[Qemu-devel] Re: [PATCH v2 1/2] hw/arm_sysctl.c: Add the Versatile Express system registers

[Paolo Bonzini]

On 03/05/2011 05:50 PM, Peter Maydell wrote:
> (1) Is there supposed to be any kind of guard on trying to
> do a vmsave on a system with devices that don't implement
> save/load? IME it just produces a snapshot which doesn't
> work when you reload it...

I think you're right, devices currently have to call 
register_device_unmigratable manually.  I guess you could add support to 
qdev, so that qdev-ified devices could specify a special "forbid 
migration" value for the vmsd field.

Alternatively, you could have NULL mean "forbid migration" and a special 
value for "do not save anything, it will just work".

> (2) How do you track incompatible changes at the machine
> level? For instance, suppose we accidentally forgot to
> model a NOT gate in an IRQ line, so we add a qemu_irq_invert()
> to the machine init function. None of the devices have
> changed, but you can't load the state of an old version
> of the machine into a new version, because then the
> devices on either end of the inverter would be inconsistent
> about the state of the line. But there's no version number
> for a machine as far as I can see.

You can change the machine model and keep the incompatible machine as a 
separate model.  A machine can specify compatibility properties that are 
meant exactly for this kind of bug-compatible behavior.  Reloading the 
VM must be done with the correct -M switch for the old model.

Examples of how to do this are in hw/pc_piix.c (which will provide good 
ideas for further grepping, too :)).

Paolo