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

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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

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[PATCH v4 00/10] make L2's kvm-clock stable, get rid of pvclock_gtod_copy in KVM Denis Plotnikov <dplotnikov@virtuozzo.com> - 2017-08-02 17:00 +0200
  [PATCH v4 10/10] kvmclock: implement the extended reading function Denis Plotnikov <dplotnikov@virtuozzo.com> - 2017-08-02 17:00 +0200
  [PATCH v4 07/10] KVM: x86: remove not used pvclock_gtod_copy Denis Plotnikov <dplotnikov@virtuozzo.com> - 2017-08-02 17:00 +0200
    Re: [PATCH v4 07/10] KVM: x86: remove not used pvclock_gtod_copy Marcelo Tosatti <mtosatti@redhat.com> - 2017-08-03 01:30 +0200
      Re: [PATCH v4 07/10] KVM: x86: remove not used pvclock_gtod_copy Paolo Bonzini <pbonzini@redhat.com> - 2017-08-03 14:40 +0200
  [PATCH v4 09/10] pvclock: add clocksource change notification on changing of tsc stable bit Denis Plotnikov <dplotnikov@virtuozzo.com> - 2017-08-02 17:00 +0200
    Re: [PATCH v4 09/10] pvclock: add clocksource change notification on  changing of tsc stable bit Marcelo Tosatti <mtosatti@redhat.com> - 2017-08-03 01:40 +0200
  [PATCH v4 06/10] timekeeper: add clocksource change notifier Denis Plotnikov <dplotnikov@virtuozzo.com> - 2017-08-02 17:00 +0200
  Re: [PATCH v4 00/10] make L2's kvm-clock stable, get rid of  pvclock_gtod_copy in KVM Paolo Bonzini <pbonzini@redhat.com> - 2017-08-02 18:20 +0200
  Re: [PATCH v4 00/10] make L2's kvm-clock stable, get rid of  pvclock_gtod_copy in KVM John Stultz <john.stultz@linaro.org> - 2017-08-02 18:50 +0200
    Re: [PATCH v4 00/10] make L2's kvm-clock stable, get rid of  pvclock_gtod_copy in KVM Paolo Bonzini <pbonzini@redhat.com> - 2017-08-02 19:20 +0200

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