block: add BlockLimits structure to BlockDriverState

this patch adds BlockLimits which introduces discard and write_zeroes
limits and alignment information to the BlockDriverState.
Signed-off-by: NPeter Lieven <pl@kamp.de>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

include/block/block_int.h

@@ -230,6 +230,20 @@ struct BlockDriver {
     QLIST_ENTRY(BlockDriver) list;
 };
 
+typedef struct BlockLimits {
+    /* maximum number of sectors that can be discarded at once */
+    int max_discard;
+
+    /* optimal alignment for discard requests in sectors */
+    int64_t discard_alignment;
+
+    /* maximum number of sectors that can zeroized at once */
+    int max_write_zeroes;
+
+    /* optimal alignment for write zeroes requests in sectors */
+    int64_t write_zeroes_alignment;
+} BlockLimits;
+
 /*
  * Note: the function bdrv_append() copies and swaps contents of
  * BlockDriverStates, so if you add new fields to this struct, please
@@ -283,6 +297,9 @@ struct BlockDriverState {
     uint64_t total_time_ns[BDRV_MAX_IOTYPE];
     uint64_t wr_highest_sector;
 
+    /* I/O Limits */
+    BlockLimits bl;
+
     /* Whether the disk can expand beyond total_sectors */
     int growable;