well, phys_to_machine is a macro which is defined in /include/asm/mach-xen/(some directory)/page.h! so you can not export it!
so, I went to /xen-sources/linux-2.6.16-xen/include/asm/mach-xen
and copied all headers (including the ones in the subdirectories) into /xen-sources/linux-2.6.16-xen/include/asm (I know, I know
then I compiled the xen-kernel again and rebooted...
it _worked_ and nvidia's driver (with lots of warnings) loaded!
BTW, if somebody knows why the headers in mach-xen weren't used... please
leave us a _post_ why?