x86_64 platform : addresses type

[PUCCETTI Armand <armand.puccetti@cea.fr>]

Several addresses are actually on 32 bits, such as in

typedef struct {
    u32 flags;
    u32 mem_lower;
    u32 mem_upper;
    u32 boot_device;
    u32 cmdline;
    u32 mods_count;
    u32 mods_addr;
    union {
        aout_symbol_table_t aout_sym;
        elf_section_header_table_t elf_sec;
    } u;
    u32 mmap_length;
    u32 mmap_addr;
} multiboot_info_t


However, on x86_64 using gcc, addresses as well as unsigned long are 64 
bits long.

It seems that luckily mmap_addr is below 2*32-1, making the u32 type 
acceptable
so far (for instance in function __start_xen, where the memory map is 
turned into
another structure) but what would it cost to change it into u64 ?
After all, the multiboot specification (page 18) has 'unsigned long' 
instead of u32 here.