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

[Cornelia Huck]

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.  
> 
> 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;
> > >  }
> > >    
> >   
>

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