Print a page table ok

answers-page-tables.txt

@@ -1,3 +0,0 @@
-Q: Which other xv6 system call(s) could be made faster using this shared page? Explain how.
-A: sys_uptime. Make the global tick in a user access page, and map the address of tick into usyscall.
-

answers-pgtbl.txt

@@ -0,0 +1,8 @@
+Q: Which other xv6 system call(s) could be made faster using this shared page? Explain how.
+A:  sys_uptime. Make the global tick in a user access page, and map the address of tick into usyscall.
+
+Q: Explain the output of vmprint in terms of Fig 3-4 from the text. What does page 0 contain? What is in page 2? When running in user mode, could the process read/write the memory mapped by page 1? What does the third to last page contain?
+A:  Page 0 contains process's text (code). 
+    Page 2 is the guard page.
+    Page 1 could be read and write in user mode, due to its PTE_U(1 << 4), PTE_R(1 << 2) and PTE_W(1 << 3) flags being set.
+    The third to last page, aka page 255-511-509, is the usyscall page used in the last task.

kernel/defs.h

@@ -173,6 +173,7 @@ uint64          walkaddr(pagetable_t, uint64);
 int             copyout(pagetable_t, uint64, char *, uint64);
 int             copyin(pagetable_t, char *, uint64, uint64);
 int             copyinstr(pagetable_t, char *, uint64, uint64);
+void            vmprint(pagetable_t pagetable);
 
 // plic.c
 void            plicinit(void);

kernel/exec.c

@@ -128,6 +128,7 @@ exec(char *path, char **argv)
   p->trapframe->sp = sp; // initial stack pointer
   proc_freepagetable(oldpagetable, oldsz);
 
+  if (p->pid == 1) vmprint(p->pagetable);
   return argc; // this ends up in a0, the first argument to main(argc, argv)
 
  bad:

kernel/vm.c

@@ -437,3 +437,28 @@ copyinstr(pagetable_t pagetable, char *dst, uint64 srcva, uint64 max)
     return -1;
   }
 }
+
+static void _vmprint(pagetable_t pagetable, int depth)
+{
+  if (depth == 3)
+    return;
+  for (int i = 0; i < 512; ++i)
+  {
+    pte_t pte = pagetable[i];
+    if (pte & PTE_V)
+    {
+      for (int _i = 0; _i < depth; ++_i)
+      {
+        printf(".. ");
+      }
+      printf("..%d: pte %p pa %p\n", i, pte, PTE2PA(pte));
+      _vmprint((pagetable_t)PTE2PA(pte), depth + 1);
+    }
+  }
+}
+
+void vmprint(pagetable_t pagetable)
+{
+  printf("page table %p\n", pagetable);
+  _vmprint(pagetable, 0);
+}