migration: dirty-bitmap: Allow control of bitmap persistence

Bitmap's source persistence is transported over the migration stream and
the destination mirrors it. In some cases the destination might want to
persist bitmaps which are not persistent on the source (e.g. the result
of merging bitmaps from a number of layers on the source when migrating
into a squashed image) but currently it would need to create another set
of persistent bitmaps and merge them.

This patch adds a 'transform' property to the alias map which allows
overriding the persistence of migrated bitmaps both on the source and
destination sides.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Message-Id: <b20afb675917b86f6359ac3591166ac6d4233573.1613150869.git.pkrempa@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: grammar tweaks, drop dead conditional]
Signed-off-by: Eric Blake <eblake@redhat.com>

2 files changed, 44 insertions, 4 deletions

diff --git a/migration/block-dirty-bitmap.c b/migration/block-dirty-bitmap.c
index b39c13ce4e..975093610a 100644
--- a/migration/block-dirty-bitmap.c
+++ b/migration/block-dirty-bitmap.c
@@ -150,6 +150,7 @@ typedef struct DBMLoadState {
     BdrvDirtyBitmap *bitmap;
 
     bool before_vm_start_handled; /* set in dirty_bitmap_mig_before_vm_start */
+    BitmapMigrationBitmapAlias *bmap_inner;
 
     /*
      * cancelled
@@ -529,6 +530,7 @@ static int add_bitmaps_to_list(DBMSaveState *s, BlockDriverState *bs,
     }
 
     FOR_EACH_DIRTY_BITMAP(bs, bitmap) {
+        BitmapMigrationBitmapAliasTransform *bitmap_transform = NULL;
         bitmap_name = bdrv_dirty_bitmap_name(bitmap);
         if (!bitmap_name) {
             continue;
@@ -549,6 +551,9 @@ static int add_bitmaps_to_list(DBMSaveState *s, BlockDriverState *bs,
             }
 
             bitmap_alias = bmap_inner->alias;
+            if (bmap_inner->has_transform) {
+                bitmap_transform = bmap_inner->transform;
+            }
         } else {
             if (strlen(bitmap_name) > UINT8_MAX) {
                 error_report("Cannot migrate bitmap '%s' on node '%s': "
@@ -574,8 +579,15 @@ static int add_bitmaps_to_list(DBMSaveState *s, BlockDriverState *bs,
         if (bdrv_dirty_bitmap_enabled(bitmap)) {
             dbms->flags |= DIRTY_BITMAP_MIG_START_FLAG_ENABLED;
         }
-        if (bdrv_dirty_bitmap_get_persistence(bitmap)) {
-            dbms->flags |= DIRTY_BITMAP_MIG_START_FLAG_PERSISTENT;
+        if (bitmap_transform &&
+            bitmap_transform->has_persistent) {
+            if (bitmap_transform->persistent) {
+                dbms->flags |= DIRTY_BITMAP_MIG_START_FLAG_PERSISTENT;
+            }
+        } else {
+            if (bdrv_dirty_bitmap_get_persistence(bitmap)) {
+                dbms->flags |= DIRTY_BITMAP_MIG_START_FLAG_PERSISTENT;
+            }
         }
 
         QSIMPLEQ_INSERT_TAIL(&s->dbms_list, dbms, entry);
@@ -783,6 +795,7 @@ static int dirty_bitmap_load_start(QEMUFile *f, DBMLoadState *s)
     uint32_t granularity = qemu_get_be32(f);
     uint8_t flags = qemu_get_byte(f);
     LoadBitmapState *b;
+    bool persistent;
 
     if (s->cancelled) {
         return 0;
@@ -807,7 +820,15 @@ static int dirty_bitmap_load_start(QEMUFile *f, DBMLoadState *s)
         return -EINVAL;
     }
 
-    if (flags & DIRTY_BITMAP_MIG_START_FLAG_PERSISTENT) {
+    if (s->bmap_inner &&
+        s->bmap_inner->has_transform &&
+        s->bmap_inner->transform->has_persistent) {
+        persistent = s->bmap_inner->transform->persistent;
+    } else {
+        persistent = flags & DIRTY_BITMAP_MIG_START_FLAG_PERSISTENT;
+    }
+
+    if (persistent) {
         bdrv_dirty_bitmap_set_persistence(s->bitmap, true);
     }
 
@@ -1091,6 +1112,8 @@ static int dirty_bitmap_load_header(QEMUFile *f, DBMLoadState *s,
             } else {
                 bitmap_name = bmap_inner->name;
             }
+
+            s->bmap_inner = bmap_inner;
         }
 
         if (!s->cancelled) {
diff --git a/qapi/migration.json b/qapi/migration.json
index ce14d78071..6e5943fbb4 100644
--- a/qapi/migration.json
+++ b/qapi/migration.json
@@ -537,6 +537,19 @@
             { 'name': 'zstd', 'if': 'defined(CONFIG_ZSTD)' } ] }
 
 ##
+# @BitmapMigrationBitmapAliasTransform:
+#
+# @persistent: If present, the bitmap will be made persistent
+#              or transient depending on this parameter.
+#
+# Since: 6.0
+##
+{ 'struct': 'BitmapMigrationBitmapAliasTransform',
+  'data': {
+      '*persistent': 'bool'
+  } }
+
+##
 # @BitmapMigrationBitmapAlias:
 #
 # @name: The name of the bitmap.
@@ -544,12 +557,16 @@
 # @alias: An alias name for migration (for example the bitmap name on
 #         the opposite site).
 #
+# @transform: Allows the modification of the migrated bitmap.
+#             (since 6.0)
+#
 # Since: 5.2
 ##
 { 'struct': 'BitmapMigrationBitmapAlias',
   'data': {
       'name': 'str',
-      'alias': 'str'
+      'alias': 'str',
+      '*transform': 'BitmapMigrationBitmapAliasTransform'
   } }
 
 ##