[PATCH v5 2/8] x86-64: Remove unnecessary barrier in vread_tsc

[Andy Lutomirski <luto@MIT.EDU>]

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>
---
 arch/x86/kernel/tsc.c |    9 +++++----
 1 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index db697b8..1e62442 100644
--- 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;
-- 
1.7.5.1