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Re: [Xen-devel] Crash on boot with 2.6.37-rc8-git3

To: M A Young <m.a.young@xxxxxxxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Subject: Re: [Xen-devel] Crash on boot with 2.6.37-rc8-git3
From: Keir Fraser <keir@xxxxxxx>
Date: Sun, 16 Jan 2011 20:56:23 +0000
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
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Thread-topic: [Xen-devel] Crash on boot with 2.6.37-rc8-git3
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On 16/01/2011 20:48, "M A Young" <m.a.young@xxxxxxxxxxxx> wrote:

>> BIOS-e820: 0000000000000000 - 000000000009f000 (usable)
>> BIOS-e820: 000000000009f000 - 00000000000a0000 (reserved)
>> BIOS-e820: 0000000000100000 - 00000000df66d800 (usable)
>> BIOS-e820: 00000000df66d800 - 00000000e0000000 (reserved)
>> BIOS-e820: 00000000f8000000 - 00000000fc000000 (reserved)
>> BIOS-e820: 00000000fec00000 - 00000000fec10000 (reserved)
>> BIOS-e820: 00000000fed18000 - 00000000fed1c000 (reserved)
>> BIOS-e820: 00000000fed20000 - 00000000fed90000 (reserved)
>> BIOS-e820: 00000000feda0000 - 00000000feda6000 (reserved)
>> BIOS-e820: 00000000fee00000 - 00000000fee10000 (reserved)
>> BIOS-e820: 00000000ffe00000 - 0000000100000000 (reserved)
>> BIOS-e820: 0000000100000000 - 0000000120000000 (usable)
>> Which looks completly normal.. I am really at loss here.
> I have looked at this again and I am worried by the last section, which is
> a chunk from 4GB to 4.5GB. The problem is that I only have 4GB. My tests
> show that dom0_mem=max:3574MB boots, dom0_mem=max:3575MB doesn't. The
> first two "usable" chunks add up to a few KB over 3574MB so the problems
> come when it tries to use the final "usable" chunk which I interpret as
> being beyond the memory I have.
> 3574MB is a bit less than 3.5GB so I would guess that the final chunk is
> trying to make up the memory to 4GB. There are also gaps in these memory
> pieces which add up to about 445MB. Hence I think there are some issues
> with the memory allocation mechanism.

Device memory gets mapped just below 4GB, so the last piece of your RAM gets
re-mapped above 4GB by your BIOS, so that it can still be accessed. If you
add up the size of all the usable regions in the list above, it will sum to
a bit less than 4GB.

The bug will be something in the kernel code that can't handle physical
addresses wider than 32 bits (i.e., physical addresses 4GB and above).

 -- Keir

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