Re: x86/mm: Limit 2/4M size calculation to x86_32

From: Avi Kivity <avi@redhat.com>
To: Stefan Bader <stefan.bader@canonical.com>
Cc: Cong Wang <xiyou.wangcong@gmail.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Ingo Molnar <mingo@elte.hu>, Yinghai Lu <yinghai@kernel.org>,
	Tejun Heo <tj@kernel.org>
Subject: Re: x86/mm: Limit 2/4M size calculation to x86_32
Date: Wed, 25 Jul 2012 15:32:12 +0300	[thread overview]
Message-ID: <500FE74C.4040907@redhat.com> (raw)
In-Reply-To: <500FD4FA.7020904@canonical.com>

On 07/25/2012 02:14 PM, Stefan Bader wrote:
> 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@canonical.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@canonical.com>
>>>> Cc: WANG Cong <xiyou.wangcong@gmail.com>
>>>> Cc: Yinghai Lu <yinghai@kernel.org>
>>>> Cc: Tejun Heo <tj@kernel.org>
>>>
>>> Acked-by: Cong Wang <xiyou.wangcong@gmail.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.
>          */
> 

That's equally true for X86_64.

Best would be to merge the MTRRs into PAT, but that might not work for SMM.

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