block/io: bdrv_co_do_copy_on_readv: lazy allocation

Allocate bounce_buffer only if it is really needed. Also, sub-optimize
allocation size (why not?).
Signed-off-by: NVladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Acked-by: NStefan Hajnoczi <stefanha@redhat.com>
Message-id: 20190604161514.262241-7-vsementsov@virtuozzo.com
Message-Id: <20190604161514.262241-7-vsementsov@virtuozzo.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

block/io.c

@@ -1236,7 +1236,7 @@ static int coroutine_fn bdrv_co_do_copy_on_readv(BdrvChild *child,
      * modifying the image file.  This is critical for zero-copy guest I/O
      * where anything might happen inside guest memory.
      */
-    void *bounce_buffer;
+    void *bounce_buffer = NULL;
 
     BlockDriver *drv = bs->drv;
     int64_t cluster_offset;
@@ -1271,14 +1271,6 @@ static int coroutine_fn bdrv_co_do_copy_on_readv(BdrvChild *child,
     trace_bdrv_co_do_copy_on_readv(bs, offset, bytes,
                                    cluster_offset, cluster_bytes);
 
-    bounce_buffer = qemu_try_blockalign(bs,
-                                        MIN(MIN(max_transfer, cluster_bytes),
-                                            MAX_BOUNCE_BUFFER));
-    if (bounce_buffer == NULL) {
-        ret = -ENOMEM;
-        goto err;
-    }
-
     while (cluster_bytes) {
         int64_t pnum;
 
@@ -1305,6 +1297,17 @@ static int coroutine_fn bdrv_co_do_copy_on_readv(BdrvChild *child,
 
             /* Must copy-on-read; use the bounce buffer */
             pnum = MIN(pnum, MAX_BOUNCE_BUFFER);
+            if (!bounce_buffer) {
+                int64_t max_we_need = MAX(pnum, cluster_bytes - pnum);
+                int64_t max_allowed = MIN(max_transfer, MAX_BOUNCE_BUFFER);
+                int64_t bounce_buffer_len = MIN(max_we_need, max_allowed);
+
+                bounce_buffer = qemu_try_blockalign(bs, bounce_buffer_len);
+                if (!bounce_buffer) {
+                    ret = -ENOMEM;
+                    goto err;
+                }
+            }
             qemu_iovec_init_buf(&local_qiov, bounce_buffer, pnum);
 
             ret = bdrv_driver_preadv(bs, cluster_offset, pnum,