtualBox GUI on the host system. It is also different from how another virtual machine running Kali Linux behaves on the same Ubuntu host system.

On Linux, I start this virtual machine, mymachine, as an unprivileged user like so:

VBoxManage startvm --type headless mymachine

Should I start it in some particular way, or should I change the machine's configuration in some particular way (I haven't found anything that seems related to this in the machine's settings)?

The only substantial difference between the OpenBSD virtual machine on macOS (which reboots properly) and the same machine on the Ubuntu host (which halts instead of rebooting) is that the former uses a VDI disk image while the latter has a VMDK disk image.

Answer 1

Unfortunately, this seems to have resolved itself in a manner that I can't fully explain.

For unrelated reasons, I changed the disk format used by the virtual machine from VMDK to VDI (the machine's disk was previously converted to VMDK from VDI by exporting it from macOS and moving it to Ubuntu). After doing that, I discovered that the machine now does reboots properly, without halting and requiring manual startup from the host machine.

I'm leaving this as an unaccepted answer until someone is able to provide a better explanation as to why the format of the virtual disk may affect the reboot behavior of the virtual machine.