Though the discussion is unresolved yet, I made a new patch for
reference.

If this patch was appried, I can override a cpu_idle member as
follows:

  mips_locore_dumpvec.cpu_idle = vr_idle();

Index: include/locore.h
===================================================================
RCS file: /cvsroot/syssrc/sys/arch/mips/include/locore.h,v
retrieving revision 1.23
diff -r1.23 locore.h
54a55
> extern void mips_idle __P((void));
124a126
>       void (*cpu_idle) __P((void));
cvs server: Diffing mips
Index: mips/genassym.cf
===================================================================
RCS file: /cvsroot/syssrc/sys/arch/mips/mips/genassym.cf,v
retrieving revision 1.13
diff -r1.13 genassym.cf
82a83
> define        MIPSX_CPU_IDLE          offsetof(mips_locore_jumpvec_t,
cpu_idle
)
Index: mips/locore.S
===================================================================
RCS file: /cvsroot/syssrc/sys/arch/mips/mips/locore.S,v
retrieving revision 1.84
diff -r1.84 locore.S
262c262
< LEAF(idle)
---
> LEAF(mips_idle)
288c288
< END(idle)
---
> END(mips_idle)
318c318,322
<       beq     t1, zero, _C_LABEL(idle)
---
>       bne     t1, zero, 1f
>       lw      t2, _C_LABEL(mips_locore_jumpvec) + MIPSX_CPU_IDLE
>       jal     ra, t2
>       nop
> 1:
331c335,339
<       beq     t0, zero, _C_LABEL(idle)        # if none, idle
---
>       bne     t0, zero, 1f
>       lw      t3, _C_LABEL(mips_locore_jumpvec) + MIPSX_CPU_IDLE
>       jal     ra, t3
>       nop
> 1:
Index: mips/mips_machdep.c
===================================================================
RCS file: /cvsroot/syssrc/sys/arch/mips/mips/mips_machdep.c,v
retrieving revision 1.63
diff -r1.63 mips_machdep.c
151c151,152
<       mips1_cpu_switch_resume
---
>       mips1_cpu_switch_resume,
>       mips_idle
208c209,210
<       mips3_cpu_switch_resume
---
>       mips3_cpu_switch_resume,
>       mips_idle
Index: mips/trap.c
===================================================================
RCS file: /cvsroot/syssrc/sys/arch/mips/mips/trap.c,v
retrieving revision 1.118
diff -r1.118 trap.c
1159c1159
< extern void idle __P((void));
---
> extern void mips_idle __P((void));
1462c1462
<       Name(idle),
---
>       Name(mips_idle),

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