[PATCH 3/4] hv_util: use do_adjtimex() to update system time

[Vitaly Kuznetsov]

With TimeSync version 4 protocol support we started updating system time
continuously through the whole lifetime of Hyper-V guests. Every 5 seconds
there is a time sample from the host which triggers do_settimeofday[64]().
While the time from the host is very accurate such adjustments may cause
issues:
- Time is jumping forward and backward, some applications may misbehave.
- In case an NTP client is run in parallel things may go south, e.g. when
  an NTP client tries to adjust tick/frequency with ADJ_TICK/ADJ_FREQUENCY
  the Hyper-V module will not see this changes and time will oscillate and
  never converge.
- Systemd starts annoying you by printing "Time has been changed" every 5
  seconds to the system log.

Instead of calling do_settimeofday64() we can pretend being an NTP client
and use do_adjtimex().

Signed-off-by: Vitaly Kuznetsov <vkuznets at redhat.com>
---
 drivers/hv/hv_util.c | 25 ++++++++++++++++++++++---
 1 file changed, 22 insertions(+), 3 deletions(-)

diff --git a/drivers/hv/hv_util.c b/drivers/hv/hv_util.c
index 94719eb..4c0fbb0 100644
--- a/drivers/hv/hv_util.c
+++ b/drivers/hv/hv_util.c
@@ -182,9 +182,10 @@ struct adj_time_work {
 static void hv_set_host_time(struct work_struct *work)
 {
 	struct adj_time_work	*wrk;
-	s64 host_tns;
+	s64 host_tns, our_tns, delta;
 	u64 newtime;
-	struct timespec64 host_ts;
+	struct timespec64 host_ts, our_ts;
+	struct timex txc = {0};
 
 	wrk = container_of(work, struct adj_time_work, work);
 
@@ -205,7 +206,25 @@ static void hv_set_host_time(struct work_struct *work)
 	host_tns = (newtime - WLTIMEDELTA) * 100;
 	host_ts = ns_to_timespec64(host_tns);
 
-	do_settimeofday64(&host_ts);
+	getnstimeofday64(&our_ts);
+	our_tns = timespec64_to_ns(&our_ts);
+
+	/* Difference between our time and host time */
+	delta = host_tns - our_tns;
+
+	/* Try adjusting time by using phase adjustment if possible */
+	if (abs(delta) > MAXPHASE) {
+		do_settimeofday64(&host_ts);
+		return;
+	}
+
+	txc.modes = ADJ_TICK | ADJ_FREQUENCY | ADJ_OFFSET | ADJ_NANO |
+		ADJ_STATUS;
+	txc.tick = TICK_USEC;
+	txc.freq = 0;
+	txc.status = STA_PLL;
+	txc.offset = delta;
+	do_adjtimex(&txc);
 }
 
 /*
-- 
2.9.3