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SubjectRE: IOMMUs was Re: Intel vs AMD x86-64
Date

> -----Original Message-----
> From: Andi Kleen [mailto:ak@suse.de]


> On Opteron the IOMMU code (ab)uses the built in AGPv3 GART in
> the CPU, which
> was originally intended for AGP. AMD converted it to be able
> to remap PCI especially for Linux, which I think deserves applause.
>
> It works surprisingly well even though it was not designed as
> a real IOMMU. Of course one of the main advantages of a real
> IOMMU - preventing arbitary memory corruption from broken
> devices - is lost because the remapping table is just a hole
> in the memory. I'm
> secretly hoping that when there is more support for Linux at
> chipset vendors they will someday add a bit to isolate all
> traffic that doesn't go through the GART from the main
> memory. This way you could get a much more reliable system
> that can tolerate broken PCI devices at a moderate
> performance penalty.

Andi is being modest. It was he and Andrea Arcangeli who convinced
me we had a problem. We found a way to trick the AGP
GART hardware into helping, and then they turned it into a
"real" solution and helped us work the warts out of the BIOS
to enable it.

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