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 (file)
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -1624,11 +1624,12 @@ first_nmi:
         * | 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 @@ first_nmi:
        .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 @@ end_repeat_nmi:
 
        /*
         * 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