CODEWITHSUNDEEP
memory-managementoperating-systemxv6
Zarif
Zarif

Reputation: 485

What does `setupkvm` do in xv6 ? What does it really mean to "set up kernel virtual memory"?

In copyuvm function setupkvm is called to set kernel virtual memory. Why do we need to setup kernel virtual memory when we are copying user process ? Why didn't we need that when we were doing allocuvm ?

Code for copyuvm

// Given a parent process's page table, create a copy
// of it for a child.
pde_t*
copyuvm(pde_t *pgdir, uint sz)
{
  pde_t *d;
  pte_t *pte;
  uint pa, i, flags;
  char *mem;

  if((d = setupkvm()) == 0)
    return 0;
  for(i = 0; i < sz; i += PGSIZE){
    if((pte = walkpgdir(pgdir, (void *) i, 0)) == 0)
      panic("copyuvm: pte should exist");
    if(!(*pte & PTE_P))
      panic("copyuvm: page not present");
    pa = PTE_ADDR(*pte);
    flags = PTE_FLAGS(*pte);
    if((mem = kalloc()) == 0)
      goto bad;
    memmove(mem, (char*)P2V(pa), PGSIZE);
    if(mappages(d, (void*)i, PGSIZE, V2P(mem), flags) < 0) {
      kfree(mem);
      goto bad;
    }
  }
  return d;

bad:
  freevm(d);
  return 0;
}

and for allocuvm

int
allocuvm(pde_t *pgdir, uint oldsz, uint newsz)
{
  char *mem;
  uint a;

  if(newsz >= KERNBASE)
    return 0;
  if(newsz < oldsz)
    return oldsz;

  a = PGROUNDUP(oldsz);
  for(; a < newsz; a += PGSIZE){
    mem = kalloc();
    if(mem == 0){
      cprintf("allocuvm out of memory\n");
      deallocuvm(pgdir, newsz, oldsz);
      return 0;
    }
    memset(mem, 0, PGSIZE);
    if(mappages(pgdir, (char*)a, PGSIZE, V2P(mem), PTE_W|PTE_U) < 0){
      cprintf("allocuvm out of memory (2)\n");
      deallocuvm(pgdir, newsz, oldsz);
      kfree(mem);
      return 0;
    }
  }
  return newsz;
}

Upvotes: 0

Views: 446

Answers (1)

Mahdi Hasnat Siyam
Mahdi Hasnat Siyam

Reputation: 26

What copyuvm does is that copy whole virtual memory (user + kernel) from a page directory. So during copyuvm we need setupkvm for kernel part.

On the other hand, allocuvm just extends existing virtual memory ( specifically heap portion). Since there already exists kernel portion of mappings in allocuvm, we are not bound to call setupkvm.

Upvotes: 1

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