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

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Date:	Wed, 25 Jul 2012 13:14:02 +0200
From:	Stefan Bader <stefan.bader@...onical.com>
To:	Avi Kivity <avi@...hat.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 25.07.2012 12:44, Avi Kivity wrote:
> On 07/24/2012 06:52 PM, Cong Wang wrote:
> 
>>> From 6b679d1af20656929c0e829f29eed60b0a86a74f Mon Sep 17 00:00:00 2001
>>> From: Stefan Bader <stefan.bader@...onical.com>
>>> Date: Fri, 13 Jul 2012 15:16:33 +0200
>>> Subject: [PATCH] x86/mm: Limit 2/4M size calculation to x86_32
>>>
>>> commit 722bc6b (x86/mm: Fix the size calculation of mapping tables)
>>> did modify the extra space calculation for mapping tables in order
>>> to make up for the first 2/4M memory range using 4K pages.
>>> However this setup is only used when compiling for 32bit. On 64bit
>>> there is only the trailing area of 4K pages (which is already added).
>>>
>>> The code was already adapted once for things went wrong on a 8TB
>>> machine (bd2753b x86/mm: Only add extra pages count for the first memory
>>> range during pre-allocation early page table space), but it looks a bit
>>> like it currently would overdo things for 64bit.
>>> I only noticed while bisecting for the reason I could not make a crash
>>> kernel boot (which ended up on this patch).
>>>
>>> Signed-off-by: Stefan Bader <stefan.bader@...onical.com>
>>> Cc: WANG Cong <xiyou.wangcong@...il.com>
>>> Cc: Yinghai Lu <yinghai@...nel.org>
>>> Cc: Tejun Heo <tj@...nel.org>
>>
>> Acked-by: Cong Wang <xiyou.wangcong@...il.com>
>>
>> Sorry for that I was not aware of x86_64 is different with x86 in the
>> first 2/4M.
> 
> Why would there be a difference?
> 
> Shouldn't the IO space at 0xa0000-0x100000 be mapped with uncacheable
> attributes (or WC for VGA)?  If it's done later, it can be done later
> for both.
> 
arch/x86/mm/init.c

unsigned long __init_refok init_memory_mapping(...
...
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.
         */




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