diff --git a/chapter10/kernel.xml b/chapter10/kernel.xml
index 971703466..5aac36b8d 100644
--- a/chapter10/kernel.xml
+++ b/chapter10/kernel.xml
@@ -466,6 +466,26 @@
       <command>chown -R 0:0</command> on the <filename
       class="directory">linux-&linux-version;</filename> directory to ensure
       all files are owned by user <emphasis>root</emphasis>.</para>
+
+      <para>If you are updating the configuration and rebuilding the kernel
+      from a retained kernel source tree, normally you should
+      <emphasis role='bold'>not</emphasis> run the
+      <command>make mrproper</command> command.  The command would purge
+      the <filename>.config</filename> file and all the
+      <filename class='extension'>.o</filename> files from the previous
+      build.  Despite it's easy to restore <filename>.config</filename> from
+      the copy in <filename class='directory'>/boot</filename>, purging all
+      the <filename class='extension'>.o</filename> files is still a waste:
+      for a simple configuration change, often only a few
+      <filename class='extension'>.o</filename> files need to be (re)built
+      and the kernel build system will correctly skip other
+      <filename class='extension'>.o</filename> files if they are not
+      purged.</para>
+
+      <para>On the other hand, if you've upgraded GCC, you should run
+      <command>make clean</command> to purge all the
+      <filename class='extension'>.o</filename> files from the previous
+      build, or the new build may fail.</para>
     </note>
 
     <warning>