Re: [libvirt] [PATCH 15/18] qemu: Support input model=virtio-{non-}transitional

On 01/24/2019 09:48 AM, Andrea Bolognani wrote: IMO extending the XML we generate by default should only be done for very good reasons, the migration compat issue is not a trivial extension and comes with issues of its own. For example, say we turn model=DEFAULT into model=VIRTIO in the qemu driver. In generic domain format code, we would do: if (!MIGRATABLE || disk->model != VIRTIO) <format disk->model in the XML> But the MIGRATABLE checking is very crude. It doesn't consider version or capabilities of the remote libvirt. We could be migrating to an identical qemu and libvirt version, but we still drop the model=VIRTIO from the XML. Basically for the indefinite future, disk->model=VIRTIO in the XML is never going to be accounted for in migration. This can cause theoretical future issues if non-qemu drivers (or other qemu configs) support model=VIRTIO but it's not their default. For those non-qemu drivers, if disk->model=VIRTIO is present in the XML, the user explicitly requested it, but generic formatting code will never send the value during migration. Let's say we skip the MIGRATABLE check. In fact for most of the cases in this patch series (except maybe for <controller> devs), this would work fine. If we are migrating to same or newer libvirt+qemu, all is good. If we migrate to older libvirt, it won't know to check disk->model at all so won't complain, maybe migration would fail due to qemu compat issues but that's unavoidable. However, the NEXT time we make a similar change, let's say filling in disk->model=FOO for bus=ide, or more likely, input->model=QEMUTABLET for input->bus=usb + input->type=tablet. Now we have to blacklist model=QEMUTABLET from the migratable XML for the indefinite future, because older libvirt _may_ know enough to parse input->model, but doesn't have QEMUTABLET in the enum, and parsing fails. Okay, this is what the MIGRATABLE flag is all about. But it's exposing us to the issue with other drivers mentioned above. And this model=QEMUTABLET MIGRATABLE issue is theoretical but it's an exact case that has happened before, with controller models at least. Basically once you open that door of adding new default output to the XML for common cases, you can't ever close it again for that enum property. Plus there's other downsides for outputting new properties by default: * massive test suite churn: every single virtio disk in the XML2XML tests will now spit out model='virtio', plus all the other devices. This makes backporting suck * libvirt downgrade issues. Previous times we added new output, like input type=keyboard, there were lots of complaints like: I upgraded libvirt via virt-preview to see if it fixed a bug, type=keyboard was added by default, I downgraded libvirt back to regular distro version, my VM disappeared, because the XML parser choked on unknown type=keyboard. Again in this particular case it likely won't cause those issues because model= is new everywhere, but the pattern basically can't be extended forward beyond this one instance. Here's the benefits I see of outputing model=virtio by default * Reports more accurate XML. Minor IMO. * Better chance of maintaining qemu compat across libvirt/qemu upgrades. Encoding model=virtio means we will always try that. Hypothetically 5 years from now libvirt starts defaulting to model=virtio-non-transitional, you might upgrade libvirt and your windows virtio VM doesn't boot. Yeah that's not nice, but it's fixable (user sets model=virtio in the XML). Same reasoning basically applies to migration compat as well I don't think the benefits outweigh the downsides. Of course I may be missing plenty of things so please correct me if I'm wrong Thanks, Cole