timecounting: add gettime(9), getuptime(9)

time_second and time_uptime are used widely in the tree.  This is a
problem on 32-bit platforms because time_t is 64-bit, so there is a
potential split-read whenever they are used at or below IPL_CLOCK.

Here are two replacement interfaces: gettime(9) and getuptime(9).
The "get" prefix signifies that they do not read the hardware
timecounter, i.e. they are fast and low-res.  The lack of a unit
(e.g. micro, nano) signifies that they yield a plain time_t.

As an optimization on LP64 platforms we can just return time_second or
time_uptime, as a single read is atomic.  On 32-bit platforms we need
to do the lockless read loop and get the values from the timecounter.

In a subsequent diff these will be substituted for time_second and
time_uptime almost everywhere in the kernel.

With input from visa@ and dlg@.

ok kettenis@

1 files changed, 45 insertions, 1 deletions

diff --git a/sys/kern/kern_tc.c b/sys/kern/kern_tc.c
index 88d4a3379f9..54d50b1a6f7 100644
--- a/sys/kern/kern_tc.c
+++ b/sys/kern/kern_tc.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: kern_tc.c,v 1.57 2020/05/29 04:42:25 deraadt Exp $ */
+/*	$OpenBSD: kern_tc.c,v 1.58 2020/06/22 21:16:07 cheloha Exp $ */
 
 /*
  * Copyright (c) 2000 Poul-Henning Kamp <phk@FreeBSD.org>
@@ -208,6 +208,28 @@ microuptime(struct timeval *tvp)
 	BINTIME_TO_TIMEVAL(&bt, tvp);
 }
 
+time_t
+getuptime(void)
+{
+#if defined(__LP64__)
+	return time_uptime;	/* atomic */
+#else
+	time_t now;
+	struct timehands *th;
+	u_int gen;
+
+	do {
+		th = timehands;
+		gen = th->th_generation;
+		membar_consumer();
+		now = th->th_offset.sec;
+		membar_consumer();
+	} while (gen == 0 || gen != th->th_generation);
+
+	return now;
+#endif
+}
+
 void
 binruntime(struct bintime *bt)
 {
@@ -268,6 +290,28 @@ microtime(struct timeval *tvp)
 	BINTIME_TO_TIMEVAL(&bt, tvp);
 }
 
+time_t
+gettime(void)
+{
+#if defined(__LP64__)
+	return time_second;	/* atomic */
+#else
+	time_t now;
+	struct timehands *th;
+	u_int gen;
+
+	do {
+		th = timehands;
+		gen = th->th_generation;
+		membar_consumer();
+		now = th->th_microtime.tv_sec;
+		membar_consumer();
+	} while (gen == 0 || gen != th->th_generation);
+
+	return now;
+#endif
+}
+
 void
 getnanouptime(struct timespec *tsp)
 {