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From: Paul Brook <paul@codesourcery.com>
Subject: Re: [Qemu-devel] [PATCH 05/11] qemu: MSI-X support functions
Date: Wed, 10 Jun 2009 17:08:15 +0100
References: <cover.1243253205.git.mst@redhat.com>
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To: qemu-devel@nongnu.org
Cc: Carsten Otte <cotte@de.ibm.com>, kvm@vger.kernel.org, "Michael S. Tsirkin" <mst@redhat.com>, Rusty Russell <rusty@rustcorp.com.au>, virtualization@lists.linux-foundation.org, Blue Swirl <blauwirbel@gmail.com>, Christian Borntraeger <borntraeger@de.ibm.com>, Avi Kivity <avi@redhat.com>

On Wednesday 10 June 2009, Michael S. Tsirkin wrote:
> On Wed, Jun 10, 2009 at 04:15:04PM +0100, Paul Brook wrote:
> > > > That's seems just plain wrong to me.
> > > > Loading a VM shouldn't not
> > > > do anything that can't happen during normal operation.
> > >
> > > At least wrt pci, we are very far from this state: load just overwrites
> > > all registers, readonly or not, which can never happen during normal
> > > operation.
> >
> > IMO that code is wrong. We should only be loading things that the guest
> > can change (directly or indirectly).
>
> Making it work this way will mean that minor changes to a device can
> break backwards compatibility with old images, often in surprising ways.
> What are the advantages?

If you can't create an identical machine from scratch then I don't consider 
snapshot/migration to be a useful feature. i.e. as soon as you shutdown and 
restart the guest it is liable to break anyway.

It may be that the snapshot/migration code wants to include a machine config, 
and create a new machine from that. However this is a separate issue, and 
arguably something your VM manager should be handling for you.

Paul