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Date:	Wed, 11 Nov 2009 15:09:54 -0600
From:	Robert Hancock <>
To:	"Anton D. Kachalov" <>
Subject: Re: Reading /dev/mem by dd

On 11/11/2009 08:36 AM, Anton D. Kachalov wrote:
> Hello everyone!
> I've found strange behavior of reading /dev/mem:
> for i in 0 1 2; do
> echo $i
> dd if=/dev/mem of=/dev/null skip=$((6+$i)) bs=$((0x20000000)) count=1
> done
> On some systems with Supermicro X8DTU I've got several messages during
> first 512Mb starting from 0xc000_0000:
> "BUG: soft lockup - CPU#xx stuck for 61s!"
> On other systems with the sameboard I've stuck without any errors at
> last 10Mb before 0x1_0000_0000. Local APIC access?
> On E5440 (Dell PowerEdge 1950) I've just got several:
> APIC error on CPU3: 00(80)
> APIC error on CPU3: 80(80)
> ...
> APIC error on CPU3: 80(80)
> That looks like wrong register access.

I don't think that we prevent any access to device registers in /dev/mem 
- if you read something that has side effects and something breaks, well 
I guess you get to keep both pieces :-) There's a reason it's root-only..
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