On 14.01.21 15:04, Cornelia Huck wrote: > On Thu, 14 Jan 2021 12:20:48 +0000 > Daniel P. Berrangé <berrange@xxxxxxxxxx> wrote: > >> On Thu, Jan 14, 2021 at 12:50:12PM +0100, Christian Borntraeger wrote: >>> >>> >>> On 14.01.21 12:45, Dr. David Alan Gilbert wrote: >>>> * 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? >>> >>> I guess that should work. But it means that we need to add logic in libvirt >>> to disable unpack for host-passthru and host-model. Next problem is then, >>> that a future version might implement migration of such guests, which means >>> that libvirt must then stop fencing unpack. >> >> The "host-model" is supposed to always be migratable, so we should >> fence the feature there. >> >> host-passthrough is "undefined" whether it is migratable - it may or may >> not work, no guarantees made by libvirt. >> >> Ultimately I think the problem is that there ought to be an explicit >> config to enable the feature for s390, as there is for SEV, and will >> also presumably be needed for ppc. > > Yes, an explicit config is what we want; unfortunately, we have to deal > with existing setups as well... > > The options I see are > - leave things for existing setups as they are now (i.e. might become > unmigratable when the guest transitions), and make sure we're doing > the right thing with the new object > - always make the unpack feature conflict with migration requirements; > this is a guest-visible change > > The first option might be less hairy, all considered? What about a libvirt change that removes the unpack from the host-model as soon as only-migrateable is used. When that is in place, QEMU can reject the combination of only-migrateable + unpack.
