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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Cornelia Huck (cohuck@redhat.com) wrote:
> On Thu, 17 Dec 2020 15:15:30 +0100
> Greg Kurz <groug@kaod.org> wrote:
> 
> > On Thu, 17 Dec 2020 12:38:42 +0100
> > Cornelia Huck <cohuck@redhat.com> wrote:
> > 
> > > On Thu, 17 Dec 2020 16:47:36 +1100
> > > David Gibson <david@gibson.dropbear.id.au> wrote:
> > >   
> > > > On Mon, Dec 14, 2020 at 06:22:40PM +0100, Cornelia Huck wrote:  
> > > > > On Fri,  4 Dec 2020 16:44:13 +1100
> > > > > David Gibson <david@gibson.dropbear.id.au> wrote:
> > > > >     
> > > > > > We haven't yet implemented the fairly involved handshaking that will be
> > > > > > needed to migrate PEF protected guests.  For now, just use a migration
> > > > > > blocker so we get a meaningful error if someone attempts this (this is the
> > > > > > same approach used by AMD SEV).
> > > > > > 
> > > > > > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> > > > > > Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > > > > > ---
> > > > > >  hw/ppc/pef.c | 9 +++++++++
> > > > > >  1 file changed, 9 insertions(+)
> > > > > > 
> > > > > > diff --git a/hw/ppc/pef.c b/hw/ppc/pef.c
> > > > > > index 3ae3059cfe..edc3e744ba 100644
> > > > > > --- a/hw/ppc/pef.c
> > > > > > +++ b/hw/ppc/pef.c
> > > > > > @@ -38,7 +38,11 @@ struct PefGuestState {
> > > > > >  };
> > > > > >  
> > > > > >  #ifdef CONFIG_KVM
> > > > > > +static Error *pef_mig_blocker;
> > > > > > +
> > > > > >  static int kvmppc_svm_init(Error **errp)    
> > > > > 
> > > > > This looks weird?    
> > > > 
> > > > Oops.  Not sure how that made it past even my rudimentary compile
> > > > testing.
> > > >   
> > > > > > +
> > > > > > +int kvmppc_svm_init(SecurableGuestMemory *sgm, Error **errp)
> > > > > >  {
> > > > > >      if (!kvm_check_extension(kvm_state, KVM_CAP_PPC_SECURABLE_GUEST)) {
> > > > > >          error_setg(errp,
> > > > > > @@ -54,6 +58,11 @@ static int kvmppc_svm_init(Error **errp)
> > > > > >          }
> > > > > >      }
> > > > > >  
> > > > > > +    /* add migration blocker */
> > > > > > +    error_setg(&pef_mig_blocker, "PEF: Migration is not implemented");
> > > > > > +    /* NB: This can fail if --only-migratable is used */
> > > > > > +    migrate_add_blocker(pef_mig_blocker, &error_fatal);    
> > > > > 
> > > > > Just so that I understand: is PEF something that is enabled by the host
> > > > > (and the guest is either secured or doesn't start), or is it using a
> > > > > model like s390x PV where the guest initiates the transition into
> > > > > secured mode?    
> > > > 
> > > > Like s390x PV it's initiated by the guest.
> > > >   
> > > > > Asking because s390x adds the migration blocker only when the
> > > > > transition is actually happening (i.e. guests that do not transition
> > > > > into secure mode remain migratable.) This has the side effect that you
> > > > > might be able to start a machine with --only-migratable that
> > > > > transitions into a non-migratable machine via a guest action, if I'm
> > > > > not mistaken. Without the new object, I don't see a way to block with
> > > > > --only-migratable; with it, we should be able to do that. Not sure what
> > > > > the desirable behaviour is here.    
> > > >   
> > 
> > The purpose of --only-migratable is specifically to prevent the machine
> > to transition to a non-migrate state IIUC. The guest transition to
> > secure mode should be nacked in this case.
> 
> Yes, that's what happens for s390x: The guest tries to transition, QEMU
> can't add a migration blocker and fails the instruction used for
> transitioning, the guest sees the error.
> 
> The drawback is that we see the failure only when we already launched
> the machine and the guest tries to transition. If I start QEMU with
> --only-migratable, it will refuse to start when non-migratable devices
> are configured in the command line, so I see the issue right from the
> start. (For s390x, that would possibly mean that we should not even
> present the cpu feature bit when only_migratable is set?)

I see --only-migratable as refusing to start if you've enabled anything
that would stop migration.
So I'd expect:
  a) Allow the cpu flag to be turned on/off somehow
     
  b) If you ask for it (-cpu ...,_confidentialcomp or whatever) and
you've got --only-migratable then you'd fail before startup.

Dave

> > 
> > > > Hm, I'm not sure what the best option is here either.  
> > > 
> > > If we agree on anything, it should be as consistent across
> > > architectures as possible :)
> > > 
> > > If we want to add the migration blocker to s390x even before the guest
> > > transitions, it needs to be tied to the new object; if we'd make it
> > > dependent on the cpu feature bit, we'd block migration of all machines
> > > on hardware with SE and a recent kernel.
> > > 
> > > Is there a convenient point in time when PEF guests transition where
> > > QEMU can add a blocker?
> > >   
> > > >   
> > > > >     
> > > > > > +
> > > > > >      return 0;
> > > > > >  }
> > > > > >      
> > > > >     
> > > >   
> > >   
> > 
> 


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