RE: [patch 02/11] PAT x86: Map only usable memory in x86_64 identity map and kernel text

["Pallipadi, Venkatesh" <venkatesh.pallipadi@intel.com>]

 

>-----Original Message-----
>From: Linus Torvalds [mailto:torvalds@linux-foundation.org] 
>Sent: Thursday, January 10, 2008 2:15 PM
>To: Pallipadi, Venkatesh
>Cc: ak@muc.de; ebiederm@xmission.com; rdreier@cisco.com; 
>gregkh@suse.de; airlied@skynet.ie; davej@redhat.com; 
>mingo@elte.hu; tglx@linutronix.de; akpm@linux-foundation.org; 
>arjan@infradead.org; Barnes, Jesse; davem@davemloft.net; 
>linux-kernel@vger.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