When using bdrv_file, .bdrv_has_zero_init_truncate always returns 1;
therefore, we can behave just like file-posix, and always implement
BDRV_REQ_ZERO_WRITE by ignoring it since the OS gives it to us for
free (note that file-posix.c had to use an 'if' because it shared code
between regular files and block devices, but in file-win32.c,
bdrv_host_device uses a separate .bdrv_file_open).

Signed-off-by: Eric Blake <eblake@redhat.com>
---
 block/file-win32.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/block/file-win32.c b/block/file-win32.c
index a6b0dda5c302..fa569685d8bc 100644
--- a/block/file-win32.c
+++ b/block/file-win32.c
@@ -408,6 +408,9 @@ static int raw_open(BlockDriverState *bs, QDict *options, int flags,
         win32_aio_attach_aio_context(s->aio, bdrv_get_aio_context(bs));
     }

+    /* When extending regular files, we get zeros from the OS */
+    bs->supported_truncate_flags = BDRV_REQ_ZERO_WRITE;
+
     ret = 0;
 fail:
     qemu_opts_del(opts);
-- 
2.26.2