[libvirt] [PATCH/QEMU] s390x/kvm: use cpu_model_available for guarded storage on compat machines

[Christian Borntraeger]

Starting a guest with
   <os>
    <type arch='s390x' machine='s390-ccw-virtio-2.9'>hvm</type>
  </os>
  <cpu mode='host-model'/>

on an IBM z14 results in

"qemu-system-s390x: Some features requested in the CPU model are not
available in the configuration: gs"

This is because guarded storage is fenced for compat machines that did not have
guarded storage support, but libvirt expands the cpu model according to the
latest available machine.

While this prevents future migration abort (by not starting the guest at all),
not being able to start a "host-model" guest is very much unexpected.  As it
turns out, even if we would modify libvirt to not expand the cpu model to
contain "gs" for compat machines, it cannot guarantee that a migration will
succeed. For example if the kernel changes its features (or the user has
nested=1 on one host but not on the other) the migration will fail
nevertheless.  So instead of fencing "gs" for machines <= 2.9 lets allow it for
all machine types that support the CPU model. This will make "host-model"
runnable all the time, while relying on the CPU model to reject invalid
migration attempts.

Suggested-by: David Hildenbrand <david at redhat.com>
Signed-off-by: Christian Borntraeger <borntraeger at de.ibm.com>
---
 hw/s390x/s390-virtio-ccw.c         | 8 --------
 include/hw/s390x/s390-virtio-ccw.h | 3 ---
 target/s390x/kvm.c                 | 2 +-
 3 files changed, 1 insertion(+), 12 deletions(-)

diff --git a/hw/s390x/s390-virtio-ccw.c b/hw/s390x/s390-virtio-ccw.c
index fabe4a6..ae5b01a 100644
--- a/hw/s390x/s390-virtio-ccw.c
+++ b/hw/s390x/s390-virtio-ccw.c
@@ -414,7 +414,6 @@ static void ccw_machine_class_init(ObjectClass *oc, void *data)
     s390mc->ri_allowed = true;
     s390mc->cpu_model_allowed = true;
     s390mc->css_migration_enabled = true;
-    s390mc->gs_allowed = true;
     mc->init = ccw_init;
     mc->reset = s390_machine_reset;
     mc->hot_add_cpu = s390_hot_add_cpu;
@@ -495,12 +494,6 @@ bool cpu_model_allowed(void)
     return get_machine_class()->cpu_model_allowed;
 }
 
-bool gs_allowed(void)
-{
-    /* for "none" machine this results in true */
-    return get_machine_class()->gs_allowed;
-}
-
 static char *machine_get_loadparm(Object *obj, Error **errp)
 {
     S390CcwMachineState *ms = S390_CCW_MACHINE(obj);
@@ -740,7 +733,6 @@ static void ccw_machine_2_9_class_options(MachineClass *mc)
 {
     S390CcwMachineClass *s390mc = S390_MACHINE_CLASS(mc);
 
-    s390mc->gs_allowed = false;
     ccw_machine_2_10_class_options(mc);
     SET_MACHINE_COMPAT(mc, CCW_COMPAT_2_9);
     s390mc->css_migration_enabled = false;
diff --git a/include/hw/s390x/s390-virtio-ccw.h b/include/hw/s390x/s390-virtio-ccw.h
index a9a90c2..ac896e3 100644
--- a/include/hw/s390x/s390-virtio-ccw.h
+++ b/include/hw/s390x/s390-virtio-ccw.h
@@ -40,15 +40,12 @@ typedef struct S390CcwMachineClass {
     bool ri_allowed;
     bool cpu_model_allowed;
     bool css_migration_enabled;
-    bool gs_allowed;
 } S390CcwMachineClass;
 
 /* runtime-instrumentation allowed by the machine */
 bool ri_allowed(void);
 /* cpu model allowed by the machine */
 bool cpu_model_allowed(void);
-/* guarded-storage allowed by the machine */
-bool gs_allowed(void);
 
 /**
  * Returns true if (vmstate based) migration of the channel subsystem
diff --git a/target/s390x/kvm.c b/target/s390x/kvm.c
index 4c85ed8..020a7ea 100644
--- a/target/s390x/kvm.c
+++ b/target/s390x/kvm.c
@@ -363,7 +363,7 @@ int kvm_arch_init(MachineState *ms, KVMState *s)
             cap_ri = 1;
         }
     }
-    if (gs_allowed()) {
+    if (cpu_model_allowed()) {
         if (kvm_vm_enable_cap(s, KVM_CAP_S390_GS, 0) == 0) {
             cap_gs = 1;
         }
-- 
2.9.4