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

From: Cornelia Huck
Subject: Re: [for-6.0 v5 11/13] spapr: PEF: prevent migration
Date: Mon, 11 Jan 2021 17:59:14 +0100

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

Does that make sense?

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