Re: [Qemu-devel] [PULL 33/42] block: return BDRV_BLOCK_ZERO past end of backing file

[Peter Lieven]

On 06.09.2013 17:39, Stefan Hajnoczi wrote:

From: Paolo Bonzini <pbonz...@redhat.com>

If the sectors are unallocated and we are past the end of the
backing file, they will read as zero.

Signed-off-by: Paolo Bonzini <pbonz...@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefa...@redhat.com>
---
  block.c | 12 ++++++++++--
  1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/block.c b/block.c
index aa9ec83..82bbd6c 100644
--- a/block.c
+++ b/block.c
@@ -3102,8 +3102,16 @@ static int64_t coroutine_fn 
bdrv_co_get_block_status(BlockDriverState *bs,
          return ret;
      }

- if (!(ret & BDRV_BLOCK_DATA) && bdrv_has_zero_init(bs)) {

-        ret |= BDRV_BLOCK_ZERO;
+    if (!(ret & BDRV_BLOCK_DATA)) {
+        if (bdrv_has_zero_init(bs)) {

this should be bdi->discard_zeroes. bdrv_has_zero_init() does only give a valid 
result
right after bdrv_create(). i currently working on extending bdi. I can send a 
patch for this
if you agree.

+            ret |= BDRV_BLOCK_ZERO;
+        } else {
+            BlockDriverState *bs2 = bs->backing_hd;

this segfaults if there is no backing_hd. found while testing get_block_status 
with iscsi.

paolo, is this the correct fix?

@@ -3110,7 +3157,7 @@ static int64_t coroutine_fn 
bdrv_co_get_block_status(BlockDriverState *bs,
     if (!(ret & BDRV_BLOCK_DATA)) {
         if (bdrv_has_zero_init(bs)) {
             ret |= BDRV_BLOCK_ZERO;
-        } else {
+        } else if (bs->backing_hd)  {
             BlockDriverState *bs2 = bs->backing_hd;
             int64_t length2 = bdrv_getlength(bs2);
             if (length2 >= 0 && sector_num >= (length2 >> BDRV_SECTOR_BITS)) {

+            int64_t length2 = bdrv_getlength(bs2);
+            if (length2 >= 0 && sector_num >= (length2 >> BDRV_SECTOR_BITS)) {
+                ret |= BDRV_BLOCK_ZERO;
+            }
+        }
      }
      return ret;
  }