constraints on rbus_min_start?

I'd like to port the code from arch/i386 that dynamically calculates the
value for rbus_min_start from the memory size.  Are there any
particular constraints I should be aware of for that value?

The i386 code in machdep.c is

#if NCARDBUS > 0
        /* Tell RBUS how much RAM we have, so it can use heuristics.
        rbus_min_start_hint(ptoa(physmem));
#endif

and rbus_min_start_hint in rbus_machdep.c picks an appropriate value
based on that number.  I'm assuming that any amd64 has a fair amount of
memory and address lines, so simply rounding up to the next-highest 1G
boundary will work.  But -- do I need to leave any gaps between real
memory and rbus_min_start?  What boundary does it need to be at?


                --Steve Bellovin, http://www.cs.columbia.edu/~smb