GUGS_SEL
tf->tf_gs = GSEL(GUGS_SEL, SEL_UPL);
curcpu()->ci_gdt[GUGS_SEL].sd = pcb->pcb_threadsegs[TSEG_GS];
tf->tf_gs = GSEL(GUGS_SEL, SEL_UPL);
setsegment(&cpu_info_primary.ci_gdt[GUGS_SEL].sd, 0,
self->ci_gdt[GUGS_SEL].sd = pcb->pcb_threadsegs[TSEG_GS];
curcpu()->ci_gdt[which == TSEG_FS ? GUFS_SEL : GUGS_SEL].sd