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Date:	Sun, 17 Jan 2010 09:40:43 -0800
From:	Arjan van de Ven <arjan@...radead.org>
To:	Felix Rubinstein <felixru@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: /dev/mem implementation

On Sun, 17 Jan 2010 18:47:10 +0200
Felix Rubinstein <felixru@...il.com> wrote:

> I see the motivation to limit the access to DRAM from root account
> CONFIG_STRICT_DEVMEM by mmap'ing /dev/[k]mem but it's easily overruled
> by simple char driver and implementing mmap of it's own totally
> bypassing all limitations.
> 
> What do you think about it guy?
> Appreciate it.

the reason PAT bans parts of /dev/mem is simple: it is illegal to have
mapping aliases (different cachability) for the same physical page.
Normal kernel APIs take care of this for the normal case, but /dev/mem
would be a back door into that.
This is a hardware imposed requirement, and violating the rule can have
really nasty consequences... hence the PAT code just not allowing it.

If you feel that you have a valid use case where you really want do
muck with such memory, it might be a good idea to explain that
usecase....

-- 
Arjan van de Ven 	Intel Open Source Technology Centre
For development, discussion and tips for power savings, 
visit http://www.lesswatts.org
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