block: Add bdrv_try_set_aio_context()

Eventually, we want to make sure that all parents and all children of a
node are in the same AioContext as the node itself. This means that
changing the AioContext may fail because one of the other involved
parties (e.g. a guest device that was configured with an iothread)
cannot allow switching to a different AioContext.

Introduce a set of functions that allow to first check whether all
involved nodes can switch to a new context and only then do the actual
switch. The check recursively covers children and parents.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>

2 files changed, 11 insertions, 0 deletions

diff --git a/include/block/block.h b/include/block/block.h
index 5e2b98b0ee..fc0239a887 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -586,6 +586,14 @@ void bdrv_coroutine_enter(BlockDriverState *bs, Coroutine *co);
  * This function must be called with iothread lock held.
  */
 void bdrv_set_aio_context(BlockDriverState *bs, AioContext *new_context);
+int bdrv_try_set_aio_context(BlockDriverState *bs, AioContext *ctx,
+                             Error **errp);
+int bdrv_child_try_set_aio_context(BlockDriverState *bs, AioContext *ctx,
+                                   BdrvChild *ignore_child, Error **errp);
+bool bdrv_child_can_set_aio_context(BdrvChild *c, AioContext *ctx,
+                                    GSList **ignore, Error **errp);
+bool bdrv_can_set_aio_context(BlockDriverState *bs, AioContext *ctx,
+                              GSList **ignore, Error **errp);
 int bdrv_probe_blocksizes(BlockDriverState *bs, BlockSizes *bsz);
 int bdrv_probe_geometry(BlockDriverState *bs, HDGeometry *geo);
 
diff --git a/include/block/block_int.h b/include/block/block_int.h
index 94d45c9708..b150c5f047 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -691,6 +691,9 @@ struct BdrvChildRole {
      * can update its reference. */
     int (*update_filename)(BdrvChild *child, BlockDriverState *new_base,
                            const char *filename, Error **errp);
+
+    bool (*can_set_aio_ctx)(BdrvChild *child, AioContext *ctx,
+                            GSList **ignore, Error **errp);
 };
 
 extern const BdrvChildRole child_file;