Is anyone else interested in having the 64-bit kernel *not* use the CP0
watchpoint registers for storing the kernel stack pointer for the CPU's
current process?

I have a couple problems with this:
 - there are read-only bits in watchhi (according to the MIPS64 spec) so
hoping to save and restore all high 32 bits (as currently coded) seems
unjustified.
 - somebody might want to actually *use* watchpoints (a JTAG debugger,
in my case)

I put together something that works, based on the 32-bit kernel which
has an array of kernelsp's instead of keeping it in CP0.  Some notes
about this solution are:
 - processor id isn't as easy to get in the 64-bit kernel since
CP0_CONTEXT has (&pgd_current[cpu] << 23) instead of (cpu << 23) so in
this patch I use ((&pgd_current[cpu] - &pgd_current[0]) + &kernelsp)
which seems expensive.
 - smp_bootstrap tries to stash the kernelsp away, but won't work
because CP0_CONTEXT on secondary CPUs isn't set up yet.  However, I
don't think this really needs to happen at this time; the first context
switch on the CPU will save the kernelsp without any damage being done
(I think).  So in the patch, I axed this use of set_saved_sp.
 - set_saved_sp implemented using the pointer math needs two temporaries
instead of one (or am I missing a trick?)

2.4 kernel patch included.

Kip