As nifty as Apple's M4 chip is, sometimes there's still a kink or two to work out.
Owners of Apple's newest M4-equipped Macs have noticed that they are unable to run older versions of macOS in a virtual machine given a strange booting issue. Running a virtual machine with older generations of macOS can have its uses, ranging from security and development to simply being able to run software that won't work with newer macOS versions. However, there seems to be a problem when it comes to using a virtual machine in this way on an M4 chip.
The bug, which was detailed by the Eclectic Light Co. and pointed out by Csaba Fitzl, issues have been found when trying to run macOS versions released before macOS 13.4 Ventura in a virtual machine on an M4 Mac. On an M1, M2, or M3-generation Mac, there is no issue, as it only happens on the M4. During tests, running the virtual machine with an older version of macOS resulted in a black screen and a boot failure. This was found to be the case regardless of the settings used for the virtual machine itself, and even in Recovery Mode.
The actual source of the issue becomes that much harder to pin down due to an inability to access the logs to see what happened. There's also no host log failure that can be seen, as everything seems to run normally until the virtual machine is unable to successfully boot.
The only real clue that has been uncovered is seeing that, despite the allocation of multiple virtual cores, just one is actually active on the host. It is presumed that the failure happens at a point in the boot process before the virtual machine kernel boots other cores, which is early in the kernel boot phase.
It's thought that there may be a bug in the early kernel boot process, and this would require fixing the bug in the affected macOS kernels and for new IPSW image files to be created and distributed.
For anyone needing to keep older macOS versions running as a virtual machine, the immediate answer is to stick to Mac models running M1, M2, or M3 hardware.
Stay tuned for additional details as they become available.