[PATCH 1/2] x86/platform: Add a low priority low frequency NMI call chain

[Mike Travis]

Add a new NMI call chain that is called last after all other NMI handlers
have been checked and did not "handle" the NMI.  This mimics the current
NMI_UNKNOWN call chain except it eliminates the WARNING message about
multiple NMI handlers registering on this call chain.

This call chain dramatically lowers the NMI call frequency when high
frequency NMI tools are in use, notably the perf tools.  It is required
for NMI handlers that cannot sustain a high NMI call rate without
ramifications to the system operability.


Signed-off-by: Mike Travis <mike.travis@xxxxxxx>
Reviewed-by: Russ Anderson <russ.anderson@xxxxxxx>
---
 arch/x86/include/asm/nmi.h |    1 +
 arch/x86/kernel/nmi.c      |   21 ++++++++++++++++++++-
 2 files changed, 21 insertions(+), 1 deletion(-)

--- linux-4.4.orig/arch/x86/include/asm/nmi.h
+++ linux-4.4/arch/x86/include/asm/nmi.h
@@ -28,6 +28,7 @@ enum {
 	NMI_UNKNOWN,
 	NMI_SERR,
 	NMI_IO_CHECK,
+	NMI_LAST,
 	NMI_MAX
 };
 
--- linux-4.4.orig/arch/x86/kernel/nmi.c
+++ linux-4.4/arch/x86/kernel/nmi.c
@@ -57,6 +57,10 @@ static struct nmi_desc nmi_desc[NMI_MAX]
 		.lock = __SPIN_LOCK_UNLOCKED(&nmi_desc[3].lock),
 		.head = LIST_HEAD_INIT(nmi_desc[3].head),
 	},
+	{
+		.lock = __SPIN_LOCK_UNLOCKED(&nmi_desc[4].lock),
+		.head = LIST_HEAD_INIT(nmi_desc[4].head),
+	},
 
 };
 
@@ -65,6 +69,7 @@ struct nmi_stats {
 	unsigned int unknown;
 	unsigned int external;
 	unsigned int swallow;
+	unsigned int last;
 };
 
 static DEFINE_PER_CPU(struct nmi_stats, nmi_stats);
@@ -312,6 +317,20 @@ unknown_nmi_error(unsigned char reason,
 }
 NOKPROBE_SYMBOL(unknown_nmi_error);
 
+static void check_nmi_last(unsigned char reason, struct pt_regs *regs)
+{
+	int handled;
+
+	/* Check low frequency, multiple CPU NMI handlers */
+	handled = nmi_handle(NMI_LAST, regs);
+	__this_cpu_add(nmi_stats.last, handled);
+	if (handled)
+		return;
+
+	unknown_nmi_error(reason, regs);
+}
+NOKPROBE_SYMBOL(check_nmi_last);
+
 static DEFINE_PER_CPU(bool, swallow_nmi);
 static DEFINE_PER_CPU(unsigned long, last_nmi_rip);
 
@@ -423,7 +442,7 @@ static void default_do_nmi(struct pt_reg
 	if (b2b && __this_cpu_read(swallow_nmi))
 		__this_cpu_add(nmi_stats.swallow, 1);
 	else
-		unknown_nmi_error(reason, regs);
+		check_nmi_last(reason, regs);
 }
 NOKPROBE_SYMBOL(default_do_nmi);
 

--