> Actually I'm not trying to communicate with a driver. The user space 
> application is kind of a driver that lives outside of the kernel and 
> communicates with the device directly. 
OK, but isn't the hardware visible via the PCI bus? Does it have any
PCI BARs defined?
> On a linux system (non xen) the bigphysarea patch was used to get large 
> contiguous DMA memory in user space. 
> I am trying to do the equivalant on Xen capable linux in Dom0 so the same 
> application will work. Kind of port it from 'linux' to 'Xen capable linux'.
Oh boy. I think you have two options:
1) Play with stubdomains. They are based on Mini-OS and
 allow you to create applications + MiniOS (that runs under Xen).
 An example of this is the QEMU stubdomain.
 You can modify your MiniOS to get you virtual addresses that are
 mapped directly to contingous memory areas.
2). Look at fbmem.c and write a device driver that allows the userspace
 application to mmap the driver (similar to fb_mem), which in turn sets
 up a vmalloc-ed area that is physically contigous (by using the
 xen_create_contiguous_region function to exchange PFNs).
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