Patch "pvclock: Add CPU barriers to get correct version value" has been added to the 4.6-stable tree

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

pvclock: Add CPU barriers to get correct version value

to the 4.6-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:
pvclock-add-cpu-barriers-to-get-correct-version-value.patch
and it can be found in the queue-4.6 subdirectory.

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

From: Minfei Huang <[email protected]>
Date: Fri, 27 May 2016 14:17:10 +0800
Subject: pvclock: Add CPU barriers to get correct version value

From: Minfei Huang <[email protected]>

commit 749d088b8e7f4b9826ede02b9a043e417fa84aa1 upstream.

Protocol for the "version" fields is: hypervisor raises it (making it uneven) before it starts updating the fields and raises it again (making it even) when it is done. Thus the guest can make sure the time values it got are consistent by checking the version before and after reading them.

Add CPU barries after getting version value just like what function vread_pvclock does, because all of callees in this function is inline.

---
 arch/x86/include/asm/pvclock.h |    2 ++
 arch/x86/kernel/pvclock.c      |    4 ++++
 2 files changed, 6 insertions(+)

@@ -85,6 +85,8 @@ unsigned __pvclock_read_cycles(const str
 	u8 ret_flags;
 
 	version = src->version;
+	/* Make the latest version visible */
+	smp_rmb();
 
 	offset = pvclock_get_nsec_offset(src);
 	ret = src->system_time + offset;

@@ -66,6 +66,8 @@ u8 pvclock_read_flags(struct pvclock_vcp
 
 	do {
 		version = __pvclock_read_cycles(src, &ret, &flags);
+		/* Make sure that the version double-check is last. */
+		smp_rmb();
 	} while ((src->version & 1) || version != src->version);
 
 	return flags & valid_flags;

@@ -80,6 +82,8 @@ cycle_t pvclock_clocksource_read(struct
 
 	do {
 		version = __pvclock_read_cycles(src, &ret, &flags);
+		/* Make sure that the version double-check is last. */
+		smp_rmb();
 	} while ((src->version & 1) || version != src->version);
 
 	if (unlikely((flags & PVCLOCK_GUEST_STOPPED) != 0)) {

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

queue-4.6/pvclock-add-cpu-barriers-to-get-correct-version-value.patch