Re: [Qemu-devel] [PATCH] Change virtio-console to PCI_CLASS_SERIAL_OTHER

[Anthony Liguori <aliguori@us.ibm.com>]

Mark McLoughlin wrote:
> On Wed, 2009-05-27 at 09:21 -0500, Anthony Liguori wrote:
>
>   
>> We need a mechanism to toggle this for both this and virtio-blk.  The 
>> reason a toggle is needed is so that 0.11 can create the same device 
>> model as 0.10.
>>     
>
> Okay, so the scenario is:
>
>   - 0.10 guest running on source machine
>
>   - migrate to dest machine running 0.11
>
>   - the device model cannot change or the guest OS will get confused
>   

Correct, but in this case, the problem is when you shutdown and start up 
the VM again.  In the process, it may trigger something like a driver 
reinstallation or worse, reactivation.  It's necessary to be able to 
prevent this by creating exactly the same device model that was 
previously created.

This is can't really be a hard rule yet without the machine config, but 
we should do our best when we can.

> How about running qemu on the dest machine with -model-version=0.10 ?
>   

Linear versioning is a bit too fickle esp due to distro backporting.

> Nasty thing about it is that the distributions couldn't cherry-pick
> changes like this because they'd deviate from the device model contract
> for a given upstream version.
>
> Could we not put this stuff in savevm and change the device model
> post-init?
>   

It's shutdown/restart that's the problem.  Windows uses a lot of 
different information to calculate whether activation is required again 
and even small changes can end up pushing things over the edge.

Regards,

Anthony Liguori

> Cheers,
> Mark.
>
>