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

[Dr. David Alan Gilbert]

* 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?

But hang on; why is 'unpack' the name of a secure guest facility - is
it really a feature for secure guest or something else?

Dave

-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK