x86/mtrr: Add comment for set_mtrr_state() serialization

Add a comment about set_mtrr_state() needing serialization.

  [ bp: Touchups. ]
Suggested-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20220820092533.29420-2-jgross@suse.com

arch/x86/kernel/cpu/mtrr/generic.c

@@ -684,7 +684,10 @@ static u32 deftype_lo, deftype_hi;
 /**
  * set_mtrr_state - Set the MTRR state for this CPU.
  *
- * NOTE: The CPU must already be in a safe state for MTRR changes.
+ * NOTE: The CPU must already be in a safe state for MTRR changes, including
+ *       measures that only a single CPU can be active in set_mtrr_state() in
+ *       order to not be subject to races for usage of deftype_lo. This is
+ *       accomplished by taking set_atomicity_lock.
  * RETURNS: 0 if no changes made, else a mask indicating what was changed.
  */
 static unsigned long set_mtrr_state(void)