static void register_subpage(struct uc_struct *uc, FlatView *fv, MemoryRegionSection *section)
{
    AddressSpaceDispatch *d = flatview_to_dispatch(fv);
    subpage_t *subpage;
    hwaddr base = section->offset_within_address_space
        & TARGET_PAGE_MASK;
    MemoryRegionSection *existing = phys_page_find(d, base);
    MemoryRegionSection subsection = {
        .offset_within_address_space = base,
        .size = int128_make64(TARGET_PAGE_SIZE),
    };
    hwaddr start, end;

    assert(existing->mr->subpage || existing->mr == &(section->mr->uc->io_mem_unassigned));

    if (!(existing->mr->subpage)) {
        subpage = subpage_init(uc, fv, base);
        subsection.fv = fv;
        subsection.mr = &subpage->iomem;
        phys_page_set(d, base >> TARGET_PAGE_BITS, 1,
                      phys_section_add(uc, &d->map, &subsection));
    } else {
        subpage = container_of(existing->mr, subpage_t, iomem);
    }
    start = section->offset_within_address_space & ~TARGET_PAGE_MASK;
    end = start + int128_get64(section->size) - 1;
    subpage_register(uc, subpage, start, end,
                     phys_section_add(uc, &d->map, section));
}