[Qemu-devel] R: Re: [PATCH v2] target/i386: kvm: add VMX migration blocker

[Paolo Bonzini]

----- Cole Robinson <address@hidden> ha scritto:
> On 4/12/19 3:47 AM, Paolo Bonzini wrote:
> > On 10/04/19 20:26, Cole Robinson wrote:
> >> On 11/20/18 6:44 AM, Dr. David Alan Gilbert wrote:
> >>> * Paolo Bonzini (address@hidden) wrote:
> >>>> Nested VMX does not support live migration yet.  Add a blocker
> >>>> until that is worked out.
> >>>>
> >>>> Nested SVM only does not support it, but unfortunately it is
> >>>> enabled by default for -cpu host so we cannot really disable it.
> >>>>
> >>>> Signed-off-by: Paolo Bonzini <address@hidden>
> >>>
> >>> So I'm OK with this, but it does need a release note warning whenever it
> >>> goes in, because it'll surprise those who've already enabled nesting
> >>> but don't use it on all their VMs.
> >>>
> >>
> >> We are hitting this in Fedora 30. Now that nested VMX is enabled by
> >> default at the kernel level, and virt-manager/boxes will use the
> >> equivalent of -cpu host by default, libvirt managedsave (migrate to
> >> file) and virt-manager snapshots (savevm) are rejected for default
> >> created VMs on intel. That's quite unfortunate.
> >>
> >> Any ideas on how to resolve this?
> > 
> > I think the simplest solution is just to finish implementation of nested
> > VMX live migration and backport it to Fedora 30.
> > 
> 
> That would simplify things :) Any guess on the timeframe? This is kernel
> work I presume?

No, the kernel part is already in. As a contingency plan, you could just revert 
this QEMU patch.

Paolo

> 
> If changes aren't landing in the near term I think we should disable
> nested VMX by default in Fedora, maybe just with modules.d/kvm.conf
> override. (Or revert this patch downstream, but I presume that's not a
> good idea).
> 
> The alternative of just letting it sit is going to generate a lot of
> complaints I suspect. And the only solutions will be 1) disable nested
> VMx for your whole machine and reboot, or 2) run this virt-xml command
> to disable VMX in your domain config... and probably forget that it's
> there and then a year later when this is all sorted out file a bug
> asking why nested virt isn't working for this one VM ;)
> 
> I guess #2 might not be avoidable anyways for the amount of people that
> have already opted into nested VMX
> 
> Thanks,
> Cole