up with a new scheme for its use. Use PTR_LA, INT_S/INT_L, etc. Disable
interrupts when returning from exceptions. Use L_CPU(tp) to get the curcpu
pointer.
When the cpu gets an exception from kernel mode, the sscratch register will be
0 and curlwp will be in the "tp" register. When the cpu gets an exception from
user mode, the sscratch register will be a pointer to the current lwp.
When an exception happends, the sp is atomically swapped with the sscratch
register.
If the sp is zero, the exception was a kernel exception and the
kernel exception path is taken: sp and sscratch are swapped again
so sscratch is zero again and then a trapframe is allocated from
the kernel stack. The t1 register is saved and then the pre-trapframe
sp is written to the trapframe.
If sp was non-zero, the exception was from user mode. The tp register
is temporarily saved in L_MD_TP(sp) and sp is moved tp. tp now
contains a pointer to the current lwp. A pointer to the user
trapframe is loaded from L_MD_UTF(tp). Then t1 is saved in the
trapframe so it can be used. The old sp is fetched from sscratch
while sscratch is zeroed (indicated kernel mode). The old sp is
saved in the trapframe.
Upon exiting the exception, if the exception is returning to user
mode, the contents of tp is written to sscratch.
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