x86-64: Remove unnecessary barrier in vread_tsc

RDTSC is completely unordered on modern Intel and AMD CPUs.  The
Intel manual says that lfence;rdtsc causes all previous instructions
to complete before the tsc is read, and the AMD manual says to use
mfence;rdtsc to do the same thing.

From a decent amount of testing [1] this is enough to make rdtsc
be ordered with respect to subsequent loads across a wide variety
of CPUs.

On Sandy Bridge (i7-2600), this improves a loop of
clock_gettime(CLOCK_MONOTONIC) by more than 5 ns/iter.

[1] https://lkml.org/lkml/2011/4/18/350

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/%3C1c158b9d74338aa5361f96dd473d0e6a58235302.1306156808.git.luto%40mit.edu%3E
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index db697b8..1e62442 100644 (file)
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -769,13 +769,14 @@ static cycle_t __vsyscall_fn vread_tsc(void)
        cycle_t ret;
 
        /*
-        * Surround the RDTSC by barriers, to make sure it's not
-        * speculated to outside the seqlock critical section and
-        * does not cause time warps:
+        * Empirically, a fence (of type that depends on the CPU)
+        * before rdtsc is enough to ensure that rdtsc is ordered
+        * with respect to loads.  The various CPU manuals are unclear
+        * as to whether rdtsc can be reordered with later loads,
+        * but no one has ever seen it happen.
         */
        rdtsc_barrier();
        ret = (cycle_t)vget_cycles();
-       rdtsc_barrier();
 
        return ret >= VVAR(vsyscall_gtod_data).clock.cycle_last ?
                ret : VVAR(vsyscall_gtod_data).clock.cycle_last;