From: Rik van Riel <riel@redhat.com>
Date: Wed, 8 Apr 2009 15:51:02 -0400
Subject: [x86] xen: fix interaction between dom0 and NTP
Message-id: 20090408155102.2b22900c@cuia.bos.redhat.com
O-Subject: [PATCH RHEL5.4] Fix interaction between dom0 and NTP (bz 494879)
Bugzilla: 494879
RH-Acked-by: Gerd Hoffmann <kraxel@redhat.com>
RH-Acked-by: Don Dutile <ddutile@redhat.com>
RH-Acked-by: Chris Lalancette <clalance@redhat.com>
RH-Acked-by: Justin M. Forbes <jforbes@redhat.com>
RH-Acked-by: Prarit Bhargava <prarit@redhat.com>

The kernel xen time code used to call ntp_clear() (indicating that
the system lost NTP sync) any time the NTP code set the HV clock.

Oops.

Doing that only from do_settimeofday() fixes things.

Fixes bug 494879 and looks like it should help fix several
other Xen PV timer bugs. Only compile tested by me because
the fix is blindingly obvious after inspecting the code.

# HG changeset patch
# User Keir Fraser <keir.fraser@citrix.com>
# Date 1237377065 0
# Node ID c0f2f398aa3cab195b632531447e95110e9d8282
# Parent  c1f0373ff44ea3f9ba3845e05faf3d27d7d52731
x86: Fix interaction of NTP and dom0->xen time updates

Don't discard NTP sync when updating Xen wallclock time from dom0,
as that's almost the first thing we do when we become synced.
Move the call to ntp_clear() into do_settimeofday(), which is the
only caller of __update_wallclock() that looks like it should break
NTP sync.

This fixes the timer chain that sets Xen's wallclock every minute when
dom0 is NTP synced, which in turn greatly improves wallclock accuracy
in PV domU.

Signed-off-by: Tim Deegan <Tim.Deegan@citrix.com>

diff --git a/arch/i386/kernel/time-xen.c b/arch/i386/kernel/time-xen.c
index 6f23a83..8809ff8 100644
--- a/arch/i386/kernel/time-xen.c
+++ b/arch/i386/kernel/time-xen.c
@@ -282,8 +282,6 @@ static void __update_wallclock(time_t sec, long nsec)
 
 	set_normalized_timespec(&xtime, xtime_sec, xtime_nsec);
 	set_normalized_timespec(&wall_to_monotonic, wtm_sec, wtm_nsec);
-
-	ntp_clear();
 }
 
 static void update_wallclock(void)
@@ -499,6 +497,7 @@ int do_settimeofday(struct timespec *tv)
 		__normalize_time(&sec, &nsec);
 		__update_wallclock(sec, nsec);
 	}
+	ntp_clear();
 
 	/* Reset monotonic gettimeofday() timeval. */
 	spin_lock(&monotonic_lock);