(Several of us started an offline discussion on this topic, and it quickly became complicated, so we decided it should continue upstream. Here is a synopsis of the discussion so far (as *I've* interpreted it, so corrections are welcome and apologies in advance for anything I got wrong!) Some of the things are stated here as givens, but feel free to rip them apart.) Summary of the problem: 1) We want to persuade libvirt+QEMU users to move away from the i440fx machinetype in favor of Q35. (NB: Someday this *might* lead to the ability to deprecate and even remove the 440fx machinetype, but even if that were to happen, it would be a *very long* time from now, so this discussion is *not* about that!) 2) When Q35 machinetype is used, libvirt assigns virtio devices to a slot on a PCI Express controller (because why have modern PCIe controllers/slots available but force everything onto clunky old legacy controllers?). 3) When a virtio device is plugged into an Express controller, QEMU disables the device's IO port space, and it is put into "modern-only" mode (this is done to avoid a rapid exhaustion of limited IO port space). 4) modern-only virtio devices won't work with a legacy (virtio-0.9-only) guest driver, because virtio-0.9 requires IO port space. 5) Some guest OSes that we still want to support (and which would otherwise work okay on a Q35 virtual machine) have virtio drivers too old to support virtio-1.0 (CentOS6 and RHEL6 are examples of such OSes), but due to the chain of reasons listed above, the "standard" config for a Q35 guest generated by libvirt doesn't support virtio-0.9, hence doesn't support these guest OSes. And here's a list of possible solutions to this problem (note that "consumers" means management applications such as OpenStack, oVirt, virt-manager, virt-install, gnome-boxes, etc. In all cases, it's assumed that the consumer's decision on the action to take will be based on information from libosinfo). For completeness, I've included even the possibilities that have been rejected, along with a brief synopsis of (at least part of) the reason for rejection: (1) Add some way libvirt consumers can ask libvirt to place virtio devices on a legacy pci slot instead of pcie when the machinetype is q35 (qemu sets virtio devices in legacy PCI slots to transitional mode, so io port space is enabled and virtio-0.0 drivers will work). This has been proposed on libvir-list, but rejected. Here is the most elquently stated reasoning for the rejection I could find (with thanks to Dan Berrange): The domain XML is a way to express the configuration of the guest virtual machine. What we're talking about here is a policy tunable for an internal libvirt QEMU driver algorithm, as so does not belong anywhere in the domain XML. (2) Add full-blown pci enumeration support to all libvirt consumers (i.e. they will need to build a model of the PCI bus topology of each guest, and keep track of which addresses are in use). They can then manually place virtio devices on legacy pci slots (again, triggering transitional mode) when the intended guest OS doesn't support virtio-0.9. (This is seen as requiring too much duplicated effort for development and support/maintenance, since up until now libvirt has been the single point of action for PCI address assignment (well, QEMU can do it too, but for > 10 years libvirt has *always* provided full PCI addresses for all devices) (3) Add virtio-1.0 support to all guest OSes. If this is done, existing libvirt configs will work. (Aside from the difficulty of backporting, and the fact that there are going to be some OSes that don't get it *at all*, there will always be older releases that haven't gotten the backport. So this isn't a complete solution). (4) Consumers can continue using the 440fx machinetype for guest OSes that don't support virtio-0.9 (This would work, but perpetuates use of the 440fx machinetype, and all for just this one reason (at least in the case of CentOS6/RHEL6, which otherwise work just fine with Q35)). (5) Introduce virtio-0.9, virtio-1.0 models in libvirt which are explicitly legacy-only and modern-only. QEMU doesn't need to change, as libvirt can simply set the right params on existing QEMU models to force the behavior. (NB: it's unclear to me whether virtio-0.9 simply won't work without forcing the device to be on a legacy PCI slot, or if that's just "a very bad idea" because it will mean that the device uses up extra io port space) The offline discussion had basically come to the point of saying that options (4) and (5) were the only reasonable ones, with option (5) being preferred (I think). As a starter for continuing the discussion, it seems to me that for option (5): a) we don't really need the virtio-1.0 model, since that's what you currently get anyway when you ask for "virtio" on Q35 (and on 440fx, "virtio" gives you transitional, which works for everybody). b) Rather than a "legacy-only" model for virtio-0.9, it would be more useful to have "transitional". This way the config would work for older OSes that don't support virtio-1.0, and when/if the OS was upgraded such that it supported virtio-1.0, that would be automatically used without needing to change the config. c) Even if it's possible to force a device on an Express slot into transitional mode, this is extremely wasteful of io port space, so libvirt should consider virtio-0.9 devices to be legacy PCI, and thus plug them into legacy PCI slots. And once we're doing this, it's unnecessary to add any extra option to the qemu commandline to force legacy support (i.e. transitional mode), as that is what QEMU already does when the device is connected to a legacy PCI slot. So making the naive assumption that we agree on implementing option (5) and there are no objections to my points a-c (Hah! As if!), how does this sound as a plan: A) libosinfo starts telling consumers that the preferred virtio device model for the relevant OSes is "virtio-0.9", and leaves the recommendation for other OSes as "virtio". B) libvirt adds a "virtio-0.9" model for all virtio devices that actually have virtio-0.9 support (a couple of devices never existed prior to virtio-1.0 (rng and ???) so virtio-0.9 would be nonsensical for them). C) inside libvirt, the implementation of the "virtio-0.9" model is identical to "virtio", except that the VIR_PCI_CONNECT_TYPE flags for these devices contain VIR_PCI_CONNECT_TYPE_PCI rather than VIR_PCI_CONNECT_TYPE_PCIE, resulting in those devices being assigned to a legacy PCI slot, and thus they would be transitional mode by default. (If there is disagreement about putting these devices on a legacy PCI slot, then (C) could be changed to add "disable-legacy=off" to the qemu commandline. But again, even if that works, it would use up 4k of IO port space for each device, causing it to rapidly run out, and I don't think that should be the default mode of operation).