BlockLimits: introduce max_transfer_length

Signed-off-by: Peter Lieven <pl@kamp.de>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

block.c

diff --git a/block.c b/block.c
index 88f6d9b..76fcc1d 100644
--- a/block.c
+++ b/block.c
@@ -519,6 +519,7 @@
             return;
         }
         bs->bl.opt_transfer_length = bs->file->bl.opt_transfer_length;
+        bs->bl.max_transfer_length = bs->file->bl.max_transfer_length;
         bs->bl.opt_mem_alignment = bs->file->bl.opt_mem_alignment;
     } else {
         bs->bl.opt_mem_alignment = 512;
@@ -533,6 +534,9 @@
         bs->bl.opt_transfer_length =
             MAX(bs->bl.opt_transfer_length,
                 bs->backing_hd->bl.opt_transfer_length);
+        bs->bl.max_transfer_length =
+            MIN_NON_ZERO(bs->bl.max_transfer_length,
+                         bs->backing_hd->bl.max_transfer_length);
         bs->bl.opt_mem_alignment =
             MAX(bs->bl.opt_mem_alignment,
                 bs->backing_hd->bl.opt_mem_alignment);

include/block/block_int.h

diff --git a/include/block/block_int.h b/include/block/block_int.h
index 8898c6c..a293e92 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -289,6 +289,9 @@
     /* optimal transfer length in sectors */
     int opt_transfer_length;
 
+    /* maximal transfer length in sectors */
+    int max_transfer_length;
+
     /* memory alignment so that no bounce buffer is needed */
     size_t opt_mem_alignment;
 } BlockLimits;