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RE: [Xen-devel] allocating AGP aperture memory

  • To: "Langsdorf, Mark" <mark.langsdorf@xxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxx>
  • From: "Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>
  • Date: Wed, 19 Oct 2005 07:35:31 +0100
  • Delivery-date: Wed, 19 Oct 2005 06:38:03 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>
  • Thread-index: AcXUItEVdqWcbj10TmOmktd1+s3AmQADf3OQAAEy7AAAEAj1QA==
  • Thread-topic: [Xen-devel] allocating AGP aperture memory

> > > I'm still working on getting IOMMU support for Opterons under Xen.
> > > 
> > > During the boot sequence, Dom0 is finding the aperture 
> through the 
> > > AGP controller at address 0xe8000000 like it should.  During AGP 
> > > controller initialization, though, it fails to reserve 
> the address 
> > > space because the necessary pages are PageReserved in the mem_map.
> > 
> > Trying to looks a bus (machine) address up in dom0's mem_map
> > (pseudo-physical) won't yield anything sensible.
> So how do I do this?

Don't! Bus addresses should never be looked up in mem_map. The mem_map
array is indexed by pseudo-physical address, and refers just to the DRAM
the domain has.

If the existing driver is trying to lookup a io bus address in mem_map,
that's a bug and you'll need to fix it.
> > Presumably the driver isn't expecting to find memory behind the 
> > aperture? Is it just trying to reserve the bus address range for 
> > purposes of ensuring that other PCI devices don't get 
> allocated to it?
> The aperture is an address space that is not supported by 
> DRAM.  Writes to the aperture get shifted to other physical 
> addresses through the magic of the GART.  

Exactly -- the adress shouldn't be looked up in mem_map.

I suspect what the driver should be doing is just bumping pci_mem_start
to avoid the aperture clashing with other pci resources.


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