On Thu, Jan 14, 2021 at 10:36:43AM +0000, 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. I believe, the proposed behavior, does follow the above rule. The VM started with --only-migratable will always be migratable. Any behavior; in the guest, to the contrary will disallow the behavior or terminate the guest, but will never let the VM transition to a non-migratable state. RP
