Add possibility for devices to hook into top of migrate-incoming QMP
command. It's a place, where migration capabilities and parameters
are already set, but migration downtime is not yet started (source
is still running). So here devices may do some remaining initializations
dependent on migration capabilities. This will be used in further commit
to support backend-transfer migration feature for vhost-user-blk.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Acked-by: Peter Xu <peterx@redhat.com>
---
 include/migration/vmstate.h |  1 +
 migration/migration.c       |  4 ++++
 migration/savevm.c          | 15 +++++++++++++++
 migration/savevm.h          |  1 +
 4 files changed, 21 insertions(+)

diff --git a/include/migration/vmstate.h b/include/migration/vmstate.h
index 63ccaee07a..f243518fb5 100644
--- a/include/migration/vmstate.h
+++ b/include/migration/vmstate.h
@@ -217,6 +217,7 @@ struct VMStateDescription {
     int version_id;
     int minimum_version_id;
     MigrationPriority priority;
+    bool (*pre_incoming)(void *opaque, Error **errp);
     int (*pre_load)(void *opaque);
     int (*pre_load_errp)(void *opaque, Error **errp);
     int (*post_load)(void *opaque, int version_id);
diff --git a/migration/migration.c b/migration/migration.c
index a63b46bbef..6ed6a10f57 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -1983,6 +1983,10 @@ void qmp_migrate_incoming(const char *uri, bool has_channels,
         return;
     }
 
+    if (!qemu_pre_incoming(errp)) {
+        return;
+    }
+
     if (!yank_register_instance(MIGRATION_YANK_INSTANCE, errp)) {
         return;
     }
diff --git a/migration/savevm.c b/migration/savevm.c
index 7b35ec4dd0..6e240ea100 100644
--- a/migration/savevm.c
+++ b/migration/savevm.c
@@ -1268,6 +1268,21 @@ bool qemu_savevm_state_blocked(Error **errp)
     return false;
 }
 
+bool qemu_pre_incoming(Error **errp)
+{
+    SaveStateEntry *se;
+
+    QTAILQ_FOREACH(se, &savevm_state.handlers, entry) {
+        if (se->vmsd && se->vmsd->pre_incoming) {
+            if (!se->vmsd->pre_incoming(se->opaque, errp)) {
+                return false;
+            }
+        }
+    }
+
+    return true;
+}
+
 void qemu_savevm_non_migratable_list(strList **reasons)
 {
     SaveStateEntry *se;
diff --git a/migration/savevm.h b/migration/savevm.h
index c337e3e3d1..4ad8997f94 100644
--- a/migration/savevm.h
+++ b/migration/savevm.h
@@ -29,6 +29,7 @@
 #define QEMU_VM_COMMAND              0x08
 #define QEMU_VM_SECTION_FOOTER       0x7e
 
+bool qemu_pre_incoming(Error **errp);
 bool qemu_savevm_state_blocked(Error **errp);
 void qemu_savevm_non_migratable_list(strList **reasons);
 int qemu_savevm_state_prepare(Error **errp);
-- 
2.48.1