[libvirt] [PATCH] qemu: Add USB and memory balloon by default for aarch64/virt guests

[Daniel P. Berrangé]

On Thu, Feb 01, 2018 at 02:44:03PM +0100, Andrea Bolognani wrote:
> On Thu, 2018-02-01 at 12:54 +0000, Daniel P. Berrangé wrote:
> > > If an application breaks based on the USB controller being either
> > > present or not present, then they shouldn't be relying on libvirt's
> > > default but rather explicitly opt either in or out.
> > 
> > It is not merely the mgmt application that may break, but the
> > guest OS inside too. When we suddenly expose new hardware to a
> > guest that was not previously present you can certainly trigger
> > latent problems in the guest OS. It could slow boot at a key
> > phase, or trigger loading of bad drivers, or any number of other
> > things that can occurr when you change hardware visible to an OS.
> 
> Note that I'm not advocating adding controllers or any other
> hardware to *existing* guests - that would clearly be a guest ABI
> breakage and thus Extremely Bad™. For newly-defined guests, however,
> none of the above applies AFAICT.

There's no practical way to distinguish an existing guest from a
new guest being provisioned. With transient domains they are one &
the same. Even with persistent guests the distinction is far from
clear.

Regards,
Daniel
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