qemu-img: add chunk size parameter to compare_buffers()

Add @chsize param to the function which, if non-zero, would represent
the chunk size to be used for comparison.  If it's zero, then
BDRV_SECTOR_SIZE is used as default chunk size, which is the previous
behaviour.

In particular, we're going to use this param in img_rebase() to make the
write requests aligned to a predefined alignment value.

Signed-off-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Hanna Czenczek <hreitz@redhat.com>
Message-ID: <20230919165804.439110-5-andrey.drobyshev@virtuozzo.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

qemu-img.c

@@ -1274,23 +1274,29 @@ static int is_allocated_sectors_min(const uint8_t *buf, int n, int *pnum,
 }
 
 /*
- * Compares two buffers sector by sector. Returns 0 if the first
- * sector of each buffer matches, non-zero otherwise.
+ * Compares two buffers chunk by chunk, where @chsize is the chunk size.
+ * If @chsize is 0, default chunk size of BDRV_SECTOR_SIZE is used.
+ * Returns 0 if the first chunk of each buffer matches, non-zero otherwise.
  *
- * pnum is set to the sector-aligned size of the buffer prefix that
- * has the same matching status as the first sector.
+ * @pnum is set to the size of the buffer prefix aligned to @chsize that
+ * has the same matching status as the first chunk.
  */
 static int compare_buffers(const uint8_t *buf1, const uint8_t *buf2,
-                           int64_t bytes, int64_t *pnum)
+                           int64_t bytes, uint64_t chsize, int64_t *pnum)
 {
     bool res;
-    int64_t i = MIN(bytes, BDRV_SECTOR_SIZE);
+    int64_t i;
 
     assert(bytes > 0);
 
+    if (!chsize) {
+        chsize = BDRV_SECTOR_SIZE;
+    }
+    i = MIN(bytes, chsize);
+
     res = !!memcmp(buf1, buf2, i);
     while (i < bytes) {
-        int64_t len = MIN(bytes - i, BDRV_SECTOR_SIZE);
+        int64_t len = MIN(bytes - i, chsize);
 
         if (!!memcmp(buf1 + i, buf2 + i, len) != res) {
             break;
@@ -1559,7 +1565,7 @@ static int img_compare(int argc, char **argv)
                     ret = 4;
                     goto out;
                 }
-                ret = compare_buffers(buf1, buf2, chunk, &pnum);
+                ret = compare_buffers(buf1, buf2, chunk, 0, &pnum);
                 if (ret || pnum != chunk) {
                     qprintf(quiet, "Content mismatch at offset %" PRId64 "!\n",
                             offset + (ret ? 0 : pnum));
@@ -3887,7 +3893,7 @@ static int img_rebase(int argc, char **argv)
                 int64_t pnum;
 
                 if (compare_buffers(buf_old + written, buf_new + written,
-                                    n - written, &pnum))
+                                    n - written, 0, &pnum))
                 {
                     if (buf_old_is_zero) {
                         ret = blk_pwrite_zeroes(blk, offset + written, pnum, 0);