On Thu, 6 Jun 2002, Kevin D. Kissell wrote:
> It's been a while since I worked on the code,
> but I'm not sure why last_task_used_math
> needs to be cleared if there is no FPU.
> The way the FPU emulator was integrated,
> the FPU register storage *is* the thread
> context, so if there is no FPU, there was no
> FPU context switching, lazy or otherwise.
> Sounds like someone broke this.
If you check do_cpu in traps.c you'll find:
fp_emul:
if (last_task_used_math != current) {
if (!current->used_math) {
fpu_emulator_init_fpu();
current->used_math = 1;
}
}
sig = fpu_emulator_cop1Handler(0, regs, ¤t->thread.fpu.soft);
last_task_used_math = current;
if (sig)
fpu_emulator_init_fpu() is not called, when two processes are created like
I described. Alternatively the test "if (last_task_used_math != current)"
could be removed.
> Beyond that, the CFC1 instruction is presumably
> there because in the R4000 (at least) it is specified
> as the means to force the FP pipeline to drain
> before the context switch. I would suppose this
> would be done in Linux to avoid mis-attribution
> of FP exceptions. (See chapter 6 of the R4000
> User's manual, page 160 in the Second Edition).
Great. It was exactly something like that I was looking for. Perhaps a
comment would be nice in the code here ;-)
/Kjeld
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