* Cornelia Huck (cohuck@xxxxxxxxxx) wrote: > On Thu, 14 Jan 2021 11:52:11 +0100 > Christian Borntraeger <borntraeger@xxxxxxxxxx> wrote: > > > On 14.01.21 11:36, Dr. David Alan Gilbert wrote: > > > * Christian Borntraeger (borntraeger@xxxxxxxxxx) wrote: > > >> > > >> > > >> On 13.01.21 13:42, Dr. David Alan Gilbert wrote: > > >>> * Cornelia Huck (cohuck@xxxxxxxxxx) wrote: > > >>>> On Tue, 5 Jan 2021 12:41:25 -0800 > > >>>> Ram Pai <linuxram@xxxxxxxxxx> 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@xxxxxxxxxx> 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 > > >>>> combination. > > >>>> > > >>>> Does that make sense? > > >>> > > >>> It's a little unusual; I don't think we have any other cases where > > >>> --only-migratable changes the behaviour; I think it normally only stops > > >>> you doing something that would have made it unmigratable or causes > > >>> an operation that would make it unmigratable to fail. > > >> > > >> I would like to NOT block this feature with --only-migrateable. A guest > > >> can startup unprotected (and then is is migrateable). the migration blocker > > >> is really a dynamic aspect during runtime. > > > > > > But the point of --only-migratable is to turn things that would have > > > blocked migration into failures, so that a VM started with > > > --only-migratable is *always* migratable. > > > > Hmmm, fair enough. How do we do this with host-model? The constructed model > > would contain unpack, but then it will fail to startup? Or do we silently > > drop unpack in that case? Both variants do not feel completely right. > > Failing if you explicitly specified unpacked feels right, but failing > if you just used the host model feels odd. Removing unpack also is a > bit odd, but I think the better option if we want to do anything about > it at all. 'host-model' feels a bit special; but breaking the rule that only-migratable doesn't change behaviour is weird. Can you do host,-unpack to make that work explicitly? But hang on; why is 'unpack' the name of a secure guest facility - is it really a feature for secure guest or something else? Dave -- Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK
