This address_sizes.diff export CPU Physical and Virtual address sizes through sysctl
interface in the machdep oid. The oids are the following:

% sysctl -d machdep.cpu_physical_address_bits
machdep.cpu_physical_address_bits: CPU physical address bits
% sysctl -d machdep.cpu_virtual_address_bits
machdep.cpu_virtual_address_bits: CPU virtual address bits

On my system below:

FreeBSD ogoshi.lan 10.0-CURRENT FreeBSD 10.0-CURRENT #3 r250287M: Sun May  5 21:59:15 CEST 2013 root@ogoshi.lan:/usr/obj/usr/home/sbz/freebsd/svn/src/sys/GENERIC  amd64

Both have the following values:

% sysctl machdep.cpu_physical_address_bits
machdep.cpu_physical_address_bits: 36
% sysctl machdep.cpu_virtual_address_bits
machdep.cpu_virtual_address_bits: 48

You can verify it's the same with the Makefile and cas.c files.

It also patch sys/modules/linprocfs to export the both values in
/usr/compat/linux/proc/cpuinfo output like it's done in Linux kernel in /proc/cpuinfo.

% (sudo mount -t linprocfs linprocfs /compat/linux/proc && cat /compat/linux/proc/cpuinfo)
...
flags		: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 b19 b21 mmxext mmx fxsr xmm sse2 b27 b28 b29 3dnow
cpu MHz		: 2494.39
bogomips	: 2494.39
address sizes	: 36 bits physical, 48 bits virtual

PR:	http://www.freebsd.org/cgi/query-pr.cgi?pr=178357 (amd64/178357)
ML:	http://lists.freebsd.org/pipermail/freebsd-hackers/2013-May/042667.html (freebsd-hackers@)