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

[Ram Pai]

On Thu, Jan 14, 2021 at 10:36:43AM +0000, 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.

I believe, the proposed behavior, does follow the above rule. The
VM started with --only-migratable will always be migratable. Any
behavior; in the guest, to the contrary will disallow the behavior or
terminate the guest, but will never let the VM transition to a
non-migratable state.


RP