[Qemu-devel] [PATCH 1/3] block: add function deappend()

[Wenchao Xia <xiawenc@linux.vnet.ibm.com>]

  This function should be used to remove contents of active *bs on the
top of backing chain, when top *bs was committed to bs->backing_hd or
*bs was just appended.

Signed-off-by: Wenchao Xia <xiawenc@linux.vnet.ibm.com>
---
 block.c               |   66 +++++++++++++++++++++++++++++++++++++++++++++++++
 include/block/block.h |    1 +
 2 files changed, 67 insertions(+), 0 deletions(-)

diff --git a/block.c b/block.c
index 0ae2e93..0cacc2f 100644
--- a/block.c
+++ b/block.c
@@ -1536,6 +1536,72 @@ void bdrv_append(BlockDriverState *bs_new, BlockDriverState *bs_top)
             bs_new->drv ? bs_new->drv->format_name : "");
 }
 
+/*
+ * Remove top bs contents in the image backing chain while the chain is live.
+ * Required fields on the top layer will be kept to let qemu continue use
+ * @bs_top.
+ *
+ * This will modify the BlockDriverState fields, and swap contents between
+ * bs_top and bs_top->backing_hd. Both bs_top and bs_top->backing_hd are
+ * modified, and bs_top->backing_hd will be deleted.
+ *
+ * This function does not delete any image files. If @bs_top have no backing
+ * image, the function will do nothing.
+ *
+ * Warning: Caller must guarantee @bs_top have same contents with its backing
+ * image, that is, when @bs_top was just appended or committed to
+ * bs_top->backing_hd.
+ */
+void bdrv_deappend(BlockDriverState *bs_top)
+{
+    BlockDriverState tmp;
+    BlockDriverState *bs_backing = bs_top->backing_hd;
+
+    if (!bs_backing) {
+        return;
+    }
+
+    BlockDriverState *bs_bottom = bs_backing->backing_hd;
+    /* Now only support deappend when bs_top is in following conditions. */
+    g_assert(bs_top->device_name[0] != '\0');
+    g_assert(bs_top->dev != NULL);
+
+    g_assert(bs_top->dirty_bitmap == NULL);
+    g_assert(bs_top->job == NULL);
+    g_assert(bs_top->in_use == 0);
+    g_assert(bs_top->io_limits_enabled == false);
+    g_assert(bs_top->block_timer == NULL);
+
+
+    tmp = *bs_top;
+    *bs_top = *bs_backing;
+    *bs_backing = tmp;
+
+    /* there are some fields which should not be swapped, move them back */
+    bdrv_move_feature_fields(&tmp, bs_top);
+    bdrv_move_feature_fields(bs_top, bs_backing);
+    bdrv_move_feature_fields(bs_backing, &tmp);
+
+    /* bs_top should keep device attached  */
+    g_assert(bs_top->device_name[0] != '\0');
+    g_assert(bs_top->dev != NULL);
+
+    /* check a few fields that should remain */
+    g_assert(bs_top->job == NULL);
+    g_assert(bs_top->in_use == 0);
+    g_assert(bs_top->io_limits_enabled == false);
+    g_assert(bs_top->block_timer == NULL);
+    g_assert(bs_top->backing_hd == bs_bottom);
+
+    /* delete bs_backing */
+    bs_backing->backing_hd = NULL;
+    bs_backing->backing_file[0] = 0;
+    bs_backing->backing_format[0] = 0;
+    bdrv_delete(bs_backing);
+
+    bdrv_rebind(bs_top);
+}
+
 void bdrv_delete(BlockDriverState *bs)
 {
     assert(!bs->dev);
diff --git a/include/block/block.h b/include/block/block.h
index 9dc6aad..4450121 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -132,6 +132,7 @@ BlockDriverState *bdrv_new(const char *device_name);
 void bdrv_make_anon(BlockDriverState *bs);
 void bdrv_swap(BlockDriverState *bs_new, BlockDriverState *bs_old);
 void bdrv_append(BlockDriverState *bs_new, BlockDriverState *bs_top);
+void bdrv_deappend(BlockDriverState *bs_top);
 void bdrv_delete(BlockDriverState *bs);
 int bdrv_parse_cache_flags(const char *mode, int *flags);
 int bdrv_parse_discard_flags(const char *mode, int *flags);
-- 
1.7.1