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

[Ram Pai]

On Tue, Jan 12, 2021 at 09:19:43AM +0100, Cornelia Huck wrote:
> On Mon, 11 Jan 2021 11:58:30 -0800
> Ram Pai <linuxram@us.ibm.com> wrote:
> 
> > On Mon, Jan 11, 2021 at 05:59:14PM +0100, Cornelia Huck 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).  
> > 
> > 
> > On POWER, secure-execution is not **automatically** enabled even when
> > the host supports it.  The feature is enabled only if the secure-object
> > is configured, and the host supports it.
> 
> Yes, the cpu feature on s390x is simply pre-existing.
> 
> > 
> > However the behavior proposed above will be consistent on POWER and
> > on s390x,  when '--only-migratable' is specified and 'secure-object'
> > is NOT specified.
> > 
> > So I am in agreement till now. 
> > 
> > 
> > > We'd still fail starting QEMU for the secure object + --only-migratable
> > > combination.  
> > 
> > Why fail? 
> > 
> > Instead, print a warning and  disable the secure-object; which will
> > disable your cpu-feature. Guests that do not transition to secure, will
> > continue to operate, and guests that transition to secure, will fail.
> 
> But that would be consistent with how other non-migratable objects are
> handled, no? It's simply a case of incompatible options on the command
> line.

Actually the two options are inherently NOT incompatible.  Halil also
mentioned this in one of his replies.

Its just that the current implementation is lacking, which will be fixed
in the near future. 

We can design it upfront, with the assumption that they both are compatible.
In the short term  disable one; preferrably the secure-object, if both 
options are specified. In the long term, remove the restriction, when
the implemetation is complete.


-- 
Ram Pai