Freebios2 recovery progress... pci64 mem optimization

YhLu YhLu at
Thu Oct 21 21:33:01 CEST 2004

You are right

4G installed: 
PCI-DMA: Disabling IOMMU.


8G installed:
PCI-DMA: Disabling AGP.
PCI-DMA: aperture base @ 8000000 size 65536 KB
PCI-DMA: Reserving 64MB of IOMMU area in the AGP aperture

-----Original Message-----
From: ebiederman at [mailto:ebiederman at] 
Sent: Thursday, October 21, 2004 7:40 PM
To: YhLu
Cc: 'LinuxBIOS'
Subject: Re: Freebios2 recovery progress... pci64 mem optimization

YhLu <YhLu at> writes:

> Iommu is only needed when system installed memory > 4G?

IOMMU's have 2 uses.
1) let devices with small DMA ranges reach all of memory
   (pci devices that don't implement DAC can only talk to the first 4G)
2) Isolation.  Allowing the OS to controll which memory may be touched
   by I/O devices.

The current Opteron IOMMU does not appear to be general enough to 
perform isolation so it is only needed when you have more than 4GB of

So there may be some point in disabling it on systems with 4GB.

Plus the linux kernel seems to be able to properly allocate the iommu
if it needs it.


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