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.
