Project_Description.md

Project 1: Get Physical Addresses

Description

In this project, you need to write a new system call void * my_get_physical_addresses(void *) so that a process can use it to get the physical address of a virtual address of a process.

The return value of this system call is either 0 or an address value. 0 means that there is no physical address assigned to the logical address currently. A non-zero value means the physical address of the logical address submitted to the system call as its parameter (in fact, this address is only the offset of a logical address).

Question 1

The following is an example code which you can use to see the effect of copy-on-write.

#include <stdio.h>

//void * my_get_physical_addresses(void *);

int global_a = 123;  //global variable

void hello(void)
{                    
   printf("======================================================================================================\n");
}  

int main()
{ 
  int loc_a;
  void *parent_use, *child_use;  

  printf("===========================Before Fork==================================\n");             
  parent_use = my_get_physical_addresses(&global_a);
  printf("pid=%d: global variable global_a:\n", getpid());  
  prinft("Offest of logical address:[%p]   Physical address:[%p]\n", &global_a, parent_use);              
  printf("========================================================================\n");  

  if(fork())
  { /*parent code*/
    printf("vvvvvvvvvvvvvvvvvvvvvvvvvv  After Fork by parent  vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv\n"); 
    parent_use = my_get_physical_addresses(&global_a);
    printf("pid=%d: global variable global_a:\n", getpid()); 
    prinft("******* Offset of logical address:[%p]   Physical address:[%p]\n", &global_a, parent_use); 
    printf("vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv\n");                      
    wait();                    
  }
  else
  { /*child code*/
    printf("llllllllllllllllllllllllll  After Fork by child  llllllllllllllllllllllllllllllll\n"); 
    child_use = my_get_physical_addresses(&global_a);
    printf("******* pid=%d: global variable global_a:\n", getpid());  
    prinft("******* Offset of logical address:[%p]   Physical address:[%p]\n", &global_a, child_use); 
    printf("llllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllll\n");  
    printf("____________________________________________________________________________\n");  

    /*----------------------- trigger CoW (Copy on Write) -----------------------------------*/    
    global_a = 789;

    printf("iiiiiiiiiiiiiiiiiiiiiiiiii  Test copy on write in child  iiiiiiiiiiiiiiiiiiiiiiii\n"); 
    child_use = my_get_physical_addresses(&global_a);
    printf("******* pid=%d: global variable global_a:\n", getpid());  
    prinft("******* Offset of logical address:[%p]   Physical address:[%p]\n", &global_a, child_use); 
    printf("iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii\n");  
    printf("____________________________________________________________________________\n");                  
    sleep(1000);
  }
}

Question 2

The following is an example code which you can use to check whether a loader loads all data of a process before executing it.

#include <stdio.h>

//void * my_get_physical_addresses(void *);

int a[2000000]; 

int main()
{ 
  int loc_a;
  void *phy_add;  

  phy_add = my_get_physical_addresses(&a[0]);
  printf("global element a[0]:\n");  
  prinft("Offest of logical address:[%p]   Physical address:[%p]\n", &a[0], phy_add);              
  printf("========================================================================\n"); 
  phy_add = my_get_physical_addresses(&a[1999999]);
  printf("global element a[1999999]:\n");  
  prinft("Offest of logical address:[%p]   Physical address:[%p]\n", &a[1999999], phy_add);              
  printf("========================================================================\n"); 
}

Hints

1. Two threads show a physical memory cell (one byte) if both of them have a virtual address that is translated into the physical address of the memory cell.
2. The kernel usually does not allocate physical memories to store all code and data of a process when the process starts execution.
3. Inside the Linux kernel, you need to use function copy_from_user() and function copy_to_user() to copy data from/to a user address buffer.
4. Check the "Referenced Material" part of the Course web site to see how to add a new system call in Linux.