[tip: x86/cpu] x86/mtrr: Add comment for set_mtrr_state() serialization

[tip-bot2 for Juergen Gross]

The following commit has been merged into the x86/cpu branch of tip:

Commit-ID:     01c97c7303580682751b5aaae043b639bdcbacb3
Gitweb:        https://git.kernel.org/tip/01c97c7303580682751b5aaae043b639bdcbacb3
Author:        Juergen Gross <jgross@xxxxxxxx>
AuthorDate:    Tue, 04 Oct 2022 10:10:08 +02:00
Committer:     Borislav Petkov <bp@xxxxxxx>
CommitterDate: Wed, 19 Oct 2022 20:37:23 +02:00

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@xxxxxxxxx>
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Link: https://lore.kernel.org/r/20220820092533.29420-2-jgross@xxxxxxxx
---
 arch/x86/kernel/cpu/mtrr/generic.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/mtrr/generic.c b/arch/x86/kernel/cpu/mtrr/generic.c
index 5581082..c8f8951 100644
--- a/arch/x86/kernel/cpu/mtrr/generic.c
+++ b/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)