Re: [libvirt] [PATCH] qemu: ensure "pc" machine is always used as default if available

There's multiple things at play. First all the canonical machine type names which are versioned pc-i440fx-2.5.0 pc-i440fx-2.6.0 pc-i440fx-2.7.0 ... pc-i440fx-2.10.0 pc-q35-2.5.0 pc-q35-2.6.0 pc-q35-2.7.0 ... pc-q35-2.10.0 Then there are the convenient aliases mapping to the most recent versioned machine type pc -> pc-i440fx-2.10.0 q35 -> pc-q35-2.10.0 Finally, the versioned machine type that is resolved by the "pc" alias is listed as the default. Even when distros ship custom machine types this has little to no impact on applications. It generally just means the version number part of the machine type changes. 'pc' still resolves to the most recent versioned machine type, and is listed as the default. The key thing is that applicatons (virt-install, virt-manager, oVirt, OpenStack, etc) need to know whether they're using "pc" or "q35" when building guest XML, as that difference is used to trigger different code paths. In looking at the code for various mgmt apps we see alot of patterns like if machine != NULL & strstr(machine, "q35") ...write XML suitable for q35... else ...write XML suitable for pc... IOW, the application is assuming that if the user hasn't requested an explicit type, they'll get "i440fx" based "pc". If QEMU changes its default so that the "q35" alias is marked as the default, then this will break every single mgmt application that we have looked at. This is exactly the kind of thing that libvirt promises will not happen to mgmt apps, so we must guarantee that if no machine type is listed in the XML, then app will always get the i440fx based "pc" machine, and not "q35". WRT your point about the "ubuntu" machine type. I think using such a machine type name is not a desirable thing todo. No application that I've looked at, nor libvirt itself, has logic to know whether "ubuntu" machine type is based on i440fx or q35 chipsets. I expect that, by luck, they'll mostly end up treating it as i440fx based, since most apps do an explicit check for "q35" in the name and assume everything else is i440fx. So the general guidance we give is it that distros should honour the QEMU machine type name prefixes "pc-q35-" and "pc-i440fx-", and only ever change the version suffix if they want to add custom distro specific variants. ie don't invent new prefixes, nor new aliases, as no application will know what todo with those. On that basis I'd ecourage you to look at whether you can phase out the "ubuntu" machine type alias.