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

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Message-ID: <20160126101921-mutt-send-email-mst@redhat.com>
Date:	Tue, 26 Jan 2016 10:20:14 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	"H. Peter Anvin" <hpa@...or.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>,
	virtualization <virtualization@...ts.linux-foundation.org>
Subject: Re: [PATCH v2 0/3] x86: faster mb()+other barrier.h tweaks

On Tue, Jan 12, 2016 at 02:25:24PM -0800, H. Peter Anvin wrote:
> On 01/12/16 14:10, 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 $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
> > 
> 
> I would like to get feedback from the hardware team about the
> implications of this change, first.
> 
> 	-hpa
> 

Hi hpa,
Any luck getting some feedback on this one?

Thanks,

-- 
MST