SMP Large Buffers

I'm working with a driver that needs dma buffers larger than 128K.  The
system is a 2 processor SMP with 2 gig total ram.

Using GRUB, we set mem= to be 1984M, leaving 64M reserved.

doing  ioremap(0x7c000000, 0x4000000)
       followed by an access to x7c000000 results in an OOPS.

is this an SMP issue regarding how memory in each node is mapped? (in that
you can't do it this way) as it works fine on a uniprocessor machine.