Why page enable in Kernel space?

[craig]

In general a address in a process is just a linear address which refer to physical address indirectly  by page directory.  This is reasonable in user space. However is it necessary to do such thing in kernel? It is sure to have penalty when converting a linear address to physical thing. Is it worth doing such thing in kernel.   I think the performance is the most important in kernel, other thing is second. I remember in linux linear address is real physical address in kernel space(is it true?). Why freebsd does not do in the same way?     craiglei