s390x: Fence huge pages prior to 3.1

As the kernel has no way of disallowing the start of a huge page
backed VM, we can migrate a running huge backed VM to a host that has
no huge page KVM support.

Let's glue huge page support support to the 3.1 machine, so we do not
migrate to a destination host that doesn't have QEMU huge page support
and can stop migration if KVM doesn't indicate support.

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Message-Id: <20180928093435.198573-1-frankja@linux.ibm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>

hw/s390x/s390-virtio-ccw.c

@@ -456,6 +456,7 @@ 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->hpage_1m_allowed = true;
     mc->init = ccw_init;
     mc->reset = s390_machine_reset;
     mc->hot_add_cpu = s390_hot_add_cpu;
@@ -535,6 +536,12 @@ bool cpu_model_allowed(void)
     return get_machine_class()->cpu_model_allowed;
 }
 
+bool hpage_1m_allowed(void)
+{
+    /* for "none" machine this results in true */
+    return get_machine_class()->hpage_1m_allowed;
+}
+
 static char *machine_get_loadparm(Object *obj, Error **errp)
 {
     S390CcwMachineState *ms = S390_CCW_MACHINE(obj);
@@ -747,6 +754,9 @@ static void ccw_machine_3_0_instance_options(MachineState *machine)
 
 static void ccw_machine_3_0_class_options(MachineClass *mc)
 {
+    S390CcwMachineClass *s390mc = S390_MACHINE_CLASS(mc);
+
+    s390mc->hpage_1m_allowed = false;
     ccw_machine_3_1_class_options(mc);
     SET_MACHINE_COMPAT(mc, CCW_COMPAT_3_0);
 }

include/hw/s390x/s390-virtio-ccw.h

@@ -39,12 +39,15 @@ typedef struct S390CcwMachineClass {
     bool ri_allowed;
     bool cpu_model_allowed;
     bool css_migration_enabled;
+    bool hpage_1m_allowed;
 } S390CcwMachineClass;
 
 /* runtime-instrumentation allowed by the machine */
 bool ri_allowed(void);
 /* cpu model allowed by the machine */
 bool cpu_model_allowed(void);
+/* 1M huge page mappings allowed by the machine */
+bool hpage_1m_allowed(void);
 
 /**
  * Returns true if (vmstate based) migration of the channel subsystem

target/s390x/kvm.c

@@ -292,6 +292,12 @@ static int kvm_s390_configure_mempath_backing(KVMState *s)
         return 0;
     }
 
+    if (!hpage_1m_allowed()) {
+        error_report("This QEMU machine does not support huge page "
+                     "mappings");
+        return -EINVAL;
+    }
+
     if (path_psize != 1 * MiB) {
         error_report("Memory backing with 2G pages was specified, "
                      "but KVM does not support this memory backing");