[PATCH v4 09/10] pvclock: add clocksource change notification on changing of tsc stable bit

[Denis Plotnikov <dplotnikov@virtuozzo.com>]

It's needed to notify the KVM guest about critical changes in pvclock
and make it to update its masterclock.

This is a part of the work aiming to make kvmclock be a clocksource
providing valid cycles value for KVM masterclock, another words
make possible to use KVM masterclock over kvmclock clocksource.

Signed-off-by: Denis Plotnikov <dplotnikov@virtuozzo.com>
---
 arch/x86/kernel/pvclock.c | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/pvclock.c b/arch/x86/kernel/pvclock.c
index bece384..5898f20 100644
--- a/arch/x86/kernel/pvclock.c
+++ b/arch/x86/kernel/pvclock.c
@@ -22,6 +22,7 @@
 #include <linux/gfp.h>
 #include <linux/bootmem.h>
 #include <linux/nmi.h>
+#include <linux/cs_notifier.h>
 
 #include <asm/fixmap.h>
 #include <asm/pvclock.h>
@@ -73,6 +74,8 @@ u8 pvclock_read_flags(struct pvclock_vcpu_time_info *src)
 	return flags & valid_flags;
 }
 
+static atomic_t clocksource_stable = ATOMIC_INIT(0);
+
 u64 pvclock_clocksource_read(struct pvclock_vcpu_time_info *src,
 				u64 *cycles_stamp, u8 *flags_stamp)
 {
@@ -102,10 +105,20 @@ u64 pvclock_clocksource_read(struct pvclock_vcpu_time_info *src,
 		pvclock_touch_watchdogs();
 	}
 
-	if ((valid_flags & PVCLOCK_TSC_STABLE_BIT) &&
-		(flags & PVCLOCK_TSC_STABLE_BIT))
-		return ret;
+	if (likely(valid_flags & PVCLOCK_TSC_STABLE_BIT)) {
+		bool stable_now = !!(flags & PVCLOCK_TSC_STABLE_BIT);
+		bool stable_last = (bool) atomic_read(&clocksource_stable);
+
+		if (unlikely(stable_now != stable_last)) {
+			/* send notification once */
+			if (stable_last == atomic_cmpxchg(
+				&clocksource_stable, stable_last, stable_now))
+				clocksource_changes_notify();
+		}
 
+		if (stable_now)
+			return ret;
+	}
 	/*
 	 * Assumption here is that last_value, a global accumulator, always goes
 	 * forward. If we are less than that, we should not be much smaller.
-- 
2.7.4