The system's capability to bypass the firmware when booting a new OS image has dependencies on device drivers' implementation of a new device operation entry point, quiesce. On supported drivers, this implementation quiesces a device, so that at completion of the function, the driver no longer generates interrupts or access memory. This implementation also resets the device to a hardware state, from which the device can be correctly configured by the driver's attach routine, without a power cycle of the system or being configured by the firmware. For more information about this functionality, see the quiesce(9E) and dev_ops(9S) man pages.
Not all device drivers implement the quiesce function. For troubleshooting instructions, see x86: Troubleshooting Conditions That Might Prevent Fast Reboot From Working.