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

[Dr. David Alan Gilbert]

* 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.

Dave

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