H . J . Lu (hjl@lucon.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. Dominic Sweetman Algorithmics Ltd.
