linux-kernel - RE: [patch 02/11] PAT x86: Map only usable memory in x86

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Date:	Thu, 10 Jan 2008 14:15:25 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>
cc:	ak@....de, ebiederm@...ssion.com, rdreier@...co.com,
	gregkh@...e.de, airlied@...net.ie, davej@...hat.com, mingo@...e.hu,
	tglx@...utronix.de, akpm@...ux-foundation.org, arjan@...radead.org,
	"Barnes, Jesse" <jesse.barnes@...el.com>, davem@...emloft.net,
	linux-kernel@...r.kernel.org,
	"Siddha, Suresh B" <suresh.b.siddha@...el.com>
Subject: RE: [patch 02/11] PAT x86: Map only usable memory in x86_64 identity
 map and kernel text



On Thu, 10 Jan 2008, Pallipadi, Venkatesh wrote:
> 
> Yes. I had those pages not mapped at all earlier. The reason I switched
> to zero page is to continue support cases like:
>  BIOS-e820: 0000000000000000 - 000000000009cc00 (usable)
>  BIOS-e820: 000000000009cc00 - 00000000000a0000 (reserved)
>  BIOS-e820: 00000000000cc000 - 00000000000d0000 (reserved)
>  BIOS-e820: 00000000000e4000 - 0000000000100000 (reserved)
>  BIOS-e820: 0000000000100000 - 00000000cff60000 (usable)
> 
> In this case if some one does a dd of /dev/mem before they can read the
> contents of usable memory in 0x100000-0xcff60000 range.

Well, I think that /dev/mem should simply give them the right info. That's 
what people use /dev/mem for - doing things like reading BIOS images etc. 

So returning *either* a zero page *or* stopping at the first hole is both 
equally wrong. 

			Linus
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