[ale] RE: IOMMUs was Re: Intel vs AMD x86-64 (fwd)

Chris Ricker kaboom at gatech.edu
Tue Feb 24 12:29:24 EST 2004

This is interesting, especially if you read the whole thread. People were
ranting about the suckage of the newly announced Intel 64-bit extensions for
P4 and their potential lack of an IOMMU. From the message below, it appears
Opteron had a similar problem, but the Linux kernel people doing the port
convinced AMD to modify their CPU / BIOS design to work around it....


---------- Forwarded message ----------
Date: Tue, 24 Feb 2004 09:50:02 -0600
From: richard.brunner at amd.com
To: linux-kernel at vger.kernel.org
Subject: RE: IOMMUs was Re: Intel vs AMD x86-64

> -----Original Message-----
> From: Andi Kleen [mailto:ak at 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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