kern: clarify boot time

In FreeBSD, the current time is computed from uptime + boottime. Uptime
is a continuous, smooth function that's monotonically increasing. To
effect changes to the current time, boottime is adjusted.  boottime is
mutable and shouldn't be cached against future need. Document the
current implementation, with the caveat that we may stop stepping
boottime on resume in the future and will step uptime instead (noted in
the commit message, but not in the code).

Sponsored by:		Netflix
Reviewed by:		phk, rpokala
Differential Revision:	https://reviews.freebsd.org/D30116

2 files changed, 21 insertions, 3 deletions

diff --git a/share/man/man9/time.9 b/share/man/man9/time.9
index 4a2b2f7240e1..443608dd3f5b 100644
--- a/share/man/man9/time.9
+++ b/share/man/man9/time.9
@@ -31,7 +31,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd September 17, 2004
+.Dd May 4, 2021
 .Dt TIME 9
 .Os
 .Sh NAME
@@ -48,7 +48,18 @@
 .Sh DESCRIPTION
 The
 .Va boottime
-variable holds the system boot time.
+variable holds the estimated system boot time.
+This time is initially set when the system boots, either from the RTC, or from a
+time estimated from the system's root filesystem.
+When the current system time is set, stepped by
+.Xr ntpd 8 ,
+or a new time is read from the RTC as the system resumes,
+.Va boottime
+is recomputed as new_time - uptime.
+The
+.Xr sysctl 8
+.Va kern.boottime
+returns this value.
 .Pp
 The
 .Va time_second
@@ -83,6 +94,7 @@ and in an atomic manner.
 The
 .Va boottime
 variable may be read and written without special precautions.
+It is adjusted when the phase of the system time changes.
 .Sh SEE ALSO
 .Xr clock_settime 2 ,
 .Xr ntp_adjtime 2 ,
diff --git a/sys/kern/kern_tc.c b/sys/kern/kern_tc.c
index b1dcb18e31be..4d4e20ef1934 100644
--- a/sys/kern/kern_tc.c
+++ b/sys/kern/kern_tc.c
@@ -103,11 +103,17 @@ int tc_min_ticktock_freq = 1;
 volatile time_t time_second = 1;
 volatile time_t time_uptime = 1;
 
+/*
+ * The system time is always computed by summing the estimated boot time and the
+ * system uptime. The timehands track boot time, but it changes when the system
+ * time is set by the user, stepped by ntpd or adjusted when resuming. It
+ * is set to new_time - uptime.
+ */
 static int sysctl_kern_boottime(SYSCTL_HANDLER_ARGS);
 SYSCTL_PROC(_kern, KERN_BOOTTIME, boottime,
     CTLTYPE_STRUCT | CTLFLAG_RD | CTLFLAG_MPSAFE, NULL, 0,
     sysctl_kern_boottime, "S,timeval",
-    "System boottime");
+    "Estimated system boottime");
 
 SYSCTL_NODE(_kern, OID_AUTO, timecounter, CTLFLAG_RW | CTLFLAG_MPSAFE, 0,
     "");