On Mon, 2016-10-24 at 17:47 +0200, Pavel Hrdina wrote: > > > 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. You raise very good points, thanks for your input! :) AFAICT the only use case that we'd risk breaking is installing a legacy guest OS that doesn't support virtio-pci without requiring the user to explicitly ask for virtio-mmio addresses. Once libosinfo has learned about this, and virt-manager has been updated to query libosinfo and switch to virtio-mmio automatically if required, would you be okay with this change? I think for aarch64 we're still in a phase where we can afford to take some tradeoffs when it comes to compatibility, if they're properly motivated: in this specific case, seeing as basic stuff like device hotplug has simply never worked for virtio-mmio, I'm fairly confident nobody will want to stick with virtio-mmio for very long now that virtio-pci is finally viable. -- Andrea Bolognani / Red Hat / Virtualization -- libvir-list mailing list libvir-list@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/libvir-list