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Date: Wed, 24 Jun 2015 16:56:18 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 0/2] target-i386: "custom" CPU model +
 script to dump existing CPU models
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To: Peter Maydell <peter.maydell@linaro.org>
Cc: Michael Mueller <mimu@linux.vnet.ibm.com>, "Michael S. Tsirkin" <mst@redhat.com>, QEMU Developers <qemu-devel@nongnu.org>, Alexander Graf <agraf@suse.de>, Christian Borntraeger <borntraeger@de.ibm.com>, Igor Mammedov <imammedo@redhat.com>, Jiri Denemark <jdenemar@redhat.com>, Richard Henderson <rth@twiddle.net>, =?UTF-8?B?QW5kcmVhcyBGw6RyYmVy?= <afaerber@suse.de>, Eduardo Habkost <ehabkost@redhat.com>



On 24/06/2015 16:54, Peter Maydell wrote:
> > It's certainly okay for libvirt and OpenStack to use the host CPU
> > features in order to check whether a node will run a given VM.  However,
> > libvirt should trust that QEMU developers will not prevent a VM from
> > running on a previously viable host, just because you change the machine
> > type.
> 
> Note that this might be true for x86 but isn't (in full generality)
> necessarily so for other architectures like ARM. For instance if
> you have a host machine with a GICv3 with no v2-back-compatibility
> support, then you can run a VM whose machine type specifies a GICv3,
> but not machine types that require a GICv2...

Hmm, that was too terse.  Just because you change the machine type *to a
newer version* with the same chipset.

Paolo