diff --git a/libc/kernel/arch-x86/asm/percpu.h b/libc/kernel/arch-x86/asm/percpu.h
index 7740e67..a51075b 100644
--- a/libc/kernel/arch-x86/asm/percpu.h
+++ b/libc/kernel/arch-x86/asm/percpu.h
@@ -7,6 +7,13 @@
  ***   structures, and macros generated from the original header, and thus,
  ***   contains no copyrightable information.
  ***
+ ***   To edit the content of this header, modify the corresponding
+ ***   source file (e.g. under external/kernel-headers/original/) then
+ ***   run bionic/libc/kernel/tools/update_all.py
+ ***
+ ***   Any manual change here will be lost the next time this script will
+ ***   be run. You've been warned!
+ ***
  ****************************************************************************
  ****************************************************************************/
 #include "percpu_64.h"