Re: [Qemu-devel] [PATCH v2 2/2] s390x/ais: disable ais for compat machines

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

On 09/27/2017 09:12 AM, Christian Borntraeger wrote:
> 
> 
> On 09/26/2017 03:51 PM, David Hildenbrand wrote:
>> On 26.09.2017 15:36, Christian Borntraeger wrote:
>>> With newer kernels that do support the ais feature (4.13) a qemu 2.11
>>> will not only enable the ais feature for the 2.11 machine, but also
>>> for a <=2.10 compat machine. As this feature is not available in
>>> QEMU <=2.9 (and QEMU 2.10.1), this guest will fail to migrate
>>> back to an older qemu like 2.9 with:
>>>
>>> _snip_
>>> error while loading state for instance 0x0 of device 's390-flic'
>>> _snip_
>>>
>>> making the whole compat machine dis-functional. As a permanent fix, we
>>> need to fence the ais feature for machines <= 2.10
>>>
>>> Due to ais being enabled on 2.10.0 (fixed in 2.10.1) this will prevent
>>> migration of ais-enabled guests from 2.10.0 with
>>>
>>> _snip_
>>> qemu-system-s390x: Failed to load s390-flic/ais:tmp
>>> qemu-system-s390x: error while loading state for instance 0x0 of device 's390-flic'
>>> qemu-system-s390x: load of migration failed: Function not implemented
>>> _snip_
>>>
>>> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
>>> Cc: Yi Min Zhao <zyimin@linux.vnet.ibm.com>
>>> Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
>>> ---
>>>  hw/intc/s390_flic_kvm.c            |  4 +++-
>>
>>
>> As discussed, I think we should use cpu_model_allowed() instead.
> I think I still prefer the explicit check for ais-enabled to make managedsave
> (on the same system) continue to work if the user does not specify a cpu model 
> at all (which will then fallback to the host model). We already fence other
> things (like guarded storage) and yes it will grow over time but the 
> *_allowed things seem to be the smallest maintenance issue in this area.

Hmm, on the other hand this fails only when we migrate to an older qemu during
managedsave. So yes, maybe this is just an "will not work anyway"