Re: [Qemu-devel] Live migration from Qemu 2.12 hosts to Qemu 3.2 hosts, with VMX flag enabled in the guest?

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

On Fri, Jan 18, 2019 at 10:16:34AM +0000, Dr. David Alan Gilbert wrote:
> * Paolo Bonzini (pbonzini@redhat.com) wrote:
> > On 18/01/19 11:02, Dr. David Alan Gilbert wrote:
> > >>
> > >> It fails if the flag is set, rather than if any nested virtualization has
> > >> been used before.
> > >>
> > >> I'm concerned I will end up with a requirement for *all* guests to be
> > >> restarted in order to migrate them to the new hosts, rather than just the
> > >> ones that would have a problem.
> > > I think you should be able to migrate from 2.12->3.1 like this, but
> > > you'd hit the problem when you then try and migrate again between your
> > > new QEMUs.
> > > 
> > > I guess we could modify it to wire it to machine type, so that
> > > older machine types didn't block.
> > 
> > That would also be wrong.  The old machine types _could_ be using KVM
> > and they have no way to block the live migration.
> > 
> > The solution is to restart the VM using "-cpu host,-vmx".
> 
> The problem as Christian explained in that thread is that it was common
> for them to start VMs with vmx enabled but for people not to use it
> on most of the VMs, so we break migration for most VMs even though most
> don't use it.
> 
> It might not be robust, but it worked for a lot of people most of the
> time.

Yes, this is exactly why I said we should make the migration blocker
be conditional on any L2 guest having been started. I vaguely recall
someone saying there wasn't any way to detect this situation from
QEMU though ?

Regards,
Daniel
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