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

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Date:	Thu, 10 Jan 2008 14:27:10 -0800
From:	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>
To:	"Linus Torvalds" <torvalds@...ux-foundation.org>
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

 

>-----Original Message-----
>From: Linus Torvalds [mailto:torvalds@...ux-foundation.org] 
>Sent: Thursday, January 10, 2008 2:15 PM
>To: Pallipadi, Venkatesh
>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; davem@...emloft.net; 
>linux-kernel@...r.kernel.org; Siddha, Suresh B
>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. 
>

I was not fully clear in my earlier email. Mapping /dev/mem would still
work with our changes. As they go through proper map interface. It is
the dd of dev mem which does the read that has the problem. I was
wondering of apps using dd.

Thanks,
Venki
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