block/export: Add option to allow export of inactive nodes

Add an option in BlockExportOptions to allow creating an export on an
inactive node without activating the node. This mode needs to be
explicitly supported by the export type (so that it doesn't perform any
operations that are forbidden for inactive nodes), so this patch alone
doesn't allow this option to be successfully used yet.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Acked-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20250204211407.381505-13-kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

3 files changed, 33 insertions, 11 deletions

diff --git a/block/export/export.c b/block/export/export.c
index bac42b8..f3bbf11 100644
--- a/block/export/export.c
+++ b/block/export/export.c
@@ -75,6 +75,7 @@ static const BlockExportDriver *blk_exp_find_driver(BlockExportType type)
 BlockExport *blk_exp_add(BlockExportOptions *export, Error **errp)
 {
     bool fixed_iothread = export->has_fixed_iothread && export->fixed_iothread;
+    bool allow_inactive = export->has_allow_inactive && export->allow_inactive;
     const BlockExportDriver *drv;
     BlockExport *exp = NULL;
     BlockDriverState *bs;
@@ -138,17 +139,24 @@ BlockExport *blk_exp_add(BlockExportOptions *export, Error **errp)
         }
     }
 
-    /*
-     * Block exports are used for non-shared storage migration. Make sure
-     * that BDRV_O_INACTIVE is cleared and the image is ready for write
-     * access since the export could be available before migration handover.
-     * ctx was acquired in the caller.
-     */
     bdrv_graph_rdlock_main_loop();
-    ret = bdrv_activate(bs, errp);
-    if (ret < 0) {
-        bdrv_graph_rdunlock_main_loop();
-        goto fail;
+    if (allow_inactive) {
+        if (!drv->supports_inactive) {
+            error_setg(errp, "Export type does not support inactive exports");
+            bdrv_graph_rdunlock_main_loop();
+            goto fail;
+        }
+    } else {
+        /*
+         * Block exports are used for non-shared storage migration. Make sure
+         * that BDRV_O_INACTIVE is cleared and the image is ready for write
+         * access since the export could be available before migration handover.
+         */
+        ret = bdrv_activate(bs, errp);
+        if (ret < 0) {
+            bdrv_graph_rdunlock_main_loop();
+            goto fail;
+        }
     }
     bdrv_graph_rdunlock_main_loop();
 
@@ -162,6 +170,9 @@ BlockExport *blk_exp_add(BlockExportOptions *export, Error **errp)
     if (!fixed_iothread) {
         blk_set_allow_aio_context_change(blk, true);
     }
+    if (allow_inactive) {
+        blk_set_force_allow_inactivate(blk);
+    }
 
     ret = blk_insert_bs(blk, bs, errp);
     if (ret < 0) {
diff --git a/include/block/export.h b/include/block/export.h
index f2fe0f8..4bd9531 100644
--- a/include/block/export.h
+++ b/include/block/export.h
@@ -29,6 +29,9 @@ typedef struct BlockExportDriver {
      */
     size_t instance_size;
 
+    /* True if the export type supports running on an inactive node */
+    bool supports_inactive;
+
     /* Creates and starts a new block export */
     int (*create)(BlockExport *, BlockExportOptions *, Error **);
 
diff --git a/qapi/block-export.json b/qapi/block-export.json
index ce33fe3..117b05d 100644
--- a/qapi/block-export.json
+++ b/qapi/block-export.json
@@ -372,6 +372,13 @@
 #     cannot be moved to the iothread.  The default is false.
 #     (since: 5.2)
 #
+# @allow-inactive: If true, the export allows the exported node to be inactive.
+#     If it is created for an inactive block node, the node remains inactive. If
+#     the export type doesn't support running on an inactive node, an error is
+#     returned. If false, inactive block nodes are automatically activated before
+#     creating the export and trying to inactivate them later fails.
+#     (since: 10.0; default: false)
+#
 # Since: 4.2
 ##
 { 'union': 'BlockExportOptions',
@@ -381,7 +388,8 @@
             '*iothread': 'str',
             'node-name': 'str',
             '*writable': 'bool',
-            '*writethrough': 'bool' },
+            '*writethrough': 'bool',
+            '*allow-inactive': 'bool' },
   'discriminator': 'type',
   'data': {
       'nbd': 'BlockExportOptionsNbd',