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Date: Wed, 10 Jun 2009 19:26:44 +0300
From: "Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [Qemu-devel] [PATCH 05/11] qemu: MSI-X support functions
Message-ID: <20090610162644.GJ28601@redhat.com>
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To: Paul Brook <paul@codesourcery.com>
Cc: Carsten Otte <cotte@de.ibm.com>, kvm@vger.kernel.org, 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>, Avi Kivity <avi@redhat.com>

On Wed, Jun 10, 2009 at 05:08:15PM +0100, Paul Brook wrote:
> 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.

Why is liable to break?
Configuration does not change until you load another image.
Look here:

void msix_reset(PCIDevice *dev)
{
    if (!(dev->cap_present & QEMU_PCI_CAP_MSIX))
        return;

...
}

So once you load and image with MSIX capability off,
it will stay off across guest restarts.

> 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

Since the image already has a necessary information, duplicating
it in a separate machine config will just lead to errors.

-- 
MST