Difference between 'current_machine' vs MACHINE(qdev_get_machine())

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

Hi,

"hw/boards.h" declare current_machine, and vl.c defines it:

     current_machine = 
MACHINE(object_new_with_class(OBJECT_CLASS(machine_class)));
     object_property_add_child(object_get_root(), "machine",
                               OBJECT(current_machine), &error_abort);

The bigger user of 'current_machine' is the accel/KVM code.

Recently in a0628599f..cc7d44c2e0 "Replace global smp variables with 
machine smp properties" we started to use MACHINE(qdev_get_machine()).

qdev_get_machine() resolves the machine in the QOM composition tree.
I am confused by this comment:

   /* qdev_get_machine() can return something that's not TYPE_MACHINE
    * if this is one of the user-only emulators; in that case there's
    * no need to check the ignore_memory_transaction_failures board flag.
    */

Following a0628599f..cc7d44c2e0, a5e0b33119 use 'current_machine' again.

What are the differences between both form, when should we use one or 
another (or can we use a single one?). Can this break user-only mode?

Thanks,

Phil.