migration: allow machine to enforce configuration section migration

Migration of pseries-2.3 doesn't have configuration section. Unfortunately,
QEMU 2.4/2.4.1/2.5 are buggy and always stream and expect the configuration
section, and break migration both ways.

This patch introduces a property which allows to enforce a configuration
section for machines who don't have one.

It can be set at startup:

-machine enforce-config-section=on

or later from the QEMU monitor:

qom-set /machine enforce-config-section on

It is up to the tooling to set or unset this property according to the
version of the QEMU at the other end of the pipe.

Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

hw/core/machine.c

@@ -312,6 +312,21 @@ static bool machine_get_suppress_vmdesc(Object *obj, Error **errp)
     return ms->suppress_vmdesc;
 }
 
+static void machine_set_enforce_config_section(Object *obj, bool value,
+                                             Error **errp)
+{
+    MachineState *ms = MACHINE(obj);
+
+    ms->enforce_config_section = value;
+}
+
+static bool machine_get_enforce_config_section(Object *obj, Error **errp)
+{
+    MachineState *ms = MACHINE(obj);
+
+    return ms->enforce_config_section;
+}
+
 static int error_on_sysbus_device(SysBusDevice *sbdev, void *opaque)
 {
     error_report("Option '-device %s' cannot be handled by this machine",
@@ -467,6 +482,12 @@ static void machine_initfn(Object *obj)
     object_property_set_description(obj, "suppress-vmdesc",
                                     "Set on to disable self-describing migration",
                                     NULL);
+    object_property_add_bool(obj, "enforce-config-section",
+                             machine_get_enforce_config_section,
+                             machine_set_enforce_config_section, NULL);
+    object_property_set_description(obj, "enforce-config-section",
+                                    "Set on to enforce configuration section migration",
+                                    NULL);
 
     /* Register notifier when init is done for sysbus sanity checks */
     ms->sysbus_notifier.notify = machine_init_notify;

include/hw/boards.h

@@ -127,6 +127,7 @@ struct MachineState {
     char *firmware;
     bool iommu;
     bool suppress_vmdesc;
+    bool enforce_config_section;
 
     ram_addr_t ram_size;
     ram_addr_t maxram_size;

migration/savevm.c

@@ -878,13 +878,19 @@ bool qemu_savevm_state_blocked(Error **errp)
     return false;
 }
 
+static bool enforce_config_section(void)
+{
+    MachineState *machine = MACHINE(qdev_get_machine());
+    return machine->enforce_config_section;
+}
+
 void qemu_savevm_state_header(QEMUFile *f)
 {
     trace_savevm_state_header();
     qemu_put_be32(f, QEMU_VM_FILE_MAGIC);
     qemu_put_be32(f, QEMU_VM_FILE_VERSION);
 
-    if (!savevm_state.skip_configuration) {
+    if (!savevm_state.skip_configuration || enforce_config_section()) {
         qemu_put_byte(f, QEMU_VM_CONFIGURATION);
         vmstate_save_state(f, &vmstate_configuration, &savevm_state, 0);
     }
@@ -1883,7 +1889,7 @@ int qemu_loadvm_state(QEMUFile *f)
         return -ENOTSUP;
     }
 
-    if (!savevm_state.skip_configuration) {
+    if (!savevm_state.skip_configuration || enforce_config_section()) {
         if (qemu_get_byte(f) != QEMU_VM_CONFIGURATION) {
             error_report("Configuration section missing");
             return -EINVAL;

qemu-options.hx

@@ -43,7 +43,8 @@ DEF("machine", HAS_ARG, QEMU_OPTION_machine, \
     "                aes-key-wrap=on|off controls support for AES key wrapping (default=on)\n"
     "                dea-key-wrap=on|off controls support for DEA key wrapping (default=on)\n"
     "                suppress-vmdesc=on|off disables self-describing migration (default=off)\n"
-    "                nvdimm=on|off controls NVDIMM support (default=off)\n",
+    "                nvdimm=on|off controls NVDIMM support (default=off)\n"
+    "                enforce-config-section=on|off enforce configuration section migration (default=off)\n",
     QEMU_ARCH_ALL)
 STEXI
 @item -machine [type=]@var{name}[,prop=@var{value}[,...]]