>       /*
>        * Make sure sigreturn trampoline is coherent.
>        */
> #ifdef MIPS3
>       if (CPUISMIPS3)
>               MachFlushDCache((vaddr_t)p->p_sigacts->ps_sigcode, szsigcode);
> #endif
>       MachFlushICache((vaddr_t)p->p_sigacts->ps_sigcode, szsigcode);

I think the intent of the above can be digested as;

        IF (cache is write-back) {
                make sure to drain the data cache to the target address range
                make sure Icache not to have (now possibly obsoluted)
                insns from target address range.
        }       
        else {
                /* no need to drain data cache because cache is write-thru */
                make sure Icache not to have (now possibly obsoluted)
                insns from target address range.
        }

I think the Icache op is necessary in theory but it's likely noop 
because insns on user stack has been never executed at the moment.
I guess that's the reason why various NetBSD/mips ports worked without
the Icache flush so far.

Tohru Nishimura