qed: refuse unaligned zero writes with a backing file

Zero writes have cluster granularity in QED.  Therefore they can only be
used to zero entire clusters.

If the zero write request leaves sectors untouched, zeroing the entire
cluster would obscure the backing file.  Instead return -ENOTSUP, which
is handled by block.c:bdrv_co_do_write_zeroes() and falls back to a
regular write.

The qemu-iotests 034 test cases covers this scenario.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/qed.c

@@ -1363,10 +1363,21 @@ static int coroutine_fn bdrv_qed_co_write_zeroes(BlockDriverState *bs,
                                                  int nb_sectors)
 {
     BlockDriverAIOCB *blockacb;
+    BDRVQEDState *s = bs->opaque;
     QEDWriteZeroesCB cb = { .done = false };
     QEMUIOVector qiov;
     struct iovec iov;
 
+    /* Refuse if there are untouched backing file sectors */
+    if (bs->backing_hd) {
+        if (qed_offset_into_cluster(s, sector_num * BDRV_SECTOR_SIZE) != 0) {
+            return -ENOTSUP;
+        }
+        if (qed_offset_into_cluster(s, nb_sectors * BDRV_SECTOR_SIZE) != 0) {
+            return -ENOTSUP;
+        }
+    }
+
     /* Zero writes start without an I/O buffer.  If a buffer becomes necessary
      * then it will be allocated during request processing.
      */