Re: [for-6.0 v5 11/13] spapr: PEF: prevent migration

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

On Thu, Jan 14, 2021 at 03:09:01PM +0100, Christian Borntraeger wrote:
> 
> 
> On 14.01.21 15:04, Cornelia Huck wrote:
> > On Thu, 14 Jan 2021 12:20:48 +0000
> > Daniel P. Berrangé <berrange@redhat.com> wrote:
> > 
> >> On Thu, Jan 14, 2021 at 12:50:12PM +0100, Christian Borntraeger wrote:
> >>>
> >>>
> >>> On 14.01.21 12:45, Dr. David Alan Gilbert wrote:  
> >>>> * Cornelia Huck (cohuck@redhat.com) wrote:  
> >>>>> On Thu, 14 Jan 2021 11:52:11 +0100
> >>>>> Christian Borntraeger <borntraeger@de.ibm.com> wrote:
> >>>>>  
> >>>>>> On 14.01.21 11:36, Dr. David Alan Gilbert wrote:  
> >>>>>>> * Christian Borntraeger (borntraeger@de.ibm.com) wrote:    
> >>>>>>>>
> >>>>>>>>
> >>>>>>>> On 13.01.21 13:42, Dr. David Alan Gilbert wrote:    
> >>>>>>>>> * Cornelia Huck (cohuck@redhat.com) wrote:    
> >>>>>>>>>> On Tue, 5 Jan 2021 12:41:25 -0800
> >>>>>>>>>> Ram Pai <linuxram@us.ibm.com> wrote:
> >>>>>>>>>>    
> >>>>>>>>>>> On Tue, Jan 05, 2021 at 11:56:14AM +0100, Halil Pasic wrote:    
> >>>>>>>>>>>> On Mon, 4 Jan 2021 10:40:26 -0800
> >>>>>>>>>>>> Ram Pai <linuxram@us.ibm.com> wrote:    
> >>>>>>>>>>    
> >>>>>>>>>>>>> The main difference between my proposal and the other proposal is...
> >>>>>>>>>>>>>
> >>>>>>>>>>>>>   In my proposal the guest makes the compatibility decision and acts
> >>>>>>>>>>>>>   accordingly.  In the other proposal QEMU makes the compatibility
> >>>>>>>>>>>>>   decision and acts accordingly. I argue that QEMU cannot make a good
> >>>>>>>>>>>>>   compatibility decision, because it wont know in advance, if the guest
> >>>>>>>>>>>>>   will or will-not switch-to-secure.
> >>>>>>>>>>>>>       
> >>>>>>>>>>>>
> >>>>>>>>>>>> You have a point there when you say that QEMU does not know in advance,
> >>>>>>>>>>>> if the guest will or will-not switch-to-secure. I made that argument
> >>>>>>>>>>>> regarding VIRTIO_F_ACCESS_PLATFORM (iommu_platform) myself. My idea
> >>>>>>>>>>>> was to flip that property on demand when the conversion occurs. David
> >>>>>>>>>>>> explained to me that this is not possible for ppc, and that having the
> >>>>>>>>>>>> "securable-guest-memory" property (or whatever the name will be)
> >>>>>>>>>>>> specified is a strong indication, that the VM is intended to be used as
> >>>>>>>>>>>> a secure VM (thus it is OK to hurt the case where the guest does not
> >>>>>>>>>>>> try to transition). That argument applies here as well.      
> >>>>>>>>>>>
> >>>>>>>>>>> As suggested by Cornelia Huck, what if QEMU disabled the
> >>>>>>>>>>> "securable-guest-memory" property if 'must-support-migrate' is enabled?
> >>>>>>>>>>> Offcourse; this has to be done with a big fat warning stating
> >>>>>>>>>>> "secure-guest-memory" feature is disabled on the machine.
> >>>>>>>>>>> Doing so, will continue to support guest that do not try to transition.
> >>>>>>>>>>> Guest that try to transition will fail and terminate themselves.    
> >>>>>>>>>>
> >>>>>>>>>> Just to recap the s390x situation:
> >>>>>>>>>>
> >>>>>>>>>> - We currently offer a cpu feature that indicates secure execution to
> >>>>>>>>>>   be available to the guest if the host supports it.
> >>>>>>>>>> - When we introduce the secure object, we still need to support
> >>>>>>>>>>   previous configurations and continue to offer the cpu feature, even
> >>>>>>>>>>   if the secure object is not specified.
> >>>>>>>>>> - As migration is currently not supported for secured guests, we add a
> >>>>>>>>>>   blocker once the guest actually transitions. That means that
> >>>>>>>>>>   transition fails if --only-migratable was specified on the command
> >>>>>>>>>>   line. (Guests not transitioning will obviously not notice anything.)
> >>>>>>>>>> - With the secure object, we will already fail starting QEMU if
> >>>>>>>>>>   --only-migratable was specified.
> >>>>>>>>>>
> >>>>>>>>>> My suggestion is now that we don't even offer the cpu feature if
> >>>>>>>>>> --only-migratable has been specified. For a guest that does not want to
> >>>>>>>>>> transition to secure mode, nothing changes; a guest that wants to
> >>>>>>>>>> transition to secure mode will notice that the feature is not available
> >>>>>>>>>> and fail appropriately (or ultimately, when the ultravisor call fails).
> >>>>>>>>>> We'd still fail starting QEMU for the secure object + --only-migratable
> >>>>>>>>>> combination.
> >>>>>>>>>>
> >>>>>>>>>> Does that make sense?    
> >>>>>>>>>
> >>>>>>>>> It's a little unusual; I don't think we have any other cases where
> >>>>>>>>> --only-migratable changes the behaviour; I think it normally only stops
> >>>>>>>>> you doing something that would have made it unmigratable or causes
> >>>>>>>>> an operation that would make it unmigratable to fail.    
> >>>>>>>>
> >>>>>>>> I would like to NOT block this feature with --only-migrateable. A guest
> >>>>>>>> can startup unprotected (and then is is migrateable). the migration blocker
> >>>>>>>> is really a dynamic aspect during runtime.     
> >>>>>>>
> >>>>>>> But the point of --only-migratable is to turn things that would have
> >>>>>>> blocked migration into failures, so that a VM started with
> >>>>>>> --only-migratable is *always* migratable.    
> >>>>>>
> >>>>>> Hmmm, fair enough. How do we do this with host-model? The constructed model
> >>>>>> would contain unpack, but then it will fail to startup? Or do we silently 
> >>>>>> drop unpack in that case? Both variants do not feel completely right.   
> >>>>>
> >>>>> Failing if you explicitly specified unpacked feels right, but failing
> >>>>> if you just used the host model feels odd. Removing unpack also is a
> >>>>> bit odd, but I think the better option if we want to do anything about
> >>>>> it at all.  
> >>>>
> >>>> 'host-model' feels a bit special; but breaking the rule that
> >>>> only-migratable doesn't change behaviour is weird
> >>>> Can you do host,-unpack   to make that work explicitly?  
> >>>
> >>> I guess that should work. But it means that we need to add logic in libvirt
> >>> to disable unpack for host-passthru and host-model. Next problem is then,
> >>> that a future version might implement migration of such guests, which means
> >>> that libvirt must then stop fencing unpack.  
> >>
> >> The "host-model" is supposed to always be migratable, so we should
> >> fence the feature there.
> >>
> >> host-passthrough is "undefined" whether it is migratable - it may or may
> >> not work, no guarantees made by libvirt.
> >>
> >> Ultimately I think the problem is that there ought to be an explicit
> >> config to enable the feature for s390, as there is for SEV, and will
> >> also presumably be needed for ppc. 
> > 
> > Yes, an explicit config is what we want; unfortunately, we have to deal
> > with existing setups as well...
> > 
> > The options I see are
> > - leave things for existing setups as they are now (i.e. might become
> >   unmigratable when the guest transitions), and make sure we're doing
> >   the right thing with the new object
> > - always make the unpack feature conflict with migration requirements;
> >   this is a guest-visible change
> > 
> > The first option might be less hairy, all considered?
> 
> What about a libvirt change that removes the unpack from the host-model as 
> soon as  only-migrateable is used. When that is in place, QEMU can reject
> the combination of only-migrateable + unpack.

I think libvirt needs to just unconditionally remove unpack from host-model
regardless, and require an explicit opt in. We can do that in libvirt
without compat problems, because we track the expansion of "host-model"
for existing running guests.

QEMU could introduce a deprecation warning right now, and then turn it into
an error after the deprecation cycle is complete.

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 :|