RE: IOMMUs was Re: Intel vs AMD x86-64

[richard . brunner]

> -----Original Message-----
> From: Andi Kleen [mailto:ak@xxxxxxx] 

 
> 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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