block: Add new bdrv_co_is_all_zeroes() function

There are some optimizations that require knowing if an image starts
out as reading all zeroes, such as making blockdev-mirror faster by
skipping the copying of source zeroes to the destination.  The
existing bdrv_co_is_zero_fast() is a good building block for answering
this question, but it tends to give an answer of 0 for a file we just
created via QMP 'blockdev-create' or similar (such as 'qemu-img create
-f raw').  Why?  Because file-posix.c insists on allocating a tiny
header to any file rather than leaving it 100% sparse, due to some
filesystems that are unable to answer alignment probes on a hole.  But
teaching file-posix.c to read the tiny header doesn't scale - the
problem of a small header is also visible when libvirt sets up an NBD
client to a just-created file on a migration destination host.

So, we need a wrapper function that handles a bit more complexity in a
common manner for all block devices - when the BDS is mostly a hole,
but has a small non-hole header, it is still worth the time to read
that header and check if it reads as all zeroes before giving up and
returning a pessimistic answer.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20250509204341.3553601-19-eblake@redhat.com>

block/io.c

@@ -38,10 +38,14 @@
 #include "qemu/error-report.h"
 #include "qemu/main-loop.h"
 #include "system/replay.h"
+#include "qemu/units.h"
 
 /* Maximum bounce buffer for copy-on-read and write zeroes, in bytes */
 #define MAX_BOUNCE_BUFFER (32768 << BDRV_SECTOR_BITS)
 
+/* Maximum read size for checking if data reads as zero, in bytes */
+#define MAX_ZERO_CHECK_BUFFER (128 * KiB)
+
 static void coroutine_fn GRAPH_RDLOCK
 bdrv_parent_cb_resize(BlockDriverState *bs);
 
@@ -2778,6 +2782,64 @@ int coroutine_fn bdrv_co_is_zero_fast(BlockDriverState *bs, int64_t offset,
     return 1;
 }
 
+/*
+ * Check @bs (and its backing chain) to see if the entire image is known
+ * to read as zeroes.
+ * Return 1 if that is the case, 0 otherwise and -errno on error.
+ * This test is meant to be fast rather than accurate so returning 0
+ * does not guarantee non-zero data; however, a return of 1 is reliable,
+ * and this function can report 1 in more cases than bdrv_co_is_zero_fast.
+ */
+int coroutine_fn bdrv_co_is_all_zeroes(BlockDriverState *bs)
+{
+    int ret;
+    int64_t pnum, bytes;
+    char *buf;
+    QEMUIOVector local_qiov;
+    IO_CODE();
+
+    bytes = bdrv_co_getlength(bs);
+    if (bytes < 0) {
+        return bytes;
+    }
+
+    /* First probe - see if the entire image reads as zero */
+    ret = bdrv_co_common_block_status_above(bs, NULL, false, BDRV_WANT_ZERO,
+                                            0, bytes, &pnum, NULL, NULL,
+                                            NULL);
+    if (ret < 0) {
+        return ret;
+    }
+    if (ret & BDRV_BLOCK_ZERO) {
+        return bdrv_co_is_zero_fast(bs, pnum, bytes - pnum);
+    }
+
+    /*
+     * Because of the way 'blockdev-create' works, raw files tend to
+     * be created with a non-sparse region at the front to make
+     * alignment probing easier.  If the block starts with only a
+     * small allocated region, it is still worth the effort to see if
+     * the rest of the image is still sparse, coupled with manually
+     * reading the first region to see if it reads zero after all.
+     */
+    if (pnum > MAX_ZERO_CHECK_BUFFER) {
+        return 0;
+    }
+    ret = bdrv_co_is_zero_fast(bs, pnum, bytes - pnum);
+    if (ret <= 0) {
+        return ret;
+    }
+    /* Only the head of the image is unknown, and it's small.  Read it.  */
+    buf = qemu_blockalign(bs, pnum);
+    qemu_iovec_init_buf(&local_qiov, buf, pnum);
+    ret = bdrv_driver_preadv(bs, 0, pnum, &local_qiov, 0, 0);
+    if (ret >= 0) {
+        ret = buffer_is_zero(buf, pnum);
+    }
+    qemu_vfree(buf);
+    return ret;
+}
+
 int coroutine_fn bdrv_co_is_allocated(BlockDriverState *bs, int64_t offset,
                                       int64_t bytes, int64_t *pnum)
 {

include/block/block-io.h

@@ -161,6 +161,8 @@ bdrv_is_allocated_above(BlockDriverState *bs, BlockDriverState *base,
 
 int coroutine_fn GRAPH_RDLOCK
 bdrv_co_is_zero_fast(BlockDriverState *bs, int64_t offset, int64_t bytes);
+int coroutine_fn GRAPH_RDLOCK
+bdrv_co_is_all_zeroes(BlockDriverState *bs);
 
 int GRAPH_RDLOCK
 bdrv_apply_auto_read_only(BlockDriverState *bs, const char *errmsg,