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Re: [Xen-devel] X86_64 and 4GB RAM using Flat Memory Model?

To: John Hannfield <hal9020@xxxxxxxxx>
Subject: Re: [Xen-devel] X86_64 and 4GB RAM using Flat Memory Model?
From: Keir Fraser <keir@xxxxxxxxxxxxx>
Date: Tue, 01 May 2007 14:15:23 +0100
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On 1/5/07 11:24, "John Hannfield" <hal9020@xxxxxxxxx> wrote:

> OK, I have updated my grub to the latest available from Debian stable/etch.
> When I access grub at the boot menu, it displays the following for
> 'displaymem'
> grub> displaymem
> EISA Memory BIOS Interface is present
> Address Map BIOS Interface is present
> Lower memory: 631K, Upper memory (to first chipset hole): 3144640K

So the issue here is that grub has for some reason been upset by the values
returned to it by the e820 bios command, and has decided to fall back to a
simpler bios command which only tells you amount of memory up to the first
memory hole. This wouldn't necessarily affect Linux because it interrogates
the bios itself, using different code which isn't affected by this bug
(either it's a bios bug which Linux explicitly works around or is simply not
susceptible to; or it's a grub bug which Linux doesn't have).

> Any further ideas?

It's grim news I'm afraid.

 1. Dig into why GRUB is bailing on the e820 map, and come up with a grub
patch to fix (or work around) the bug. This will require modifying,
building, installing grub, rebooting the machine, etc etc. It's a pain in
the arse.

 2. Boot via a different method (basically that would mean pxe). This might
be convenient or impossible, depending on your machine's local network

 3. There is no command-line option for overrding the e820 map. We could
lash one up, or I can give you a basic patch for you to fill in the blanks
with your memory map. You can then build special Xen with hardcoded map for
that box.

 -- Keir

> Normal debian etch booting on same machine sees all 4GB, using the same
> grub. But I guess it doesn't get the memory map from grub?  If I know the
> memory
> map from the vanilla linux boot, can I provide this another way to xen?

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