H . J . Lu (firstname.lastname@example.org) writes:
> > It would, in principle, be possible to save/restore k0
> > or k1 (but not both) if no other clever solution can be found.
> > There are other VM OSes that manage to do so for MIPS,
> > for other outside-the-old-ABI reasons. It does, of course,
> > add some instructions and some memory traffic to the
> > low-level exception handling , and we would have to look
> > at whether we would want to make such a feature standard
> > or specific to a "thread-ready" kernel build.
> I like the read-only k0 idea. We just need to make a system call to
> tell kernel what value to put in k0 before returning to the user space.
> It shouldn't be too hard to implement. I will try it next week.
You could, I guess, wire a TLB entry to map the thread register into
the highest virtual memory region of the machine (the top of 'kseg2'),
which is accessible in a single instruction as a negative offset from
$0. The kernel can write it through kseg0 or 64-bit equivalent, if
you're a bit careful about cache aliases.
Reading something out of the cache is pretty cheap: would that be
close enough to a 'register' to do the job? There's no change to
critical routines, that way.