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RE: [Xen-users] AMD or Intel?

To: "Joop Boonen" <joop_boonen@xxxxxx>
Subject: RE: [Xen-users] AMD or Intel?
From: "Petersson, Mats" <Mats.Petersson@xxxxxxx>
Date: Fri, 25 May 2007 15:38:33 +0200
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Thread-topic: [Xen-users] AMD or Intel?

> -----Original Message-----
> From: Joop Boonen [mailto:joop_boonen@xxxxxx] 
> Sent: 25 May 2007 13:50
> To: Petersson, Mats
> Cc: lists; xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: RE: [Xen-users] AMD or Intel?
> Hello Mats,
> Is the IOMMU the one mentioned in this artikle?
> http://www.amd.com/us-en/assets/content_type/white_papers_and_
> tech_docs/34434.pdf
> http://www.amd.com/us-en/Corporate/VirtualPressRoom/0,,51_104_
> 543~117440,00.html


> If so, in which processor/core will it be available?

As I said earlier, I don't know this, and even if I did know, I'd have
to shoot you if I told you ;-)

> Regards,
> Joop Boonen.
> See also:
> http://www.xensource.com/files/xs0106_xen_iommu.pdf

Yes, that's working on a IOMMU model that is produced by IBM, used in
some of their high-end x86 and PowerPC servers as far as I know. 

> What does in this artikle
> http://www.theinquirer.net/default.aspx?article=16879 the IOMMU mean?

IOMMU in this case isn't a "real" IOMMU, it's using the GART (Graphics
Aperture Routing Table), which can indeed be used to remap IO-space, but
there is at least one problem with it:
You can only remap one guest physical address to one machine physical
address for all PCI devices. Let's say you want Xen to use a PCI device
for Dom0, and another from DomU, then the IOMMU would have to remap DomU
memory accesses, but not Dom0. If the guest physical memory range
overlaps the machine range for Dom0, then Dom0's PCI accesses would
potentially be translated and thus use the wrong area of memory.

There may be other problems too, but this one will prevent it from doing
what the real IOMMU does. 

The "real" IOMMU is intended to allow different devices or groups of
devices to have separate "guest to machine" mappings, and also allow
protecting one device from using the others memory and such (so also
gives increased security for the system). 


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