Re: [for-6.0 v5 11/13] spapr: PEF: prevent migration

[Christian Borntraeger <borntraeger@de.ibm.com>]

On 14.01.21 11:36, Dr. David Alan Gilbert wrote:
> * Christian Borntraeger (borntraeger@de.ibm.com) wrote:
>>
>>
>> On 13.01.21 13:42, Dr. David Alan Gilbert wrote:
>>> * Cornelia Huck (cohuck@redhat.com) wrote:
>>>> On Tue, 5 Jan 2021 12:41:25 -0800
>>>> Ram Pai <linuxram@us.ibm.com> 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@us.ibm.com> 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.