AMD64/3ware/8Gig RAM and iommu=fullflush

I've been trying to find information about the iommu=fullflush kernel switch and was wondering if anyone had any information on what the actual performance hit is, forcing it to write out its disk cache every write, or whatever it does?

It must slow things down, but I was wondering how much.

Also, as this was admittedly a kludge, how far off is a more permanent solution to the 3ware 4+gig/32bit only on a 64bit system problem?

Thanks