linux-kernel - [PATCH v2 0/3] x86: faster mb()+other barrier.h tweaks

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Date:	Wed, 13 Jan 2016 00:10:10 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Davidlohr Bueso <dave@...olabs.net>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	Davidlohr Bueso <dbueso@...e.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	virtualization <virtualization@...ts.linux-foundation.org>
Subject: [PATCH v2 0/3] x86: faster mb()+other barrier.h tweaks

mb() typically uses mfence on modern x86, but a micro-benchmark shows that it's
2 to 3 times slower than lock; addl $0,(%%e/rsp) that we use on older CPUs.

So let's use the locked variant everywhere - helps keep the code simple as
well.

While I was at it, I found some inconsistencies in comments in
arch/x86/include/asm/barrier.h

I hope I'm not splitting this up too much - the reason is I wanted to isolate
the code changes (that people might want to test for performance) from comment
changes approved by Linus, from (so far unreviewed) comment change I came up
with myself.

Lightly tested on my system.

Michael S. Tsirkin (3):
  x86: drop mfence in favor of lock+addl
  x86: drop a comment left over from X86_OOSTORE
  x86: tweak the comment about use of wmb for IO

 arch/x86/include/asm/barrier.h | 10 +++-------
 1 file changed, 3 insertions(+), 7 deletions(-)

-- 
MST