linux-kernel - Re: x86_64 ten times slower than i386

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Date:	Mon, 05 Nov 2007 17:19:44 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Ray Lee <ray-lk@...rabbit.org>,
	Bo Brantén <bosse@....umu.se>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jesse Barnes <jesse.barnes@...el.com>,
	linux-kernel@...r.kernel.org
Subject: Re: x86_64 ten times slower than i386

Andi Kleen wrote:
>> Jesse Barnes (cc:d) wrote a patch to address this, I think (x86: trim
>> memory not covered by WB MTRRs), but as far as I can tell it hasn't
>> been merged yet. System is Intel, 4gb of RAM.
> 
> It wasn't merged because it broke booting on some systems.
> Besides the memory would be still lost -- all it did was to automate
> the "mem=XXXX" line.

There really are only two ways to deal with this -- drop the memory 
(which should be automated, and a warning printed) or adjust the MTRRs. 
  The problem is that at some point we run out of MTRRs, partially 
because they're masks instead of base/limit.

Even use of PAT doesn't trivially resolve this issue with less than 
doing MTRR emulation via PAT (setting the default MTRR to WB); however, 
that is bound to cause trouble with SMM.

	-hpa
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