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Re: The impact to change page size to 16k for cache alias

To: Ralf Baechle <ralf@linux-mips.org>
Subject: Re: The impact to change page size to 16k for cache alias
From: Zhang Le <r0bertz@gentoo.org>
Date: Wed, 1 Apr 2009 02:09:37 +0800
Cc: 林建安 <colin@realtek.com.tw>, linux-mips@linux-mips.org
In-reply-to: <20090331171225.GC24154@linux-mips.org>
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On 19:12 Tue 31 Mar     , Ralf Baechle wrote:
> On Wed, Apr 01, 2009 at 12:54:12AM +0800, Zhang Le wrote:
> 
> > > > Linux on Loongson 2E and 2F uses 16k page size to avoid cache alias 
> > > > problem, too.
> > > > However, I haven't encountered any problem on Linux kernel itself due 
> > > > to 16k page
> > > > size.
> > > > 
> > > > Anyway, I am not 100% familiar with Loongson patches, so I am not sure 
> > > > whether
> > > > the page size problem is already been taken care of in the patch. If 
> > > > you are
> > > > interested to find out yourself, you can get the whole source here:
> > > > http://repo.or.cz/w/linux-2.6/linux-loongson.git
> > > 
> > > I've got a report that Fulong is currently only working with 16k pages.  
> > > So
> > > 4k is no longer the bullet proof choice for all cases :)
> > 
> > Yes, at least from what I can tell.
> > I have tried 4k before, because I heard someone told me the aliasing problem
> > already can be taken care by software, namely Linux. But as it turned out, 
> > 16k
> > is still necessary for the Loongson boxes to function properly.
> 
> Linux knows how to handle aliasing caches - the vast majority of MIPS
> systems have aliasing caches and are running at 4k page size.  So the
> issue is Loongson-specific.  This might be due to processor detection
> not detecting the cache size, cache line size, number of ways correctly
> or because loongson's caches has some unusual cache properties which the
> Linux kernel is not designed to handle yet.
> 
> Btw, I just noticed that there is no
> arch/mips/include/asm/mach-lemote/cpu-feature-overrides.h which is going
> to impact code size and performance.

Thanks for the reminder. I didn't know it before. I am gonna take a look.

Zhang, Le
http://zhangle.is-a-geek.org

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