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

[Mark Lassiter <mackdaddylassiter@xxxxxxxxx>]

Ah, that explains so much!  Thank you for all the
help.

This list rocks!


--- Mark Williamson <mark.williamson@xxxxxxxxxxxx>
wrote:

> > 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).
> 
> Yep.
> 
> > 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 
> regions.
> 
> 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 
> rises!
> 
> Cheers,
> Mark
> 



                
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