Part 5: How to operate on page tables

This section explains 3 things:

1. How page table directory is stored.

2. How to find the address of PTE given virtual address

3. How to map page by creating PTE.

How page table directory is stored.

typedef uint64 pte_t;
typedef uint64 *pagetable_t;// 512 PTEs

pagetable_t is a pointer points to 512 PTEs. page tables are physical memory, initialized by kalloc(). The *pte stores physical memory.

How to find the address of PTE given virtual address

*// Return the address of the PTE in page table pagetable*
*// that corresponds to virtual address va.  If alloc!=0,*
*// create any required page-table pages.*
*//*
*// The risc-v Sv39 scheme has three levels of page-table*
*// pages. A page-table page contains 512 64-bit PTEs.*
*// A 64-bit virtual address is split into five fields:*
*//   39..63 — must be zero.*
*//   30..38 — 9 bits of level-2 index.*
*//   21..39 — 9 bits of level-1 index.*
*//   12..20 — 9 bits of level-0 index.*
*//    0..12 — 12 bits of byte offset within the page.*
static pte_t *
walk(pagetable_t *pagetable*, uint64 *va*, int *alloc*)
{
  if(va >= MAXVA)
    panic(“walk”);

  for(int level = 2; level > 0; level—) {
    pte_t *pte = &pagetable[PX(level, va)];
    if(*pte & PTE_V) {
      pagetable = (pagetable_t)PTE2PA(*pte);
    } else {
      if(!alloc || (pagetable = (pde_t*)kalloc()) == 0)
        return 0;
      memset(pagetable, 0, PGSIZE);
      *pte = PA2PTE(pagetable) | PTE_V;
    }
  }
  return &pagetable[PX(0, va)];
}

pagetable = (pde_t)kalloc())

Above code creates the level_X page table directly from physical memory, and set to pte.

How to map page.

*// Create PTEs for virtual addresses starting at va that refer to*
*// physical addresses starting at pa. va and size might not*
*// be page-aligned. Returns 0 on success, -1 if walk() couldn’t*
*// allocate a needed page-table page.*
int
mappages(pagetable_t *pagetable*, uint64 *va*, uint64 *size*, uint64 *pa*, int *perm*)
{
  uint64 a, last;
  pte_t *pte;

  a = PGROUNDDOWN(va);
  last = PGROUNDDOWN(va + size - 1);
  for(;;){
    if((pte = walk(pagetable, a, 1)) == 0)
      return -1;
    if(*pte & PTE_V)
      panic("remap");
    *pte = PA2PTE(pa) | perm | PTE_V;
    if(a == last)
      break;
    a += PGSIZE;
    pa += PGSIZE;
  }
  return 0;
}