Here goes the sketchy code segment.  The issue is how to implement
lazy FP context scheme given no FP register MIPS processor.

        case T_COP_UNUSABLE+T_USER:
                if (cpuinfo.fpuirr != 0) {
                        savefpregs(fpcurproc);
                        loadfpregs(p);
                }
                fpcurproc = p;
                if (cpuinfo.fpirr == 0)
                        dealfpu(status, cause, opc);
                else {
                        struct frame *f = p->p_md.md_regs;
                        f->f_regs[SR] |= MIPS_SR_COP_1_BIT;
                }
                p->p_md.md_flags |= MDP_FPUSED;
                userret(p, opc, sticks);
                return; /* GEN */
        case T_FPE+T_USER:
                dealfpu(status, cause, opc);
                userret(p, opc, sticks);
                return; /* GEN */

where dealfpu() will handle CP1 load/store insns, too.

This change would overload the meaning of fpuirr value 0, which is now
meant no FP regs.  It'd be rather nice to introduce -1 or something
special.

Tohru Nishimura