4 level page table traversal

pagetables.c

// from josx64, but this code is pretty generic.
// [kernel] fills in the pagetable entries from the given address 
//
// see for example Intel Dev Guide Vol 3 4.2
_JOS_API_FUNC void    pagetables_traverse_tables(void* at, uintptr_t * entries, size_t num_entries) {
    _JOS_ASSERT(entries &&_4_level_paging && num_entries>=4);
    
    memset(entries, 0, 4*sizeof(uintptr_t));

    //NOTE: this relies on identity mapping in order to get access to the pagetables themselves 
    //      but this is not essential for understanding the traversal.
    uintptr_t address =(uintptr_t)at;
    mapping_table_t* cr3 = (mapping_table_t*)x86_64_get_pml4();
    uintptr_t pml4e = cr3->entries[PML4_IDX(address)];
    entries[0] = pml4e;
    if ( pml4e & PAGE_BIT_P_PRESENT ) {
        mapping_table_t*  pml4 = (mapping_table_t*)(pml4e & PAGE_ADDR_MASK);         
        uintptr_t pdpte = pml4->entries[PDPT_IDX(address)];
        entries[1] = pdpte;

        if ( (pdpte & PAGE_HUGE)==0 ) {
            if ( pdpte & PAGE_BIT_P_PRESENT ) {
                mapping_table_t* pdpt = (mapping_table_t*)(pdpte & PAGE_ADDR_MASK);
                uintptr_t pde = pdpt->entries[PD_IDX(address)];
                entries[2] = pde;

                if ( (pde & PAGE_HUGE)==0 ) {
                    // 4KB pages
                    mapping_table_t* pd = (mapping_table_t*)(pde & PAGE_ADDR_MASK);
                    uintptr_t pte = pd->entries[PT_IDX(address)];
                    entries[3] = pte;
                }
                // else 2MB pages    
            }
        }
        // else 1GB pages
    }
}