A single lseek(SEEK_DATA) is sufficient to tell us if a raw file is
completely sparse, in which case it reads as all zeroes.  Not done
here, but possible extension for the future: when working with block
devices instead of files, there may be various ways with ioctl or
similar to quickly probe if a given block device is known to be
completely unmapped where unmapped regions read as zero.  But for now,
block devices remain without a .bdrv_known_zeroes, because most block
devices have random content without an explicit pre-zeroing pass.

Signed-off-by: Eric Blake <eblake@redhat.com>
---
 block/file-posix.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/block/file-posix.c b/block/file-posix.c
index ff9e39ab882f..b4d73dd0363b 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -2541,6 +2541,19 @@ static int find_allocation(BlockDriverState *bs, off_t start,
 #endif
 }

+static int raw_known_zeroes(BlockDriverState *bs)
+{
+    /* This callback is only installed for files, not block devices. */
+    int r = BDRV_ZERO_CREATE | BDRV_ZERO_TRUNCATE;
+    off_t data, hole;
+
+    if (find_allocation(bs, 0, &data, &hole) == -ENXIO) {
+        r |= BDRV_ZERO_OPEN;
+    }
+
+    return r;
+}
+
 /*
  * Returns the allocation status of the specified offset.
  *
@@ -3071,7 +3084,7 @@ BlockDriver bdrv_file = {
     .bdrv_close = raw_close,
     .bdrv_co_create = raw_co_create,
     .bdrv_co_create_opts = raw_co_create_opts,
-    .bdrv_known_zeroes = bdrv_known_zeroes_truncate,
+    .bdrv_known_zeroes = raw_known_zeroes,
     .bdrv_co_block_status = raw_co_block_status,
     .bdrv_co_invalidate_cache = raw_co_invalidate_cache,
     .bdrv_co_pwrite_zeroes = raw_co_pwrite_zeroes,
-- 
2.24.1