Re: [libvirt] [RFC] qemu: Use virtio-pci by default for mach-virt guests

> > I like that this makes pci truly the default in a simple manner, but  > > still allows switching back to mmio if necessary. On the other hand, it  > > puts the potential "switch" to decide whether or not to use mmio for all  > > devices down into the config of a single device, which is a bit weird to  > > explain. (On the other hand, how often will mmio be used in the future?  > > Maybe it doesn't matter if it's weird to explain...) >  > We really want to push for virtio-pci going forward as it has > a number of advantages, but there are still legacy guest OSs > out there that don't support virtio-pci at all yet we still > want to be able to run.   Changing the default is usually a tricky part and may break some users. I'm not sure that this will save the need to adapt management applications and users.  They will have to adapt in both cases to support legacy and new OSes based on libosinfo or another tool/database.  If we make virtio-pci the default one, which I think is the way it should be used, they will have to make sure that with new libvirt for the same OS they will fallback to virtio-mmio.   From what I can remember we've never done such change of default device model or default address and we always left it no management application or user to change the default to better model.  In case of management applications it's not an issue because they will make sure that the best device models and device address are used.   I'm not against changing the default to virtio-pci, but we may break things for some users and management tools like virt-manager unless they will adapt to this change.