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

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Date:   Wed,  2 Aug 2017 17:38:09 +0300
From:   Denis Plotnikov <dplotnikov@...tuozzo.com>
To:     pbonzini@...hat.com, rkrcmar@...hat.com, kvm@...r.kernel.org,
        john.stultz@...aro.org, tglx@...utronix.de
Cc:     mingo@...hat.com, hpa@...or.com, linux-kernel@...r.kernel.org,
        x86@...nel.org, rkagan@...tuozzo.com, den@...tuozzo.com
Subject: [PATCH v4 09/10] pvclock: add clocksource change notification on changing of tsc stable bit

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@...tuozzo.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