[Qemu-devel] [PATCH for-2.8 1/3] migration: alternative way to set instance_id in SaveStateEntry

[Michael Roth]

From: Jianjun Duan <address@hidden>

Currently migrated Devices are identified with an idstr which is
calculated automatically using their path in the QOM composition
tree. In some cases these Devices may have a more reliable
identifier that may be preferable in situations where there's a
chance their path in the composition tree might change in the
future as a resulting of changes in how the device is modeled
in QEMU.

One such Device is the "spapr-dr-connector" used to handle hotplug
for various resources on pseries machine types, where the PAPR
specification mandates that each DRC have a 32-bit globally unique
identifier associated with it. As such, this identifier is also ideal
as a reliable way to identify a particular DRC in the migration
stream, so we introduce support here for using a caller-side supplied
instance_id for Devices in preparation for that.

register_savevm_live() and vmstate_register_with_alias_id() already
provide a means to let the caller supply an instance_id instead of
relying on the migration subsystem to generate one automatically,
but in cases where we're registering SaveVMHandlers or VMSDs
(respectively) that are associated with a Device, the instance_id is
ignored in favor of a string identifier based solely on the QOM path.

This patch generalizes this so that an instance_id can also be
supplied by the caller for Devices. Since VMSD registration for
Devices is generally handled automatically by qdev, we also introduce
a DeviceClass->dev_get_instance_id() method that, when set, is called
by qdev to obtain the corresponding instance_id that should be used
for a particular Device. Otherwise we maintain the original behavior
of passing instance_id == -1 and thus falling back to the previous
logic of using the QOM path.

Signed-off-by: Jianjun Duan <address@hidden>
* moved usage of DeviceClass->dev_get_instance_id() out of savevm.c
  and into caller (qdev) instead
* clarified usage/intent in comments and commit msg
Signed-off-by: Michael Roth <address@hidden>
---
 hw/core/qdev.c         | 6 +++++-
 include/hw/qdev-core.h | 9 +++++++++
 migration/savevm.c     | 4 ++--
 3 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/hw/core/qdev.c b/hw/core/qdev.c
index 5783442..c8c0c44 100644
--- a/hw/core/qdev.c
+++ b/hw/core/qdev.c
@@ -933,7 +933,11 @@ static void device_set_realized(Object *obj, bool value, 
Error **errp)
         }
 
         if (qdev_get_vmsd(dev)) {
-            vmstate_register_with_alias_id(dev, -1, qdev_get_vmsd(dev), dev,
+            int instance_id = DEVICE_GET_CLASS(dev)->dev_get_instance_id
+                ? DEVICE_GET_CLASS(dev)->dev_get_instance_id(dev)
+                : -1;
+
+            vmstate_register_with_alias_id(dev, instance_id, 
qdev_get_vmsd(dev), dev,
                                            dev->instance_id_alias,
                                            dev->alias_required_for_version);
         }
diff --git a/include/hw/qdev-core.h b/include/hw/qdev-core.h
index 2c97347..8ba82af 100644
--- a/include/hw/qdev-core.h
+++ b/include/hw/qdev-core.h
@@ -139,6 +139,15 @@ typedef struct DeviceClass {
     qdev_initfn init; /* TODO remove, once users are converted to realize */
     qdev_event exit; /* TODO remove, once users are converted to unrealize */
     const char *bus_type;
+
+    /* When this field is set, instead of using the device's QOM path,
+     * SaveStateEntry's for devices will be identified using a combination
+     * of the corresponding VMSD name and an instance_id returned by this
+     * function. This should only be necessary for situations where the
+     * QOM path is anticipated to change and a more stable identifier is
+     * desired to identify a device in the migration stream.
+     */
+    int (*dev_get_instance_id)(DeviceState *dev);
 } DeviceClass;
 
 typedef struct NamedGPIOList NamedGPIOList;
diff --git a/migration/savevm.c b/migration/savevm.c
index 0363372..a95fff9 100644
--- a/migration/savevm.c
+++ b/migration/savevm.c
@@ -556,7 +556,7 @@ int register_savevm_live(DeviceState *dev,
         se->is_ram = 1;
     }
 
-    if (dev) {
+    if (dev && instance_id == -1) {
         char *id = qdev_get_dev_path(dev);
         if (id) {
             pstrcpy(se->idstr, sizeof(se->idstr), id);
@@ -640,7 +640,7 @@ int vmstate_register_with_alias_id(DeviceState *dev, int 
instance_id,
     se->vmsd = vmsd;
     se->alias_id = alias_id;
 
-    if (dev) {
+    if (dev && instance_id == -1) {
         char *id = qdev_get_dev_path(dev);
         if (id) {
             pstrcpy(se->idstr, sizeof(se->idstr), id);
-- 
1.9.1