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

[Daniel P . Berrangé]

On Wed, Jan 13, 2021 at 12:42:26PM +0000, 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 agree,  --only-migratable is supposed to be a *behavioural* toggle
for QEMU. It must /not/ have any impact on the guest ABI.

A management application needs to be able to add/remove --only-migratable
at will without changing the exposing guest ABI.

Regards,
Daniel
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