linux-kernel - Re: [rfc][patch 2/3] x86: fix IO write barriers

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Message-Id: <200710042021.59704.ak@suse.de>
Date:	Thu, 4 Oct 2007 20:21:59 +0200
From:	Andi Kleen <ak@...e.de>
To:	Dave Jones <davej@...hat.com>
Cc:	Nick Piggin <npiggin@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [rfc][patch 2/3] x86: fix IO write barriers

On Thursday 04 October 2007 20:10:44 Dave Jones wrote:
> On Thu, Oct 04, 2007 at 07:53:16PM +0200, Andi Kleen wrote:
>  > 
>  > > The only vendor that ever implemented OOSTOREs was Centaur, and they
>  > > only did in the Winchip generation of the CPUs.  When they dropped it
>  > > from the C3, I asked whether they intended to bring it back, and the
>  > > answer was "extremely unlikely".
>  > >
>  > 
>  > Do you know if it made a big performance difference?
> 
> On the winchip, it was a huge win. I can't remember exact numbers,
> but pretty much every benchmark I threw at it at the time showed
> significant improvement.

Significant as in >10%?


>  > But yes we should probably just remove this special case to make 
>  > maintenance easier.
> 
> It's CONFIG_SMP anyway, which none of the winchips were.

It's not. And we need memory barriers even without SMP 
when talking to device drivers. Only the smp_*b()s get noped
on UP.

-Andi
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