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

[Mark McLoughlin <markmc@redhat.com>]

On Wed, 2009-05-27 at 17:37 -0500, Anthony Liguori wrote:
> 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.

Okay.

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

There's not much point in doing this unless there's some practical way
to use it.

If we e.g. add a 'class' parameter for virtio-blk and virtio-console,
we'd also need to add some way for a management tool to figure out what
the class value is when it initially creates the guest and, for ever
more, supply that value.

Options I see:

  1) Add a monitor command to query the class values and delay actually 
     changing the values until the next release cycle

  2) Use 'info version' to achieve this - management tools would need to
     start retaining the version that guests were created with and
     supplying that version to qemu on the command line. As you say,
     though, linear versioning sucks

  3) Decide we can only make these guarantees with the machine config
     stuff - management tools would need to query a running qemu for a
     detailed machine config when initially creating the guest and
     always supply that when later starting the guest

Cheers,
Mark.