linux-kernel - [PATCH v3 16/18] x86/tsc: In read_tsc, use rdtsc_ordered() instead of get

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Date:	Tue, 16 Jun 2015 17:36:04 -0700
From:	Andy Lutomirski <luto@...nel.org>
To:	x86@...nel.org
Cc:	Borislav Petkov <bp@...e.de>,
	Peter Zijlstra <peterz@...radead.org>,
	John Stultz <john.stultz@...aro.org>,
	linux-kernel@...r.kernel.org, Len Brown <lenb@...nel.org>,
	Huang Rui <ray.huang@....com>,
	Denys Vlasenko <dvlasenk@...hat.com>, kvm@...r.kernel.org,
	Ralf Baechle <ralf@...ux-mips.org>,
	Andy Lutomirski <luto@...nel.org>
Subject: [PATCH v3 16/18] x86/tsc: In read_tsc, use rdtsc_ordered() instead of get_cycles()

There are two logical changes here.  First, this removes a check for
cpu_has_tsc.  That check is unnecessary, as we don't register the
TSC as a clocksource on systems that have no TSC.  Second, it adds a
barrier, thus preventing observable non-monotonicity.

I suspect that the missing barrier was never a problem in practice
because system calls themselves were heavy enough barriers to
prevent user code from observing time warps due to speculation.
(Without the corresponding barrier in the vDSO, however,
non-monotonicity is easy to detect.)

Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
 arch/x86/kernel/tsc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 21d6e04e3e82..451bade0d320 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -961,7 +961,7 @@ static struct clocksource clocksource_tsc;
  */
 static cycle_t read_tsc(struct clocksource *cs)
 {
-	return (cycle_t)get_cycles();
+	return (cycle_t)rdtsc_ordered();
 }
 
 /*
-- 
2.4.2

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