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

From: Christian Borntraeger
Subject: Re: [for-6.0 v5 11/13] spapr: PEF: prevent migration
Date: Thu, 14 Jan 2021 15:09:01 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0

On 14.01.21 15:04, Cornelia Huck wrote:
> On Thu, 14 Jan 2021 12:20:48 +0000
> Daniel P. Berrangé <berrange@redhat.com> 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@redhat.com) wrote:  
>>>>> On Thu, 14 Jan 2021 11:52:11 +0100
>>>>> Christian Borntraeger <borntraeger@de.ibm.com> wrote:
>>>>>> 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.   
>>>>> 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.

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