[tip:x86/asm] x86/asm/tsc/sync: Use rdtsc_ordered() in check_tsc_warp() and drop extra barriers

tip-bot for Andy Lutomirski
2015-07-06T15:45:38Z

Commit-ID: eee6946e44510b61c35cf754f5505537c7a8eb77
Gitweb: http://git.kernel.org/tip/eee6946e44510b61c35cf754f5505537c7a8eb77
Author: Andy Lutomirski <luto@kernel.org>
AuthorDate: Thu, 25 Jun 2015 18:44:09 +0200
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Mon, 6 Jul 2015 15:23:29 +0200

x86/asm/tsc/sync: Use rdtsc_ordered() in check_tsc_warp() and drop extra barriers

Using get_cycles was unnecessary: check_tsc_warp() is not called
on TSC-less systems. Replace rdtsc_barrier(); get_cycles() with
rdtsc_ordered().

While we're at it, make the somewhat more dangerous change of
removing barrier_before_rdtsc after RDTSC in the TSC warp check
code. This should be okay, though -- the vDSO TSC code doesn't
have that barrier, so, if removing the barrier from the warp
check would cause us to detect a warp that we otherwise wouldn't
detect, then we have a genuine bug.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Huang Rui <ray.huang@amd.com>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Len Brown <lenb@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: kvm ML <kvm@vger.kernel.org>
Link: http://lkml.kernel.org/r/387c4c3a75f875bcde6cd68cee013273a744f364.1434501121.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
arch/x86/kernel/tsc_sync.c | 14 ++++++--------
1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kernel/tsc_sync.c b/arch/x86/kernel/tsc_sync.c
index dd8d079..78083bf 100644
--- a/arch/x86/kernel/tsc_sync.c
+++ b/arch/x86/kernel/tsc_sync.c
@@ -39,16 +39,15 @@ static cycles_t max_warp;
static int nr_warps;

/*
- * TSC-warp measurement loop running on both CPUs:
+ * TSC-warp measurement loop running on both CPUs. This is not called
+ * if there is no TSC.
*/
static void check_tsc_warp(unsigned int timeout)
{
cycles_t start, now, prev, end;
int i;

- rdtsc_barrier();
- start = get_cycles();
- rdtsc_barrier();
+ start = rdtsc_ordered();
/*
* The measurement runs for 'timeout' msecs:
*/
@@ -63,9 +62,7 @@ static void check_tsc_warp(unsigned int timeout)
*/
arch_spin_lock(&sync_lock);
prev = last_tsc;
- rdtsc_barrier();
- now = get_cycles();
- rdtsc_barrier();
+ now = rdtsc_ordered();
last_tsc = now;
arch_spin_unlock(&sync_lock);

@@ -126,7 +123,7 @@ void check_tsc_sync_source(int cpu)

/*
* No need to check if we already know that the TSC is not
- * synchronized:
+ * synchronized or if we have no TSC.
*/
if (unsynchronized_tsc())
return;
@@ -190,6 +187,7 @@ void check_tsc_sync_target(void)
{
int cpus = 2;

+ /* Also aborts if there is no TSC. */
if (unsynchronized_tsc() || tsc_clocksource_reliable)
return;