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

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

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 5c9d215..2e40968 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);
     }
 
diff --git a/hw/s390x/s390-virtio-ccw.c b/hw/s390x/s390-virtio-ccw.c
index 98c82c2..41b770a 100644
--- a/hw/s390x/s390-virtio-ccw.c
+++ b/hw/s390x/s390-virtio-ccw.c
@@ -435,6 +435,7 @@ static void ccw_machine_class_init(ObjectClass *oc, void *data)
     s390mc->cpu_model_allowed = true;
     s390mc->css_migration_enabled = true;
     s390mc->gs_allowed = true;
+    s390mc->ais_allowed = true;
     mc->init = ccw_init;
     mc->reset = s390_machine_reset;
     mc->hot_add_cpu = s390_hot_add_cpu;
@@ -519,6 +520,11 @@ bool gs_allowed(void)
     return get_machine_class()->gs_allowed;
 }
 
+bool ais_allowed(void)
+{
+    return get_machine_class()->ais_allowed;
+}
+
 static char *machine_get_loadparm(Object *obj, Error **errp)
 {
     S390CcwMachineState *ms = S390_CCW_MACHINE(obj);
@@ -742,6 +748,10 @@ static void ccw_machine_2_10_instance_options(MachineState *machine)
 
 static void ccw_machine_2_10_class_options(MachineClass *mc)
 {
+    S390CcwMachineClass *s390mc = S390_MACHINE_CLASS(mc);
+
+    s390mc->ais_allowed = false;
+
     ccw_machine_2_11_class_options(mc);
     SET_MACHINE_COMPAT(mc, CCW_COMPAT_2_10);
 }
diff --git a/include/hw/s390x/s390-virtio-ccw.h b/include/hw/s390x/s390-virtio-ccw.h
index a9a90c2..d25a98f 100644
--- a/include/hw/s390x/s390-virtio-ccw.h
+++ b/include/hw/s390x/s390-virtio-ccw.h
@@ -41,6 +41,7 @@ typedef struct S390CcwMachineClass {
     bool cpu_model_allowed;
     bool css_migration_enabled;
     bool gs_allowed;
+    bool ais_allowed;
 } S390CcwMachineClass;
 
 /* runtime-instrumentation allowed by the machine */
@@ -49,6 +50,8 @@ bool ri_allowed(void);
 bool cpu_model_allowed(void);
 /* guarded-storage allowed by the machine */
 bool gs_allowed(void);
+/* ais allowed by the machine */
+bool ais_allowed(void);
 
 /**
  * Returns true if (vmstate based) migration of the channel subsystem
-- 
2.9.4