Re: [Qemu-devel] [QEMU PATCH v2 0/2]: KVM: i386: Add support for save and restore nested state

[Dr. David Alan Gilbert]

* Daniel P. Berrangé (address@hidden) 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
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--
Dr. David Alan Gilbert / address@hidden / Manchester, UK