diff --git a/include/xen/xen.h b/include/xen/xen.h
index fdf0fc448..4c21fb5ef 100644
--- a/include/xen/xen.h
+++ b/include/xen/xen.h
@@ -823,6 +823,11 @@ struct start_info {
                                 /* (PFN of pre-loaded module if           */
                                 /*  SIF_MOD_START_PFN set in flags).      */
     unsigned long mod_len;      /* Size (bytes) of pre-loaded module.     */
+    /*
+     * cmd_line will contain a NUL-termianted string if it contains valid
+     * data, but it MAY be invalid and not contain a NUL byte at all. Code
+     * that accesses cmd_line MUST NOT assume it is NUL-terminated.
+     */
 #define GRUB_XEN_MAX_GUEST_CMDLINE 1024
     int8_t cmd_line[GRUB_XEN_MAX_GUEST_CMDLINE];
     /* The pfn range here covers both page table and p->m table frames.   */