block drivers: expose requirement for write same alignment from formats

This will let misaligned but large requests use zero clusters.  This
is important because the cluster size is not guest visible.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Reviewed-by: NPeter Lieven <pl@kamp.de>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

block/qcow2.c

@@ -718,6 +718,7 @@ static int qcow2_open(BlockDriverState *bs, QDict *options, int flags,
     }
 
     qemu_opts_del(opts);
+    bs->bl.write_zeroes_alignment = s->cluster_sectors;
 
     if (s->use_lazy_refcounts && s->qcow_version < 3) {
         error_setg(errp, "Lazy refcounts require a qcow2 image with at least "

block/qed.c

@@ -495,6 +495,7 @@ static int bdrv_qed_open(BlockDriverState *bs, QDict *options, int flags,
         }
     }
 
+    bs->bl.write_zeroes_alignment = s->header.cluster_size >> BDRV_SECTOR_BITS;
     s->need_check_timer = timer_new_ns(QEMU_CLOCK_VIRTUAL,
                                             qed_need_check_timer_cb, s);
 

block/vmdk.c

@@ -428,6 +428,10 @@ static int vmdk_add_extent(BlockDriverState *bs,
     extent->l2_size = l2_size;
     extent->cluster_sectors = flat ? sectors : cluster_sectors;
 
+    if (!flat) {
+        bs->bl.write_zeroes_alignment =
+            MAX(bs->bl.write_zeroes_alignment, cluster_sectors);
+    }
     if (s->num_extents > 1) {
         extent->end_sector = (*(extent - 1)).end_sector + extent->sectors;
     } else {