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Re: [Xen-users] Dom0 seeing 2Gb, but not 4GB ram

To: Mark Lassiter <mackdaddylassiter@xxxxxxxxx>
Subject: Re: [Xen-users] Dom0 seeing 2Gb, but not 4GB ram
From: Mark Williamson <mark.williamson@xxxxxxxxxxxx>
Date: Thu, 10 Nov 2005 03:40:08 +0000
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> I pasted it below again.  I have since recompiled with
> PAE support and it works!

Excellent stuff!

> Only problem was that I had to manually edit the
> Config.mk file in the root of the xen source tree and
> specify the XEN_TARGET_X86_PAE ?= y (was n).


> The instructions  (README) indicate that I only need
> to run menuconfig from the xen0 and xenU source
> directories, but apparently this is necessary,
> otherwise you get an error about xen and dom0 PAE mode
> mismatch on boot.
> I can now see all 4GB.

The editof the toplevel makefile is necessary to make Xen itself be PAE 
enabled...  I guess the README mightn't have mentioned that.

> I still wonder why I can't with CONFIG_HIGHMEM4G?
> Perhaps something along the lines of the PAE config
> problem above whereby the xen kernel is not actually
> being built with the same options as dom0 and domU?
> Perhaps I'm misreading the README on  building the
> dist.

CONFIG_HIGHMEM4G won't fix your problem: access to 4Gigs of RAM isn't enough 
on your system.  See below...

> Nov  9 19:02:03 localhost kernel:  BIOS-e820:
> 0000000000100000 - 0000000080000000 (usable)

OK, that's about 2 gig of your memory, mapped into a sane place in the 
physical address space...

> Nov  9 19:02:03 localhost kernel:  BIOS-e820:
> 00000000ff780000 - 0000000100000000 (reserved)

And this is an enormous hole, taking up the next 2 gigs of your physical 
address space.

Even with 4G himem, your CPU will only be able to access this far: 2Gigs of 
your real memory, 2Gigs that your BIOS (for some reason) has reserved for IO 

This means that your other 2Gig is mapped too high in the physical address 
space: with 4G himem there is simply no way to address it.

> Nov  9 19:02:03 localhost kernel:  BIOS-e820:
> 0000000100000000 - 0000000180000000 (usable)

And he's the other 2Gig.  This is mapped from 4Gig-6Gig in your physical 
address space, so isn't mappable without PAE support.  With PAE you can 
easily address this range, so the memory becomes usable for you.

I've never *seen* a memory hole 2gigs in size.  Maybe there's a reason for it 
or maybe I just had a sheltered upbringing.

Anyhow, glad you got it working.  I really gotta go sleep before the sun 


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