[PULL 03/27] qemu-img: rebase: use backing files' BlockBackend for buffer alignment

[Kevin Wolf]

From: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>

Since commit bb1c05973cf ("qemu-img: Use qemu_blockalign"), buffers for
the data read from the old and new backing files are aligned using
BlockDriverState (or BlockBackend later on) referring to the target image.
However, this isn't quite right, because buf_new is only being used for
reading from the new backing, while buf_old is being used for both reading
from the old backing and writing to the target.  Let's take that into account
and use more appropriate values as alignments.

Signed-off-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
Message-ID: <20230919165804.439110-4-andrey.drobyshev@virtuozzo.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 qemu-img.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/qemu-img.c b/qemu-img.c
index 2b2a3a86ca..e61d996e0f 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -3759,8 +3759,13 @@ static int img_rebase(int argc, char **argv)
         int64_t n;
         float local_progress = 0;
 
-        buf_old = blk_blockalign(blk, IO_BUF_SIZE);
-        buf_new = blk_blockalign(blk, IO_BUF_SIZE);
+        if (blk_old_backing && bdrv_opt_mem_align(blk_bs(blk_old_backing)) >
+            bdrv_opt_mem_align(blk_bs(blk))) {
+            buf_old = blk_blockalign(blk_old_backing, IO_BUF_SIZE);
+        } else {
+            buf_old = blk_blockalign(blk, IO_BUF_SIZE);
+        }
+        buf_new = blk_blockalign(blk_new_backing, IO_BUF_SIZE);
 
         size = blk_getlength(blk);
         if (size < 0) {
-- 
2.41.0