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.
