target/s390x/kvm: Enable adapter interruption suppression again

The AIS feature has been disabled late in the v2.10 development cycle since
there were some issues with migration (see commit 3f2d07b3 -
"s390x/ais: for 2.10 stable: disable ais facility"). We originally wanted
to enable it again for newer machine types, but apparently we forgot to do
this so far. Let's do it now for the machines that support proper CPU models.

Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1756946Signed-off-by: NThomas Huth <thuth@redhat.com>
Message-Id: <20200122101437.5069-1-thuth@redhat.com>
Reviewed-by: NDavid Hildenbrand <david@redhat.com>
Tested-by: NMatthew Rosato <mjrosato@linux.ibm.com>
Signed-off-by: NCornelia Huck <cohuck@redhat.com>

target/s390x/kvm.c

@@ -365,10 +365,13 @@ int kvm_arch_init(MachineState *ms, KVMState *s)
     /*
      * The migration interface for ais was introduced with kernel 4.13
      * but the capability itself had been active since 4.12. As migration
-     * support is considered necessary let's disable ais in the 2.10
-     * machine.
+     * support is considered necessary, we only try to enable this for
+     * newer machine types if KVM_CAP_S390_AIS_MIGRATION is available.
      */
-    /* kvm_vm_enable_cap(s, KVM_CAP_S390_AIS, 0); */
+    if (cpu_model_allowed() && kvm_kernel_irqchip_allowed() &&
+        kvm_check_extension(s, KVM_CAP_S390_AIS_MIGRATION)) {
+        kvm_vm_enable_cap(s, KVM_CAP_S390_AIS, 0);
+    }
 
     kvm_set_max_memslot_size(KVM_SLOT_MAX_BYTES);
     return 0;