[PATCH v3 1/2] qcow2: Report BDRV_BLOCK_ZERO more accurately in bdrv_co_block_status()

[Alberto Garcia]

If a BlockDriverState supports backing files but has none then any
unallocated area reads back as zeroes.

bdrv_co_block_status() is only reporting this is if want_zero is true,
but this is an inexpensive test and there is no reason not to do it in
all cases.

Suggested-by: Vladimir Sementsov-Ogievskiy <vsement...@virtuozzo.com>
Signed-off-by: Alberto Garcia <be...@igalia.com>
---
 block/io.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/block/io.c b/block/io.c
index ad3a51ed53..1b0ae29610 100644
--- a/block/io.c
+++ b/block/io.c
@@ -2408,16 +2408,16 @@ static int coroutine_fn 
bdrv_co_block_status(BlockDriverState *bs,
 
     if (ret & (BDRV_BLOCK_DATA | BDRV_BLOCK_ZERO)) {
         ret |= BDRV_BLOCK_ALLOCATED;
-    } else if (want_zero && bs->drv->supports_backing) {
-        if (bs->backing) {
+    } else if (bs->drv->supports_backing) {
+        if (!bs->backing) {
+            ret |= BDRV_BLOCK_ZERO;
+        } else if (want_zero) {
             BlockDriverState *bs2 = bs->backing->bs;
             int64_t size2 = bdrv_getlength(bs2);
 
             if (size2 >= 0 && offset >= size2) {
                 ret |= BDRV_BLOCK_ZERO;
             }
-        } else {
-            ret |= BDRV_BLOCK_ZERO;
         }
     }
 
-- 
2.20.1