linux-kernel - Re: x86/mm: Limit 2/4M size calculation to x86

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Message-ID: <500FF767.8020507@redhat.com>
Date:	Wed, 25 Jul 2012 16:40:55 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Stefan Bader <stefan.bader@...onical.com>
CC:	Cong Wang <xiyou.wangcong@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>, Yinghai Lu <yinghai@...nel.org>,
	Tejun Heo <tj@...nel.org>
Subject: Re: x86/mm: Limit 2/4M size calculation to x86_32

On 07/25/2012 04:24 PM, Stefan Bader wrote:
>>> ...
>>> ifdef CONFIG_X86_32
>>>         /*
>>>          * Don't use a large page for the first 2/4MB of memory
>>>          * because there are often fixed size MTRRs in there
>>>          * and overlapping MTRRs into large pages can cause
>>>          * slowdowns.
>>>          */
>>>
>> 
>> That's equally true for X86_64.
>> 
>> Best would be to merge the MTRRs into PAT, but that might not work for SMM.
>> 
>> 
> Ok, true. Not sure why this was restricted to 32bit when reconsidering. Except
> if in 64bit it was assumed (or asserted) that the regions are aligned to 2M...
> But maybe this can be answered by someone knowing the details. I would not mind
> either way (have the first range with 4K pages in all cases or fixing the
> additional PTE allocation). Just as it is now it is inconsistent.

Sometimes CONFIG_X86_32 is used as an alias for "machines so old they
don't support x86_64".  As a 32-bit kernel can be run on a machine that
does support x86_64, it should be replaced by a runtime test for
X86_FEATURE_LM, until a more accurate test can be found.

-- 
error compiling committee.c: too many arguments to function


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