TSEG_FS
setsegment(&pcb->pcb_threadsegs[TSEG_FS], 0,
curcpu()->ci_gdt[GUFS_SEL].sd = pcb->pcb_threadsegs[TSEG_FS];
self->ci_gdt[GUFS_SEL].sd = pcb->pcb_threadsegs[TSEG_FS];
curcpu()->ci_gdt[which == TSEG_FS ? GUFS_SEL : GUGS_SEL].sd
uint32_t base = i386_get_threadbase(p, TSEG_FS);
error = i386_set_threadbase(p, base, TSEG_FS);