Kernel 3: Virtual memory and page tables

Review

The vmiter class examines and modifies the page table structure that implements virtual memory on x86-64
vmiter(PAGETABLE, VA) constructs a vmiter pointing at virtual address VA in pagetable PAGETABLE
it.va() returns the vmiter’s current virtual address (i.e., the key)
it.pa() and it.perm() retrieve $\mathscr{P}(\texttt{it.va()}, \textit{priv})$
- it.pa() is the physical address result (if there is one)
- it.perm() determines which accesses fault
  - If (it.perm() & PTE_P) == 0, all accesses fault
  - If (it.perm() & PTE_W) == 0, write accesses fault
  - If (it.perm() & PTE_U) == 0, unprivileged accesses fault
it.find(va) and it += N redirect vmiter to point to a different virtual address

it.try_map(pa, perm) changes the mapping for it.va()
You may only call it.try_map(pa, perm) if it.va() is page-aligned (a multiple of PAGESIZE)
If it.try_map(pa, perm) succeeds, then afterwards, it.pa() == pa and it.perm() == perm
it.try_map(pa, perm) can fail (return -1)
- Because it.try_map might require allocating kernel memory using kalloc!