diff --git a/lib/libc/sys/Makefile.inc b/lib/libc/sys/Makefile.inc
index 49ac92c5fb1b..cc4b45ca1bda 100644
--- a/lib/libc/sys/Makefile.inc
+++ b/lib/libc/sys/Makefile.inc
@@ -118,7 +118,7 @@ MLINKS+=intro.2 errno.2
 MLINKS+=jail.2 jail_attach.2
 MLINKS+=kqueue.2 kevent.2
 MLINKS+=kse.2 kse_create.2 kse.2 kse_exit.2 kse.2 kse_release.2 \
-	kse.2 kse_wakeup.2 kse.2 kse_thr_interrupt.2
+	kse.2 kse_switchin.2 kse.2 kse_thr_interrupt.2 kse.2 kse_wakeup.2
 MLINKS+=madvise.2 posix_madvise.2
 MLINKS+=mlock.2 munlock.2
 MLINKS+=mlockall.2 munlockall.2
diff --git a/lib/libc/sys/kse.2 b/lib/libc/sys/kse.2
index be81f7f2408c..a1151dcc5935 100644
--- a/lib/libc/sys/kse.2
+++ b/lib/libc/sys/kse.2
@@ -53,9 +53,11 @@
 .Ft int
 .Fn kse_release "struct timespec *timeout"
 .Ft int
-.Fn kse_wakeup "struct kse_mailbox *mbx"
+.Fn kse_switchin "mcontext_t *mcp" "long val" "long *loc"
 .Ft int
 .Fn kse_thr_interrupt "struct kse_thr_mailbox *tmbx"
+.Ft int
+.Fn kse_wakeup "struct kse_mailbox *mbx"
 .Sh DESCRIPTION
 These system calls implement kernel support for multi-threaded processes.
 .\"
@@ -286,6 +288,16 @@ If successful,
 does not return to the caller.
 .Pp
 The
+.Fn kse_switchin
+system call can be used by the UTS, when it has selected a new thread,
+to switch to the context of that thread.
+The use of
+.Fn kse_switchin
+is machine dependent.
+Some platforms do not need a system call to switch to a new context,
+while others require its use in particular cases.
+.Pp
+The
 .Fn kse_wakeup
 system call
 is the opposite of