block: honour BlockLimits in bdrv_co_discard

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

block.c

@@ -4243,6 +4243,11 @@ static void coroutine_fn bdrv_discard_co_entry(void *opaque)
     rwco->ret = bdrv_co_discard(rwco->bs, rwco->sector_num, rwco->nb_sectors);
 }
 
+/* if no limit is specified in the BlockLimits use a default
+ * of 32768 512-byte sectors (16 MiB) per request.
+ */
+#define MAX_DISCARD_DEFAULT 32768
+
 int coroutine_fn bdrv_co_discard(BlockDriverState *bs, int64_t sector_num,
                                  int nb_sectors)
 {
@@ -4264,7 +4269,37 @@ int coroutine_fn bdrv_co_discard(BlockDriverState *bs, int64_t sector_num,
     }
 
     if (bs->drv->bdrv_co_discard) {
-        return bs->drv->bdrv_co_discard(bs, sector_num, nb_sectors);
+        int max_discard = bs->bl.max_discard ?
+                          bs->bl.max_discard : MAX_DISCARD_DEFAULT;
+
+        while (nb_sectors > 0) {
+            int ret;
+            int num = nb_sectors;
+
+            /* align request */
+            if (bs->bl.discard_alignment &&
+                num >= bs->bl.discard_alignment &&
+                sector_num % bs->bl.discard_alignment) {
+                if (num > bs->bl.discard_alignment) {
+                    num = bs->bl.discard_alignment;
+                }
+                num -= sector_num % bs->bl.discard_alignment;
+            }
+
+            /* limit request size */
+            if (num > max_discard) {
+                num = max_discard;
+            }
+
+            ret = bs->drv->bdrv_co_discard(bs, sector_num, num);
+            if (ret) {
+                return ret;
+            }
+
+            sector_num += num;
+            nb_sectors -= num;
+        }
+        return 0;
     } else if (bs->drv->bdrv_aio_discard) {
         BlockDriverAIOCB *acb;
         CoroutineIOCompletion co = {