linux-kernel - Re: [PATCH v3 0/4] x86: faster mb()+documentation tweaks

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Date:	Thu, 14 Jan 2016 12:39:34 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	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: Re: [PATCH v3 0/4] x86: faster mb()+documentation tweaks

On Wed, Jan 13, 2016 at 10:12:22PM +0200, Michael S. Tsirkin wrote:
> mb() typically uses mfence on modern x86, but a micro-benchmark shows that it's
> 2 to 3 times slower than lock; addl that we use on older CPUs.
> 
> So let's use the locked variant everywhere.
> 
> While I was at it, I found some inconsistencies in comments in
> arch/x86/include/asm/barrier.h
> 
> The documentation fixes are included first - I verified that
> they do not change the generated code at all. They should be
> safe to apply directly.
> 
> The last patch changes mb() to lock addl. I was unable to
> measure a speed difference on a macro benchmark,
> but I noted that even doing
> 	#define mb() barrier()
> seems to make no difference for most benchmarks
> (it causes hangs sometimes, of course).
> 
> HPA asked that the last patch is deferred until we hear back from
> intel, which makes sense of course. So it needs HPA's ack.
> 
> 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) changes
> I came up with myself.
> 
> Changes from v2:
> 	add patch adding cc clobber for addl
> 	tweak commit log for patch 2
> 	use addl at SP-4 (as opposed to SP) to reduce data dependencies
> 
> Michael S. Tsirkin (4):
>   x86: add cc clobber for addl
>   x86: drop a comment left over from X86_OOSTORE
>   x86: tweak the comment about use of wmb for IO

First three look ok to me regardless of what happens with 4. So applied.

Thanks.

-- 
Regards/Gruss,
    Boris.

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