[ale] IOMMU

[ups at tree.com (Stephan Uphoff)]

> > Good afternnon!
> > 
> >   I was wondering if someone could explain what an IOMMU is?
> > We've got some IBM e325 Opteron boxes with RHEL and we're
> > being told to reboot with 'iommu=off' and I'm curious if this will
> > hurt performance.
> 
> The only guess I can come up with is:
> 
> I/O Memory Management Unit
> 
> now why they would have an MMU just for I/O is beyond me.
 
Do you remember bouncing buffers for ISA ....
To avoid the same problem with 32Bit only PCI cards on 64Bit architectures
you need to map addresses > 2^32 into the 32bit space of the PCI DMA.
This is done by mapping arbitrary 64bit Memory addresses into a reserved
32bit space addressed by PCI DMA => IOMMU.

I don't think you will have 32bit only PCI devices on a e325 - otherwise
bouncing buffers could reduce the performance (Memory > 4GB).


	Stephan