* Daniel P. Berrangé (berrange@xxxxxxxxxx) wrote: > On Sun, Nov 04, 2018 at 11:19:57PM +0100, Paolo Bonzini wrote: > > On 02/11/2018 17:54, Daniel P. Berrangé wrote: > > > We have usually followed a rule that new machine types must not > > > affect runability of a VM on a host. IOW new machine types should > > > not introduce dependancies on specific kernels, or hardware features > > > such as CPU flags. > > > > > Anything that requires a new kernel feature thus ought to be an > > > opt-in config tunable on the CLI, separate from machine type > > > choice. > > > > Unless someone tinkered with the module parameters, they could not even > > use nested virtualization before 4.20. So for everyone else, "-cpu > > ...,+vmx" does count as an "opt-in config tunable on the CLI" that > > requires 4.20. > > > > For those that did tinker with module parameters, we can grandfather in > > the old machine types, so that they can use nested virtualization with > > no live migration support. For those that did not, however, I don't > > think it makes sense to say "oh by the way I really want to be able to > > migrate this VM" on the command line, or even worse on the monitor. > > IIUC, 4.20 is only required from POV of migration state. Is it thus > possible to just register a migration blocker if QEMU is launched > on a host with kernel < 4.20. > > Migration has always been busted historically, so those people using > nested VMX already won't be hurt by not having ability to live migrate > their VM, but could otherwise continue using them without being forced > to upgrade their kernel to fix a feature they're not even using. Yes, although I am a bit worried we might have a population of users that: a) Have enabled nesting b) Run VMs with vmx enabled c) Don't normally actually run nested guests d) Currently happily migrate. Dave > Regards, > Daniel > -- > |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| > |: https://libvirt.org -o- https://fstop138.berrange.com :| > |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :| -- Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK
