Enabling DMA32 in 2.6.x
 

I've read a good amount on this topic, the why's and what for's, but for the life of me I can't find anything referring to how to use it. I know that some hardware are plagued by not making proper use of DMA calls in 2.6.x x86_64 releases and because of this a 4gig DMA32 zone was added, but how do you turn it on? or better yet do you still have to explicitly turn it on in the latest kernel releases by recompiling and adding the config dma32 in Kconfig, or is it enough to just use iommu switches as long as you've compiled them in? Also which switch should I use if thats the case?

A Lil Background:
The main reason behind me asking this question is because I've used the xen kernel and the regular public release and the dma addressing in dmesg is different in both. The xen kernel always assigns a single dma zone, with the address in the lower 4gig, but the default kernel even though it says its creating a DMA32 zone, its not necessarily in the lower 4gig, which can cause problems with some hardware. The xen kernel out of the box forces 32bit address for dma use, but how? since the xen .config and regular release .config read exactly the same with the exception of the xen specific options - (although I read somewhere that they have a slightly different swiotlb implementation - and that could be the key to this week long quest), but I'm at a lost for now, because I think I understand everything except for how to use the thing. Any help/insight/comment would be much appreciated.