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