2021 MIT6.S081 Lab: page table

Lab: page table

2021的Lab3在设计上相比2020精简了许多内容，很快就做完了，第三个assignment的边界检查没做也能过就懒得做了

Speed up system calls

1. 增加结构体struct proc字段：

   struct usyscall* usyscall;
   

2. 在函数allocproc中增加字段：

   static struct proc*
   allocproc(void)
   {
        
       struct proc *p;
   
       for(p = proc; p < &proc[NPROC]; p++) {
        
           acquire(&p->lock);
           if(p->state == UNUSED) {
        
               goto found;
           } else {
        
               release(&p->lock);
           }
       }
       return 0;
   
       found:
       p->pid = allocpid();
       p->state = USED;
   
       // Allocate a trapframe page.
       if((p->trapframe = (struct trapframe *)kalloc()) == 0){
        
           freeproc(p);
           release(&p->lock);
           return 0;
       }
   
       if((p->usyscall = (struct usyscall*)kalloc()) == 0) {
        
           freeproc(p);
           release(&p->lock);
           return 0;
       }
   
       // An empty user page table.
       p->pagetable = proc_pagetable(p);
       if(p->pagetable == 0){
        
           freeproc(p);
           release(&p->lock);
           return 0;
       }
   
       copyout(p->pagetable, USYSCALL, (char*)&(p->pid), sizeof(p->pid));
   
       // Set up new context to start executing at forkret,
       // which returns to user space.
       memset(&p->context, 0, sizeof(p->context));
       p->context.ra = (uint64)forkret;
       p->context.sp = p->kstack + PGSIZE;
   
       return p;
   }
   

3. 在freeproc中增加字段：

   if(p->usyscall)
       kfree((void*)p->usyscall);
   p->usyscall = 0;
   

4. 在函数proc_pagetable中增加字段：

   if(mappages(pagetable, USYSCALL, PGSIZE, (uint64)(p->usyscall), PTE_U|PTE_R) < 0) {
        
       uvmunmap(pagetable, USYSCALL, 1, 0);
       uvmfree(pagetable, 0);
       return 0;
   }
   

5. 在函数proc_freepagetable中增加字段：

   uvmunmap(pagetable, USYSCALL, 1, 0);
   

Print a page table

1. 在vm.c中定义函数_vmprint和vmprint：

   void _vmprint(pagetable_t pgtbl, int level) {
        
       for(int i = 0; i<512; ++i)
           if(pgtbl[i] & PTE_V)
               switch(level) {
        
                   case 0:
                       printf("..%d: pte %p pa %p\n", i, pgtbl[i], PTE2PA(pgtbl[i]));
                       _vmprint((pagetable_t)PTE2PA(pgtbl[i]), level+1);
                       break;
                   case 1:
                       printf(".. ..%d: pte %p pa %p\n", i, pgtbl[i], PTE2PA(pgtbl[i]));
                       _vmprint((pagetable_t)PTE2PA(pgtbl[i]), level+1);
                       break;
                   case 2:
                       printf(".. .. ..%d: pte %p pa %p\n", i, pgtbl[i], PTE2PA(pgtbl[i]));
                       _vmprint((pagetable_t)PTE2PA(pgtbl[i]), level+1);
                       break;
           }
   
       return;
   }
   
   void vmprint(pagetable_t pgtbl) {
        
       printf("page table %p\n", pgtbl);
       _vmprint(pgtbl, 0);
       return;
   }
   

2. 在def.h中增加函数声明：

   void            vmprint(pagetable_t);
   

3. 在函数exec中增加字段：

   if(p->pid == 1)
       vmprint(p->pagetable);
   

Detecting which pages have been accessed

1. 在def.h中增加函数声明：

   pte_t* walk(pagetable_t, uint64, int);
   

2. 在riscv.h中增加宏定义：

   #define PTE_A (1L << 6)
   

3. 修改函数sys_pgaccess内容：

   int
   sys_pgaccess(void)
   {
        
       uint64 uva, dst;
       int len;
       uint buffer = 0;
       pte_t* pte;
       struct proc* p = myproc();
   
       if(argaddr(0, &uva) | argint(1, &len) | argaddr(2, &dst))
           return -1;
   
       uva = PGROUNDDOWN(uva);
   
       for(int i = 0; i<len; ++i, uva+=PGSIZE) {
        
           pte = walk(p->pagetable, uva, 0);
           if(*pte & PTE_A) {
        
               buffer |= (1L << i);
               *pte &= ~PTE_A;
           }
       }
   
       copyout(p->pagetable, dst, (char*)&buffer, sizeof(buffer));
   
       return 0;
   }
   

结果