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Subject: Re: Configuration vs. compat hints [was Re: [Qemu-devel] [PATCHv3
	03/13] qemu: add routines to manage PCI capabilities]
From: Mark McLoughlin <markmc@redhat.com>
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Date: Fri, 12 Jun 2009 17:48:53 +0100
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To: Anthony Liguori <anthony@codemonkey.ws>
Cc: Carsten Otte <cotte@de.ibm.com>, kvm@vger.kernel.org, "Michael S. Tsirkin" <mst@redhat.com>, Glauber Costa <glommer@redhat.com>, Rusty Russell <rusty@rustcorp.com.au>, qemu-devel@nongnu.org, virtualization@lists.linux-foundation.org, Blue Swirl <blauwirbel@gmail.com>, Christian Borntraeger <borntraeger@de.ibm.com>, Paul Brook <paul@codesourcery.com>, Avi Kivity <avi@redhat.com>

On Fri, 2009-06-12 at 11:12 -0500, Anthony Liguori wrote:
> Mark McLoughlin wrote:
> > On Fri, 2009-06-12 at 09:55 -0500, Anthony Liguori wrote:
> >   
> >> Mark McLoughlin wrote:
> >>     
> >>> On Wed, 2009-06-10 at 20:27 +0100, Jamie Lokier wrote:
> >>>   
> >>> = Solution - Separate configuration from compat hints =
> >>>
> >>> As I suggested before:
> >>>
> >>>   - Allow the VM manager to dump compat hints; this would be an opaque 
> >>>     file format, more like the savevm format than a config file
> >>>   
> >>>       
> >> How is compat hints different from a device tree?
> >>
> >> In my mind, that's what compat hints is.  I don't see another sane way 
> >> to implement it.
> >>     
> >
> > A device tree with a different purpose than a config file.
> >
> > In its simplest form it could be a device tree with a version number for
> > each device[1]. 
> >   
> 
> I think the point is that you don't need version numbers if you have a 
> proper device tree.

How do you add a new attribute to the device tree and, when a supplied
device tree lacking said attribute, distinguish between a device tree
from an old version of qemu (i.e. use the old default) and a partial
device tree from the VM manager (i.e. use the new default) ?

> NB the device tree contains no host configuration information.

So, it wouldn't e.g. include the path to the image file for a block
device? That would always be specified on the command line?

Cheers,
Mark.