[Qemu-block] [PATCH 8/9] block: Introduce BlockDriver.bdrv_drain callback

[Fam Zheng]

Drivers can have internal request sources that generate IO, like the
need_check_timer in QED. Since we want quiesced periods that contain
nested event loops in block layer, we need to have a way to disable such
event sources.

Block drivers must implement the "bdrv_drain" callback if it has any
internal sources that can generate I/O activity, like a timer or a
worker thread (even in a library) that can schedule QEMUBH in an
asynchronous callback.

Update the comments of bdrv_drain and bdrv_drained_begin accordingly.

Signed-off-by: Fam Zheng <address@hidden>
Reviewed-by: Jeff Cody <address@hidden>
Reviewed-by: Kevin Wolf <address@hidden>
---
 block/io.c                | 6 +++++-
 include/block/block_int.h | 6 ++++++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/block/io.c b/block/io.c
index 358c3c4..a740827 100644
--- a/block/io.c
+++ b/block/io.c
@@ -234,7 +234,8 @@ bool bdrv_requests_pending(BlockDriverState *bs)
 }
 
 /*
- * Wait for pending requests to complete on a single BlockDriverState subtree
+ * Wait for pending requests to complete on a single BlockDriverState subtree,
+ * and suspend block driver's internal I/O until next request arrives.
  *
  * Note that unlike bdrv_drain_all(), the caller must hold the BlockDriverState
  * AioContext.
@@ -247,6 +248,9 @@ void bdrv_drain(BlockDriverState *bs)
 {
     bool busy = true;
 
+    if (bs->drv && bs->drv->bdrv_drain) {
+        bs->drv->bdrv_drain(bs);
+    }
     while (busy) {
         /* Keep iterating */
          bdrv_flush_io_queue(bs);
diff --git a/include/block/block_int.h b/include/block/block_int.h
index d3b2411..ade5387 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -294,6 +294,12 @@ struct BlockDriver {
      */
     int (*bdrv_probe_geometry)(BlockDriverState *bs, HDGeometry *geo);
 
+    /**
+     * Drain and stop any internal sources of requests in the driver, and
+     * remain so until next I/O callback (e.g. bdrv_co_writev) is called.
+     */
+    void (*bdrv_drain)(BlockDriverState *bs);
+
     QLIST_ENTRY(BlockDriver) list;
 };
 
-- 
2.4.3