x86: Fix the NMI nesting comments
Some of the comments for the nesting NMI algorithm were stale and
had some references to some prototypes that were first tried.
I also updated the comments to be a little easier to understand
the flow of the code. It definitely needs the documentation.
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index e0eca00..2de3e45 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -1624,11 +1624,12 @@
* | pt_regs |
* +-------------------------+
*
- * The saved RIP is used to fix up the copied RIP that a nested
- * NMI may zero out. The original stack frame and the temp storage
+ * The saved stack frame is used to fix up the copied stack frame
+ * that a nested NMI may change to make the interrupted NMI iret jump
+ * to the repeat_nmi. The original stack frame and the temp storage
* is also used by nested NMIs and can not be trusted on exit.
*/
- /* Do not pop rdx, nested NMIs will corrupt it */
+ /* Do not pop rdx, nested NMIs will corrupt that part of the stack */
movq (%rsp), %rdx
CFI_RESTORE rdx
@@ -1641,6 +1642,8 @@
.endr
CFI_DEF_CFA_OFFSET SS+8-RIP
+ /* Everything up to here is safe from nested NMIs */
+
/*
* If there was a nested NMI, the first NMI's iret will return
* here. But NMIs are still enabled and we can take another
@@ -1667,9 +1670,8 @@
/*
* Everything below this point can be preempted by a nested
- * NMI if the first NMI took an exception. Repeated NMIs
- * caused by an exception and nested NMI will start here, and
- * can still be preempted by another NMI.
+ * NMI if the first NMI took an exception and reset our iret stack
+ * so that we repeat another NMI.
*/
pushq_cfi $-1 /* ORIG_RAX: no syscall to restart */
subq $ORIG_RAX-R15, %rsp
