Re: [Qemu-devel] [PATCH 28/35] kvm: x86: Introduce kvmclock device to save/restore its state

[Anthony Liguori]

On 01/19/2011 11:19 AM, Daniel P. Berrange wrote:
> In our past experiance though, *not* specifying attributes like
> these has also been pretty bad from a forward compatibility
> perspective too. We're kind of damned either way, so on balance
> we decided we'd specify every attribute in qdev that's related
> to unique identification of devices&  their inter-relationships.
> By strictly locking down the topology we were defining, we ought
> to have a more stable ABI in face of future changes. I accept
> this might not always work out, so we may have to adjust things
> over time still. Predicting the future is hard :-)
>    

There are two distinct things here:

1) creating exactly the same virtual machine (like for migration) given 
a newer version of QEMU

2) creating a reasonably similar virtual machine given a newer version 
of QEMU

For (1), you cannot use -M pc.  You should use things like bus=X,addr=Y 
much better is for QEMU to dump a device file and to just reuse that 
instead of guessing what you need.

For (2), you cannot use bus=X,addr=Y because it makes assumptions about 
the PCI topology which may change in newer -M pc's.

I think libvirt needs to treat this two scenarios differently to support 
forwards compatibility.

Regards,

Anthony Liguori

> Daniel
>