Patch "Prevent timer value 0 for MWAITX" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

Prevent timer value 0 for MWAITX

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
prevent-timer-value-0-for-mwaitx.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Janakarajan Natarajan <[email protected]>
Date: Tue, 25 Apr 2017 16:44:03 -0500
Subject: Prevent timer value 0 for MWAITX

From: Janakarajan Natarajan <[email protected]>

commit 88d879d29f9cc0de2d930b584285638cdada6625 upstream.

Newer hardware has uncovered a bug in the software implementation of using MWAITX for the delay function. A value of 0 for the timer is meant to indicate that a timeout will not be used to exit MWAITX. On newer hardware this can result in MWAITX never returning, resulting in NMI soft lockup messages being printed. On older hardware, some of the other conditions under which MWAITX can exit masked this issue. The AMD APM does not currently document this and will be updated.

Please refer to http://marc.info/?l=kvm&m=148950623231140 for information regarding NMI soft lockup messages on an AMD Ryzen 1800X. This has been root-caused as a 0 passed to MWAITX causing it to wait indefinitely.

This change has the added benefit of avoiding the unnecessary setup of MONITORX/MWAITX when the delay value is zero.

---
 arch/x86/lib/delay.c |    7 +++++++
 1 file changed, 7 insertions(+)

@@ -93,6 +93,13 @@ static void delay_mwaitx(unsigned long _
 {
 	u64 start, end, delay, loops = __loops;
 
+	/*
+	 * Timer value of 0 causes MWAITX to wait indefinitely, unless there
+	 * is a store on the memory monitored by MONITORX.
+	 */
+	if (loops == 0)
+		return;
+
 	start = rdtsc_ordered();
 
 	for (;;) {

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/prevent-timer-value-0-for-mwaitx.patch