Attached is my first attempt at this fix.  It has been compile tested
for x86_{64,32} and so far, dev tested in so much as it does not break
backwards compatibility for 32bit dom0 on 64bit Xen.  It is untested as
far as ia64 goes, but I believe it should be correct from code inspection.

-- 
Andrew Cooper - Dom0 Kernel Engineer, Citrix XenServer
T: +44 (0)1223 225 900, http://www.citrix.com