The reasons for using TSS (32 bits) or IST (64 bits) are: concern about
the size of the regular kernel stack, and a concern that the kernel
stack pointer may not be in a usable state. The former is not a problem
here: we're doing a stack switch anyway, and so the additional overhead
on the main stack is pretty minimal, but the latter may be.
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel. I don't speak on their behalf.