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RE: [Xen-devel] [PATCH 00/02] firmware changes as part of QEMU/Xen merge

To: Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx>
Subject: RE: [Xen-devel] [PATCH 00/02] firmware changes as part of QEMU/Xen merge.
From: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
Date: Mon, 18 Oct 2010 16:10:59 +0100
Cc: Anthony Perard <anthony.perard@xxxxxxxxxx>, Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>, Xen Devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Keir Fraser <keir@xxxxxxx>
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On Mon, 18 Oct 2010, Ian Pratt wrote:
> > We cannot make changes to qemu-dm that are not backwards compatible with
> > saved guests. Not only does this affect qemu in this case, but the
> > register
> > blocks you move around also affect Xen's emulation of ACPI pmtimer. Both
> > will/would need to be configurable based on whether this is an old or new
> > guest.
> > 
> > Hence this has to be strongly NACKed in its current form.
> It's getting to the point where we might have to consider having two qemu 
> binaries, one for compatibility, one that new VMs are transitioned on to.
> Things are now so different that I'm worried that installed guests might 
> complain even when just fresh booted on the new qemu. This is something that 
> we'd need to test and devise workarounds. Trying to retain saved image 
> compatibility with a single binary looks ugly.

I agree.

However in this particular case the firmware changes are only for ACPI
support and we might be able to support both the old AND the new ioports
in qemu-xen, so that we can switch the firmware to the new interface
while keeping backward compatibility in qemu.

That said I doubt we'll be able to do the same in upstream qemu, so at
some point the old ioport interface will probably be dropped.

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