linux-kernel - [tip:x86/asm] x86/asm/tsc, x86/kvm: Drop open-coded barrier and use rdtsc

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Message-ID: <tip-502dfeff239e8313bfbe906ca0a1a6827ac8481b@git.kernel.org>
Date:	Mon, 6 Jul 2015 08:45:08 -0700
From:	tip-bot for Andy Lutomirski <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	brgerst@...il.com, peterz@...radead.org, lenb@...nel.org,
	pbonzini@...hat.com, rkrcmar@...hat.com, tglx@...utronix.de,
	dvlasenk@...hat.com, john.stultz@...aro.org, ralf@...ux-mips.org,
	kvm@...r.kernel.org, bp@...en8.de, mingo@...nel.org,
	luto@...nel.org, mtosatti@...hat.com, luto@...capital.net,
	ray.huang@....com, linux-kernel@...r.kernel.org, bp@...e.de,
	torvalds@...ux-foundation.org, hpa@...or.com
Subject: [tip:x86/asm] x86/asm/tsc, x86/kvm:
  Drop open-coded barrier and use rdtsc_ordered() in kvmclock

Commit-ID:  502dfeff239e8313bfbe906ca0a1a6827ac8481b
Gitweb:     http://git.kernel.org/tip/502dfeff239e8313bfbe906ca0a1a6827ac8481b
Author:     Andy Lutomirski <luto@...nel.org>
AuthorDate: Thu, 25 Jun 2015 18:44:11 +0200
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Mon, 6 Jul 2015 15:23:30 +0200

x86/asm/tsc, x86/kvm: Drop open-coded barrier and use rdtsc_ordered() in kvmclock

__pvclock_read_cycles() used to have two barriers, one of which was unnecessary,
which got removed after an initial version of this patch was sent.

But the barrier is still open-coded unnecessarily - get rid of
that barrier and clean up the code by just using rdtsc_ordered().

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: 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: Thomas Gleixner <tglx@...utronix.de>
Cc: kvm ML <kvm@...r.kernel.org>
Link: http://lkml.kernel.org/r/678981cc4761fb38a793c217c9cac42503cf3719.1434501121.git.luto@kernel.org
[ Ported it to v4.2-rc1. ]
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 arch/x86/include/asm/pvclock.h | 10 ++--------
 1 file changed, 2 insertions(+), 8 deletions(-)

diff --git a/arch/x86/include/asm/pvclock.h b/arch/x86/include/asm/pvclock.h
index 5c490db..7a6bed5 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);
 }
@@ -76,13 +76,7 @@ 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;
--
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