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

[David Hildenbrand]

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 <address@hidden>
> Cc: Yi Min Zhao <address@hidden>
> Cc: Dr. David Alan Gilbert <address@hidden>
> ---
>  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?


-- 

Thanks,

David