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

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.