[libvirt] libvirt default machine-type guarantees? (was Re: [Qemu-devel] [PATCH RFC] hw/pc: set q35 as the default x86 machine)

[Daniel P. Berrangé]

On Tue, Jun 05, 2018 at 07:16:56PM +0300, Michael S. Tsirkin wrote:
> On Tue, Jun 05, 2018 at 02:12:32PM +0100, Daniel P. Berrangé wrote:
> > eg old OS versions that only work with PC, or more commonly pre-existing
> > cloud disk images that were built against PC can't be assumed to just
> > work against q35, even if the OS in the image supports it.
> 
> I don't understand that part - when using libvirt domain xml records the
> machine type used for the install, so it seems that installed images
> won't be affected.

There's no libvirt XML for distributed cloud images, it is created whenever
the image is instantiated. Libvirt also doesn't require use of its persistent
guest feature - it is possible to create so called "transient" guests with
libvirt where you just provide an XML doc at each boot up, and libvirt does
not save this XML between boots. Even if an app is using persistent libvirt
XML they might not boot the VM on the same host each time, so the previous
persistent XML may not be available at next boot up. So there's many ways
existing deployed guests can be affected by changes in defaults, even when
libvirt records info in the XML.

Regards,
Daniel
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