Re: [Qemu-devel] [PATCH/RFC 3/3] s390x/ais: disable ais for compat machines

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

On 09/26/2017 02:26 PM, David Hildenbrand wrote:
> On 22.09.2017 10:38, 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 +++-
>>  hw/s390x/s390-virtio-ccw.c         | 10 ++++++++++
>>  include/hw/s390x/s390-virtio-ccw.h |  3 +++
>>  3 files changed, 16 insertions(+), 1 deletion(-)
>>
>> diff --git a/hw/intc/s390_flic_kvm.c b/hw/intc/s390_flic_kvm.c
>> index a655567..2a94bfc 100644
>> --- a/hw/intc/s390_flic_kvm.c
>> +++ b/hw/intc/s390_flic_kvm.c
>> @@ -22,6 +22,7 @@
>>  #include "hw/s390x/s390_flic.h"
>>  #include "hw/s390x/adapter.h"
>>  #include "hw/s390x/css.h"
>> +#include "hw/s390x/s390-virtio-ccw.h"
>>  #include "trace.h"
>>  
>>  #define FLIC_SAVE_INITIAL_SIZE getpagesize()
>> @@ -559,7 +560,8 @@ static void kvm_s390_flic_realize(DeviceState *dev, Error **errp)
>>                                              KVM_HAS_DEVICE_ATTR, test_attr);
>>      /* try enable the AIS facility */
>>      test_attr.group = KVM_DEV_FLIC_AISM_ALL;
>> -    if (!ioctl(flic_state->fd, KVM_HAS_DEVICE_ATTR, test_attr)) {
>> +    if (ais_allowed() &&
>> +        !ioctl(flic_state->fd, KVM_HAS_DEVICE_ATTR, test_attr)) {
>>              kvm_vm_enable_cap(kvm_state, KVM_CAP_S390_AIS, 0);
>>      }
>>  
> 
> Wondering if this is really necessary. Shouldn't the CPU model feature
> make sure that migration works?

older QEMUs complain like when migrating a 2.9 machine to 2.9

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

e.g. when using the host model.