Bug stops M4 Macs from virtualizing older macOS versions

Owners of Apple's newest Mac models running M4 chips aren't able to run older versions of macOS in a virtual machine, thanks to a mystery 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.

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.

Running the virtual machine with an older macOS installation results 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 is unknown, due to an inability to access the logs to see what actually happened. There's also no host log failure, with everything seeming to run normally, until the virtual machine's failure to start.

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.

The current best guess is that there is a bug somewhere in the early kernel boot process. This would require fixing the bug in the affected macOS kernels and for new IPSW image files to be shared.

While it has been reported to Apple in Feedback FB15774587 and reported as affecting UTM, it's unlikely that Apple will do much with this issue. The report writes that Apple probably won't go to the trouble, as it hasn't issued revised IPSWs in that way before.

For anyone needing to keep older macOS versions running as a virtual machine, the immediate answer is to stick to Mac models running M3 or earlier, and to seriously think about future Mac upgrades.