Re: [libvirt] [PATCH v3 6/8] qemu: use virDomainPCIAddressIsMulti() to determine multifunction setting

On Mon, 2016-12-19 at 10:23 -0500, Laine Stump wrote: > If the multifunction attribute isn't set in the config for the device > at function 0 of a slot used for multifunction, it would previously > have been an error. This patch will instead automatically correct the > omission (but only if it hasn't been set at all - if someone > explicitly has "multifunction='off'" on function 0, or > "multifunction='on'" when function != 0, we have to assume they have a > reason for that). >  > This effectively obsoletes the requirement of specifying > multifunction='on' in the config, although you're still free to do > so. Note that if you migrate a domain that needs an implied > "multifunction='on'" back to any older libvirt that doesn't have it, > the migration will fail. (Note that this would only be an issue with a > domain config that was *created* on a newer libvirt; any config > created on an older libvirt and then later migrated to a newer libvirt > would necessarily have multifunction explicitly set in the config, and > that will not be lost during migration). I keep forgetting our official stance on migrating to older libvirt versions...

As far as I'm concerned, the only reason you would want to do that is because you are upgrading your hypervisor pool and, at some point during the process, you realize there are issues with the upgrade and need to roll back. As you mention, that use case would work just fine because the guests have been defined using an older libvirt versions.