linux-kernel - [tip:x86/asm] x86/asm/tsc/sync: Use rdtsc_ordered() in check_tsc

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Date:	Mon, 6 Jul 2015 08:44:28 -0700
From:	tip-bot for Andy Lutomirski <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	john.stultz@...aro.org, luto@...capital.net, mingo@...nel.org,
	kvm@...r.kernel.org, lenb@...nel.org, hpa@...or.com,
	brgerst@...il.com, dvlasenk@...hat.com, ray.huang@....com,
	bp@...en8.de, torvalds@...ux-foundation.org, ralf@...ux-mips.org,
	tglx@...utronix.de, luto@...nel.org, bp@...e.de,
	peterz@...radead.org, linux-kernel@...r.kernel.org
Subject: [tip:x86/asm] x86/asm/tsc/sync: Use rdtsc_ordered()
  in check_tsc_warp() and drop extra barriers

Commit-ID:  eee6946e44510b61c35cf754f5505537c7a8eb77
Gitweb:     http://git.kernel.org/tip/eee6946e44510b61c35cf754f5505537c7a8eb77
Author:     Andy Lutomirski <luto@...nel.org>
AuthorDate: Thu, 25 Jun 2015 18:44:09 +0200
Committer:  Ingo Molnar <mingo@...nel.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@...nel.org>
Signed-off-by: Borislav Petkov <bp@...e.de>
Cc: Andy Lutomirski <luto@...capital.net>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Brian Gerst <brgerst@...il.com>
Cc: Denys Vlasenko <dvlasenk@...hat.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Huang Rui <ray.huang@....com>
Cc: John Stultz <john.stultz@...aro.org>
Cc: Len Brown <lenb@...nel.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Ralf Baechle <ralf@...ux-mips.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: kvm ML <kvm@...r.kernel.org>
Link: http://lkml.kernel.org/r/387c4c3a75f875bcde6cd68cee013273a744f364.1434501121.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@...nel.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;
 
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