x86-64: Add time to vDSO

The only fast implementation of time(2) we expose is through the
vsyscall page and we want to get userspace to stop using the
vsyscall page.  So make it available through the vDSO as well.

This is essentially a cut-n-paste job.
Signed-off-by: Andy Lutomirski <luto@mit.edu>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Borislav Petkov <bp@amd64.org>
Link: http://lkml.kernel.org/r/%3Cbf963bac5207de4b29613f27c42705e4371812a8.1306156808.git.luto%40mit.edu%3ESigned-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/vdso/vclock_gettime.c

@@ -2,7 +2,7 @@
  * Copyright 2006 Andi Kleen, SUSE Labs.
  * Subject to the GNU Public License, v.2
  *
- * Fast user context implementation of clock_gettime and gettimeofday.
+ * Fast user context implementation of clock_gettime, gettimeofday, and time.
  *
  * The code should have no internal unresolved relocations.
  * Check with readelf after changing.
@@ -160,3 +160,36 @@ notrace int __vdso_gettimeofday(struct timeval *tv, struct timezone *tz)
 }
 int gettimeofday(struct timeval *, struct timezone *)
 	__attribute__((weak, alias("__vdso_gettimeofday")));
+
+/* This will break when the xtime seconds get inaccurate, but that is
+ * unlikely */
+
+static __always_inline long time_syscall(long *t)
+{
+	long secs;
+	asm volatile("syscall"
+		     : "=a" (secs)
+		     : "0" (__NR_time), "D" (t) : "cc", "r11", "cx", "memory");
+	return secs;
+}
+
+notrace time_t __vdso_time(time_t *t)
+{
+	unsigned seq;
+	time_t result;
+	if (unlikely(!VVAR(vsyscall_gtod_data).sysctl_enabled))
+		return time_syscall(t);
+
+	do {
+		seq = read_seqbegin(&VVAR(vsyscall_gtod_data).lock);
+
+		result = VVAR(vsyscall_gtod_data).wall_time_sec;
+
+	} while (read_seqretry(&VVAR(vsyscall_gtod_data).lock, seq));
+
+	if (t)
+		*t = result;
+	return result;
+}
+int time(time_t *t)
+	__attribute__((weak, alias("__vdso_time")));

arch/x86/vdso/vdso.lds.S

@@ -23,6 +23,8 @@ VERSION {
 		__vdso_gettimeofday;
 		getcpu;
 		__vdso_getcpu;
+		time;
+		__vdso_time;
 	local: *;
 	};
 }