block, block-backend: write some hot coroutine wrappers by hand

The introduction of the graph lock is causing blk_get_geometry, a hot function
used in the I/O path, to create a coroutine.  However, the only part that really
needs to run in coroutine context is the call to bdrv_co_refresh_total_sectors,
which in turn only happens in the rare case of host CD-ROM devices.

So, write by hand the three wrappers on the path from blk_co_get_geometry to
bdrv_co_refresh_total_sectors, so that the coroutine wrapper is only created
if bdrv_nb_sectors actually calls bdrv_refresh_total_sectors.

Reported-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20230407153303.391121-9-pbonzini@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 27 insertions, 0 deletions

diff --git a/block/block-backend.c b/block/block-backend.c
index cf58d4d1b7..55efc735b4 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -1629,6 +1629,23 @@ int64_t coroutine_fn blk_co_nb_sectors(BlockBackend *blk)
     }
 }
 
+/*
+ * This wrapper is written by hand because this function is in the hot I/O path,
+ * via blk_get_geometry.
+ */
+int64_t coroutine_mixed_fn blk_nb_sectors(BlockBackend *blk)
+{
+    BlockDriverState *bs = blk_bs(blk);
+
+    IO_CODE();
+
+    if (!bs) {
+        return -ENOMEDIUM;
+    } else {
+        return bdrv_nb_sectors(bs);
+    }
+}
+
 /* return 0 as number of sectors if no device present or error */
 void coroutine_fn blk_co_get_geometry(BlockBackend *blk,
                                       uint64_t *nb_sectors_ptr)
@@ -1637,6 +1654,16 @@ void coroutine_fn blk_co_get_geometry(BlockBackend *blk,
     *nb_sectors_ptr = ret < 0 ? 0 : ret;
 }
 
+/*
+ * This wrapper is written by hand because this function is in the hot I/O path.
+ */
+void coroutine_mixed_fn blk_get_geometry(BlockBackend *blk,
+                                         uint64_t *nb_sectors_ptr)
+{
+    int64_t ret = blk_nb_sectors(blk);
+    *nb_sectors_ptr = ret < 0 ? 0 : ret;
+}
+
 BlockAIOCB *blk_aio_preadv(BlockBackend *blk, int64_t offset,
                            QEMUIOVector *qiov, BdrvRequestFlags flags,
                            BlockCompletionFunc *cb, void *opaque)