linux-kernel - [PATCH 17/19] x86/kvm/tsc: Drop extra barrier and use rdtsc

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Date:	Thu, 25 Jun 2015 18:44:11 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: [PATCH 17/19] x86/kvm/tsc: Drop extra barrier and use rdtsc_ordered() in kvmclock

From: Andy Lutomirski <luto@...nel.org>

__pvclock_read_cycles() had an unnecessary barrier. Get rid of that
barrier and clean up the code by just using rdtsc_ordered().

Signed-off-by: Andy Lutomirski <luto@...nel.org>
Cc: Denys Vlasenko <dvlasenk@...hat.com>
Cc: Huang Rui <ray.huang@....com>
Cc: John Stultz <john.stultz@...aro.org>
Cc: kvm ML <kvm@...r.kernel.org>
Cc: Len Brown <lenb@...nel.org>
Cc: Marcelo Tosatti <mtosatti@...hat.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Radim Krcmar <rkrcmar@...hat.com>
Cc: Ralf Baechle <ralf@...ux-mips.org>
Cc: x86-ml <x86@...nel.org>
Link: http://lkml.kernel.org/r/678981cc4761fb38a793c217c9cac42503cf3719.1434501121.git.luto@kernel.org
Signed-off-by: Borislav Petkov <bp@...e.de>
---
 arch/x86/include/asm/pvclock.h | 10 +---------
 1 file changed, 1 insertion(+), 9 deletions(-)

diff --git a/arch/x86/include/asm/pvclock.h b/arch/x86/include/asm/pvclock.h
index 6084bce345fc..b6c9c9eda914 100644
--- a/arch/x86/include/asm/pvclock.h
+++ b/arch/x86/include/asm/pvclock.h
@@ -62,7 +62,7 @@ static inline u64 pvclock_scale_delta(u64 delta, u32 mul_frac, int shift)
 static __always_inline
 u64 pvclock_get_nsec_offset(const struct pvclock_vcpu_time_info *src)
 {
-	u64 delta = rdtsc() - src->tsc_timestamp;
+	u64 delta = rdtsc_ordered() - src->tsc_timestamp;
 	return pvclock_scale_delta(delta, src->tsc_to_system_mul,
 				   src->tsc_shift);
 }
@@ -76,17 +76,9 @@ unsigned __pvclock_read_cycles(const struct pvclock_vcpu_time_info *src,
 	u8 ret_flags;
 
 	version = src->version;
-	/* Note: emulated platforms which do not advertise SSE2 support
-	 * result in kvmclock not using the necessary RDTSC barriers.
-	 * Without barriers, it is possible that RDTSC instruction reads from
-	 * the time stamp counter outside rdtsc_barrier protected section
-	 * below, resulting in violation of monotonicity.
-	 */
-	rdtsc_barrier();
 	offset = pvclock_get_nsec_offset(src);
 	ret = src->system_time + offset;
 	ret_flags = src->flags;
-	rdtsc_barrier();
 
 	*cycles = ret;
 	*flags = ret_flags;
-- 
2.3.5

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