diff --git a/hw/9pfs/9p.h b/hw/9pfs/9p.h
index d8f364fafd..1a309664f6 100644
--- a/hw/9pfs/9p.h
+++ b/hw/9pfs/9p.h
@@ -482,6 +482,7 @@ struct V9fsTransport {
                                          unsigned int *pniov, size_t size);
     void        (*push_and_notify)(V9fsPDU *pdu);
     size_t      (*msize_limit)(V9fsState *s);
+    size_t      (*response_buffer_size)(V9fsPDU *pdu);
 };
 
 #endif