* 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. Dave -- Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK
