time: Only do nanosecond rounding on GENERIC_TIME_VSYSCALL_OLD systems

We only do rounding to the next nanosecond so we don't see minor
1ns inconsistencies in the vsyscall implementations. Since we're
changing the vsyscall implementations to avoid this, conditionalize
the rounding only to the GENERIC_TIME_VSYSCALL_OLD architectures.

Cc: Tony Luck <[e-mail]>
Cc: Paul Mackerras <[e-mail]>
Cc: Benjamin Herrenschmidt <[e-mail]>
Cc: Andy Lutomirski <[e-mail]>
Cc: Martin Schwidefsky <[e-mail]>
Cc: Paul Turner <[e-mail]>
Cc: Steven Rostedt <[e-mail]>
Cc: Richard Cochran <[e-mail]>
Cc: Prarit Bhargava <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Signed-off-by: John Stultz <[e-mail]>

kernel/time/timekeeping.c

···
 	return offset;
 }
 
+#ifdef CONFIG_GENERIC_TIME_VSYSCALL_OLD
+static inline void old_vsyscall_fixup(struct timekeeper *tk)
+{
+	s64 remainder;
+
+	/*
+	* Store only full nanoseconds into xtime_nsec after rounding
+	* it up and add the remainder to the error difference.
+	* XXX - This is necessary to avoid small 1ns inconsistnecies caused
+	* by truncating the remainder in vsyscalls. However, it causes
+	* additional work to be done in timekeeping_adjust(). Once
+	* the vsyscall implementations are converted to use xtime_nsec
+	* (shifted nanoseconds), and CONFIG_GENERIC_TIME_VSYSCALL_OLD
+	* users are removed, this can be killed.
+	*/
+	remainder = tk->xtime_nsec & ((1ULL << tk->shift) - 1);
+	tk->xtime_nsec -= remainder;
+	tk->xtime_nsec += 1ULL << tk->shift;
+	tk->ntp_error += remainder << tk->ntp_error_shift;
+
+}
+#else
+#define old_vsyscall_fixup(tk)
+#endif
+
+
+
 /**
  * update_wall_time - Uses the current clocksource to increment the wall time
  *
···
 	cycle_t offset;
 	int shift = 0, maxshift;
 	unsigned long flags;
-	s64 remainder;
 
 	write_seqlock_irqsave(&tk->lock, flags);
 
···
 	/* correct the clock when NTP error is too big */
 	timekeeping_adjust(tk, offset);
 
-
 	/*
-	* Store only full nanoseconds into xtime_nsec after rounding
-	* it up and add the remainder to the error difference.
-	* XXX - This is necessary to avoid small 1ns inconsistnecies caused
-	* by truncating the remainder in vsyscalls. However, it causes
-	* additional work to be done in timekeeping_adjust(). Once
-	* the vsyscall implementations are converted to use xtime_nsec
-	* (shifted nanoseconds), this can be killed.
-	*/
-	remainder = tk->xtime_nsec & ((1ULL << tk->shift) - 1);
-	tk->xtime_nsec -= remainder;
-	tk->xtime_nsec += 1ULL << tk->shift;
-	tk->ntp_error += remainder << tk->ntp_error_shift;
+	 * XXX This can be killed once everyone converts
+	 * to the new update_vsyscall.
+	 */
+	old_vsyscall_fixup(tk);
 
 	/*
 	 * Finally, make sure that after the rounding