diff --git a/src/conf/domain_conf.h b/src/conf/domain_conf.h
index a1acebe9d4..eb3a5f30f8 100644
--- a/src/conf/domain_conf.h
+++ b/src/conf/domain_conf.h
@@ -2404,6 +2404,8 @@ typedef virDomainXMLPrivateDataCallbacks *virDomainXMLPrivateDataCallbacksPtr;
 struct _virDomainXMLPrivateDataCallbacks {
     virDomainXMLPrivateDataAllocFunc  alloc;
     virDomainXMLPrivateDataFreeFunc   free;
+    /* note that private data for devices are not copied when using
+     * virDomainDefCopy and similar functions */
     virDomainXMLPrivateDataNewFunc    diskNew;
     virDomainXMLPrivateDataNewFunc    hostdevNew;
     virDomainXMLPrivateDataFormatFunc format;