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

[Christian Borntraeger]

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.