what's the mean of num in address_sapce

Hi,
    in kernel 2.6.39, in /include/linux/compiler.h, I found the followed code:

    # define __percpu __attribute__((noderef, address_space(3)))
    #ifdef CONFIG_SPARSE_RCU_POINTER
    # define __rcu __attribute__((noderef, address_space(4)))
    #else
    # define __rcu 

    address_space(num), if num is 0 ,it means kernel space,
    1 , user space; 2, io space,
    so 3 and 4 means what?
     I found nothing about this by google.

Thanks,
feng


feng247 at yeah.net
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