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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* 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.


Dave

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