[Qemu-devel] [PATCH v2 3/3] migration: allow to suppress configuration section submission

[Greg Kurz <gkurz@linux.vnet.ibm.com>]

Since the addition of the configuration section in QEMU 2.4, it is impossible
to migrate a pseries-2.3 machine back to QEMU 2.3.

This patch makes it possible thanks to a new machine property which allows to
disable configuration section submission.

To disable submission, just add:

-machine suppress-config-section=on

Alternatively, if the target QEMU version isn't known at startup, this can
be done later from the QEMU monitor with:

qom-set /machine suppress-config-section on

This property won't be automatically set for pseries-2.3 because it would
then break backward migration to QEMU 2.4. If automatic behaviour is needed,
it is up to the tooling to handle this.

Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
---
 hw/core/machine.c   |   21 +++++++++++++++++++++
 include/hw/boards.h |    1 +
 migration/savevm.c  |    8 +++++++-
 qemu-options.hx     |    3 ++-
 4 files changed, 31 insertions(+), 2 deletions(-)

diff --git a/hw/core/machine.c b/hw/core/machine.c
index 7203dd260bf2..ad4ecdc65787 100644
--- a/hw/core/machine.c
+++ b/hw/core/machine.c
@@ -327,6 +327,21 @@ static bool machine_get_require_config_section(Object *obj, Error **errp)
     return ms->require_config_section;
 }
 
+static void machine_set_suppress_config_section(Object *obj, bool value,
+                                                Error **errp)
+{
+    MachineState *ms = MACHINE(obj);
+
+    ms->suppress_config_section = value;
+}
+
+static bool machine_get_suppress_config_section(Object *obj, Error **errp)
+{
+    MachineState *ms = MACHINE(obj);
+
+    return ms->suppress_config_section;
+}
+
 static int error_on_sysbus_device(SysBusDevice *sbdev, void *opaque)
 {
     error_report("Option '-device %s' cannot be handled by this machine",
@@ -489,6 +504,12 @@ static void machine_initfn(Object *obj)
     object_property_set_description(obj, "require-config-section",
                                     "Set on/off to reject/accept migration without configuration section",
                                     NULL);
+    object_property_add_bool(obj, "suppress-config-section",
+                             machine_get_suppress_config_section,
+                             machine_set_suppress_config_section, NULL);
+    object_property_set_description(obj, "suppress-config-section",
+                                    "Set on/off to enable/disable configuration section migration",
+                                    NULL);
 
     /* Register notifier when init is done for sysbus sanity checks */
     ms->sysbus_notifier.notify = machine_init_notify;
diff --git a/include/hw/boards.h b/include/hw/boards.h
index d6ff1ba4c260..35aaa6493458 100644
--- a/include/hw/boards.h
+++ b/include/hw/boards.h
@@ -129,6 +129,7 @@ struct MachineState {
     bool iommu;
     bool suppress_vmdesc;
     bool require_config_section;
+    bool suppress_config_section;
 
     ram_addr_t ram_size;
     ram_addr_t maxram_size;
diff --git a/migration/savevm.c b/migration/savevm.c
index f8dee03a3350..3fd07efeb38b 100644
--- a/migration/savevm.c
+++ b/migration/savevm.c
@@ -878,13 +878,19 @@ bool qemu_savevm_state_blocked(Error **errp)
     return false;
 }
 
+static bool should_send_configuration(void)
+{
+    MachineState *machine = MACHINE(qdev_get_machine());
+    return !machine->suppress_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 && should_send_configuration()) {
         qemu_put_byte(f, QEMU_VM_CONFIGURATION);
         vmstate_save_state(f, &vmstate_configuration, &savevm_state, 0);
     }
diff --git a/qemu-options.hx b/qemu-options.hx
index 172471c75b1c..af08fa8ec314 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -44,7 +44,8 @@ DEF("machine", HAS_ARG, QEMU_OPTION_machine, \
     "                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"
-    "                require-config-section=on|off incoming migration requires configuration section (default=on)\n",
+    "                require-config-section=on|off incoming migration requires configuration section (default=on)\n"
+    "                suppress-config-section=on|off disables configuration section migration (default=off)\n",
     QEMU_ARCH_ALL)
 STEXI
 @item -machine [type=]@var{name}[,prop=@var{value}[,...]]