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

["Daniel P. Berrangé" <berrange@redhat.com>]

On Mon, Nov 12, 2018 at 04:50:54PM +0000, Dr. David Alan Gilbert wrote:
> * Daniel P. Berrangé (berrange@redhat.com) 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.

True, and (b) would include anyone using libvirt's  host-model CPU. So if
you enabled nesting, have host-model for all guests, but only use nesting
in one of the guests, you'd be doomed.

Is it possible for QEMU to determine if there are nested guests running or
not and conditionally block migration appropriately to ensure safety ?


Regards,
Daniel
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