block: drop unallocated_blocks_are_zero

Currently this field only set by qed and qcow2. But in fact, all
backing-supporting formats (parallels, qcow, qcow2, qed, vmdk) share
these semantics: on unallocated blocks, if there is no backing file they
just memset the buffer with zeroes.

So, document this behavior for .supports_backing and drop
.unallocated_blocks_are_zero

Signed-off-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: Max Reitz <[email protected]>

Author: Vladimir Sementsov-Ogievskiy

block/io.c

@@ -2406,7 +2406,7 @@ 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) {
+    } else if (want_zero && bs->drv->supports_backing) {
         if (bs->backing) {
             BlockDriverState *bs2 = bs->backing->bs;
             int64_t size2 = bdrv_getlength(bs2);
@@ -2415,12 +2415,7 @@ static int coroutine_fn bdrv_co_block_status(BlockDriverState *bs,
                 ret |= BDRV_BLOCK_ZERO;
             }
         } else {
-            BlockDriverInfo bdi;
-            int ret2 = bdrv_get_info(bs, &bdi);
-
-            if (ret2 == 0 && bdi.unallocated_blocks_are_zero) {
-                ret |= BDRV_BLOCK_ZERO;
-            }
+            ret |= BDRV_BLOCK_ZERO;
         }
     }
 

block/qcow2.c

@@ -4987,7 +4987,6 @@ static BlockMeasureInfo *qcow2_measure(QemuOpts *opts, BlockDriverState *in_bs,
 static int qcow2_get_info(BlockDriverState *bs, BlockDriverInfo *bdi)
 {
     BDRVQcow2State *s = bs->opaque;
-    bdi->unallocated_blocks_are_zero = true;
     bdi->cluster_size = s->cluster_size;
     bdi->vm_state_offset = qcow2_vm_state_offset(s);
     return 0;

block/qed.c

@@ -1514,7 +1514,6 @@ static int bdrv_qed_get_info(BlockDriverState *bs, BlockDriverInfo *bdi)
     memset(bdi, 0, sizeof(*bdi));
     bdi->cluster_size = s->header.cluster_size;
     bdi->is_dirty = s->header.features & QED_F_NEED_CHECK;
-    bdi->unallocated_blocks_are_zero = true;
     return 0;
 }
 

include/block/block.h

@@ -21,11 +21,6 @@ typedef struct BlockDriverInfo {
     /* offset at which the VM state can be saved (0 if not possible) */
     int64_t vm_state_offset;
     bool is_dirty;
-    /*
-     * True if unallocated blocks read back as zeroes. This is equivalent
-     * to the LBPRZ flag in the SCSI logical block provisioning page.
-     */
-    bool unallocated_blocks_are_zero;
     /*
      * True if this block driver only supports compressed writes
      */

include/block/block_int.h

@@ -123,7 +123,17 @@ struct BlockDriver {
      */
     bool bdrv_needs_filename;
 
-    /* Set if a driver can support backing files */
+    /*
+     * Set if a driver can support backing files. This also implies the
+     * following semantics:
+     *
+     *  - Return status 0 of .bdrv_co_block_status means that corresponding
+     *    blocks are not allocated in this layer of backing-chain
+     *  - For such (unallocated) blocks, read will:
+     *    - fill buffer with zeros if there is no backing file
+     *    - read from the backing file otherwise, where the block layer
+     *      takes care of reading zeros beyond EOF if backing file is short
+     */
     bool supports_backing;
 
     /* For handling image reopen for split or non-split files */