This made me wonder if a better approach here would be to move the logic that determines
if LOST_TICK_POLICY_SLEW is available into the "lost_tick_policy" property
setter defined at
https://gitlab.com/qemu-project/qemu/-/blob/master/hw/core/qdev-properties-system.c#L558.
If you look at the code directly below the link above you can see how
set_blocksize() overrides the .set function for qdev_prop_blocksize to provide
additional validation, which is similar to what we are trying to do here.
I think it may be possible to come up with a similar solution for
qdev_prop_losttickpolicy which makes use of the logic you suggested before i.e.
MachineState *ms = MACHINE(qdev_get_machine());
if (!object_dynamic_cast(OBJECT(ms), TYPE_X86_MACHINE)) {
....
}
which can then emit a suitable warning or return an error accordingly. A quick
glance at hw/core/qdev-properties-system.c suggests there are a number of
similar examples showing how this could be done.