On Thu, Jan 14, 2021 at 11:25:17AM +0000, Daniel P. Berrangé wrote: > On Wed, Jan 13, 2021 at 12:42:26PM +0000, 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 agree, --only-migratable is supposed to be a *behavioural* toggle > for QEMU. It must /not/ have any impact on the guest ABI. > > A management application needs to be able to add/remove --only-migratable > at will without changing the exposing guest ABI. At the qemu level, it sounds like the right thing to do is to fail outright if all of the below are true: 1. --only-migratable is specified 2. -cpu host is specified 3. unpack isn't explicitly disabled 4. the host CPU actually does have the unpack facility That can be changed if & when migration support is added for PV. -- David Gibson | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_ | _way_ _around_! http://www.ozlabs.org/~dgibson
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