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

[Ram Pai]

On Wed, Jan 13, 2021 at 09:06:29AM +0100, Cornelia Huck wrote:
> On Tue, 12 Jan 2021 10:55:11 -0800
> Ram Pai <linuxram@us.ibm.com> wrote:
> 
> > 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.
> 
> Can't we simply mark the object as non-migratable now, and then remove
> that later? I don't see what is so special about it.

This is fine too. 

However I am told that libvirt has some assumptions, where it assumes
that the VM is guaranteed to be migratable if '--only-migratable' is
specified. Silently turning off that option can be bad.

-- 
Ram Pai