[Qemu-devel] [PATCH 1/2] migration: allow to prioritize save state entries

[Peter Xu <peterx@redhat.com>]

During migration, save state entries are saved/loaded without a specific
order - we just traverse the savevm_state.handlers list and do it one by
one. This might not be enough in the future.

There is case that we need to load specific device's vmstate first
before others. For example, VT-d IOMMU contains DMA address remapping
information, which is required by all the PCI devices to do address
translations. We need to make sure IOMMU's device state is loaded before
the rest of the PCI devices, so that DMA address translation can work
properly.

This patch provide a VMStateDescription.priority value to allow specify
the priority of the saved states. The loadvm operation will be done with
those devices with higher vmsd priority.

Current ordering logic is still naive and slow, but after all that's not
a critical path so IMO it's a workable solution for now.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 include/migration/vmstate.h |  1 +
 migration/savevm.c          | 27 +++++++++++++++++++++++----
 2 files changed, 24 insertions(+), 4 deletions(-)

diff --git a/include/migration/vmstate.h b/include/migration/vmstate.h
index 1638ee5..dd5e26a 100644
--- a/include/migration/vmstate.h
+++ b/include/migration/vmstate.h
@@ -207,6 +207,7 @@ struct VMStateDescription {
     int version_id;
     int minimum_version_id;
     int minimum_version_id_old;
+    int priority;
     LoadStateHandler *load_state_old;
     int (*pre_load)(void *opaque);
     int (*post_load)(void *opaque, int version_id);
diff --git a/migration/savevm.c b/migration/savevm.c
index 0363372..93a2837 100644
--- a/migration/savevm.c
+++ b/migration/savevm.c
@@ -257,6 +257,7 @@ typedef struct SaveStateEntry {
     void *opaque;
     CompatEntry *compat;
     int is_ram;
+    int priority;
 } SaveStateEntry;
 
 typedef struct SaveState {
@@ -532,6 +533,23 @@ static int calculate_compat_instance_id(const char *idstr)
     return instance_id;
 }
 
+static void savevm_state_handler_insert(SaveStateEntry *nse)
+{
+    SaveStateEntry *se;
+
+    QTAILQ_FOREACH(se, &savevm_state.handlers, entry) {
+        if (se->priority < nse->priority) {
+            break;
+        }
+    }
+
+    if (se) {
+        QTAILQ_INSERT_BEFORE(se, nse, entry);
+    } else {
+        QTAILQ_INSERT_TAIL(&savevm_state.handlers, nse, entry);
+    }
+}
+
 /* TODO: Individual devices generally have very little idea about the rest
    of the system, so instance_id should be removed/replaced.
    Meanwhile pass -1 as instance_id if you do not already have a clearly
@@ -551,6 +569,8 @@ int register_savevm_live(DeviceState *dev,
     se->ops = ops;
     se->opaque = opaque;
     se->vmsd = NULL;
+    se->priority = 0;
+
     /* if this is a live_savem then set is_ram */
     if (ops->save_live_setup != NULL) {
         se->is_ram = 1;
@@ -578,8 +598,7 @@ int register_savevm_live(DeviceState *dev,
         se->instance_id = instance_id;
     }
     assert(!se->compat || se->instance_id == 0);
-    /* add at the end of list */
-    QTAILQ_INSERT_TAIL(&savevm_state.handlers, se, entry);
+    savevm_state_handler_insert(se);
     return 0;
 }
 
@@ -639,6 +658,7 @@ int vmstate_register_with_alias_id(DeviceState *dev, int instance_id,
     se->opaque = opaque;
     se->vmsd = vmsd;
     se->alias_id = alias_id;
+    se->priority = vmsd->priority;
 
     if (dev) {
         char *id = qdev_get_dev_path(dev);
@@ -662,8 +682,7 @@ int vmstate_register_with_alias_id(DeviceState *dev, int instance_id,
         se->instance_id = instance_id;
     }
     assert(!se->compat || se->instance_id == 0);
-    /* add at the end of list */
-    QTAILQ_INSERT_TAIL(&savevm_state.handlers, se, entry);
+    savevm_state_handler_insert(se);
     return 0;
 }
 
-- 
2.7.4