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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 00:54:12 +0800
Cc: 林建安 <colin@realtek.com.tw>, linux-mips@linux-mips.org
In-reply-to: <20090331081113.GA17934@linux-mips.org>
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On 10:11 Tue 31 Mar     , Ralf Baechle wrote:
> On Mon, Mar 30, 2009 at 04:24:15PM +0800, Zhang Le wrote:
> 
> > > Hi all,
> > > We are willing to use 16k page size to avoid cache alias problem.
> > > The Linux version we use is 2.6.12. If we just upgrade mm system to 
> > > support 16k page size, what else problems will happen?
> > > There is already one thing we know that applications of ELF format  
> > > applications should be transformed to be 16k alignment.
> > > Another one, we think, highly suspected to be problematic is that many  
> > > drivers will be ok for 4k page size but fails for 16k.
> > > That is because 4k page size had been seemed to be natural for a very 
> > > long long time.
> > > Any other problem that shall happen for 16k page size?
> > 
> > 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.

And btw, lemote intended to call the box fuloong, as can be seen here:
http://www.lemote.com/english/fuloong.html

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

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