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Getting Started

When you reconfigure your kernel for Assignment 3, major changes will be made that will prevent your kernel from compiling. These changes include:

If you look at addrspace.c you will find empty skeletons for the addrspace methods. They used to be implemented in dumbvm.c, which is no longer being compiled. The idea is that you should put your new implementations of those functions into addrspace.c

The best way to get started on A3 is to first get back to where you can compile and run your kernel, and then start to implement the requirements for A3. If you don't do this, you will will not be able to test your A3 work incrementally as you get it done, and you'll be left with a huge mess to test at the end - almost a guarantee of testing and debugging nightmares.

One way to get started is to simply copy the implementations of the addrspace functions from dumbvm.c to addrspace.c. You will also need initial implementations of the other functions from dumbvm.c (like the fault handler, vm_fault and the various low level VM and physical memory functions, such as vm_bootstrap and getppages). For these, you can create a new source file (e.g, kern/arch/mips/mips/vm.c) or use kern/vm/addrspace.c and copy their implementations from dumbvm.c to that file. (Don't forget to add your new file to the kernel configuration and reconfigure your kernel!) Finally, you will need to patch up the addrspace structure in addrspace.h so that the fields that went away when OPT_DUMBVM stopped being defined will be present again. Once you've made these changes, make sure that you can build and run your kernel. Everything that worked after A2 should be working again.

Once you have done this, you can start working on the various parts of A3. Since you have a working kernel, you should be able to test each part of A3 as you build it.


Handling TLB Faults



The assignments webpage contains instructions for downloading and installing some new testing programs and scripts. Generally speaking, these run various combinations of the testing programs that you are already familiar with, but there are a few new ones.

The scripts are organized in a hierarchical fashion. So, higher level scripts contain lists of other scripts that they will call. So, it should be easy for anyone to determine exactly what a given script does. Make sure to look at the README file at the top level of the tar package.

Marking will depend on the output of some of the testing scripts. For many tests, we will be relying on the virtual memory statistics output by your kernel as an indication of whether your kernel is behaving as expected.

I didn't get the A2 system calls working. What can I do?

Most of the testing for A3 will involve only writes to the console and _exit. If you have at least that functionality working from A2, you can work with that, and you can ignore the rest of this section.

If you do not even have console writes and _exit working from A2, you can use the following instructions for a quick-and-dirty implementation of those two system calls. This can either replace your implementation in A2 or you can even start by adding this code to a fresh kernel. These implementations don't do much (in particular, this _exit simply reboots the machine) but it is enough to support most of the A3 testing.

First, modify the function mips_syscall(struct trapframe *tf) in kern/arch/mips/mips/syscall.c as shown below. Note that you may need to include some header files for this to compile.

mips_syscall(struct trapframe *tf)


  switch (callno) {
    case SYS_reboot:
      err = sys_reboot(tf->tf_a0);

    /* BEGIN NEW CODE ------------------------------------------- */
    /* NEW: Simple code to handle writes to console */
    /*   this *only* works for writing null-terminated */
    /*   strings, which should be sufficient to handle */
    /*   printf()s in the application code */
    case SYS_write:
      /* check that the write is to stdout */
      assert(tf->tf_a0 == STDOUT_FILENO);
      kprintf("%s", (char *) tf->tf_a1);
      retval = strlen((char *) tf->tf_a1);

    /* NEW: Simple code to handle _exit call */
    case SYS__exit:
      thread_exit(); /* NOTE: need to modify thread_exit(); */
      break;         /* may require #include <thread.h> */
    /* END NEW CODE ------------------------------------------- */

    /* don't forget to comment out or otherwise disable */
    /* any existing implementation of SYS_write and */
    /* SYS__exit that you may have */

      kprintf("Unknown syscall %d\n", callno);
      err = ENOSYS;
Next, in kern/thread/thread.c , modify the thread_exit function:
  /* BEGIN NEW CODE ------------------------------------------- */
  /* NEW: just shut everything down */
  extern int sys_reboot(int code);
  sys_reboot(RB_POWEROFF); /* may require #include <kern/unistd.h> */
  /* END NEW CODE ------------------------------------------- */

  /* leave everything else here */

Finally, in kern/arch/mips/mips/trap.c, modify kill_curthread so that thread_exit will get called.
kill_curthread(u_int32_t epc, unsigned code, u_int32_t vaddr)
   /* BEGIN NEW CODE ------------------------------------------- */
   /* New: if the current thread gets killed */
   thread_exit();         /* may require #include <thread.h> */
   /* END NEW CODE ------------------------------------------- */


Programming the TLB

The code that is used to implement the TLB routines can be found in kern/arch/mips/mips/tlb_mips1.S

A description of what these functions do as well as some useful #defines can be found in kern/arch/mips/include/tlb.h

You can find out quite a lot of detail about how the R3000 TLB operates in the R3000 manual. See "Memory Management and the TLB, Chapter 6". PDF viewer starting page number is 80 and document starting page number is "6-1".

Tracking Virtual Memory Statistics

Please do not preload and/or save and restore TLB contents.
Please only demand load the TLB. Even after a TLB invalidation.
This will allow us to more easily compare statistics.

Below we try to answer some frequently asked questions and to clarify what the stats should count.

  • "TLB Reloads": The number of TLB reloads (TLB faults that did not require a page fault)
  • "Page Faults (Zeroed)" : The number of Page Faults that did not require a disk copy.
  • "Page Faults (Disk)": The number of Page Faults that required a disk copy (e.g., loading a text/code page)

    Losing Output During Booting

  • Sometimes it can be helpful to buffer more printf output before the kprintf subsystem is initialized. To permit more kprintfs to happen before everything is intialized you can make a modification similar to the one shown below to the file kern/dev/generic/console.c
    #if OPT_A3
    /* increase the size substantially */
    #define DELAYBUFSIZE  10240
    #define DELAYBUFSIZE  1024
    static char delayed_outbuf[DELAYBUFSIZE];

    Understanding How Physical Memory is Handled

  • Look at gettppages in kern/arch/mips/mips/dumbvm.c. Notice how getppages gets more page frames when needed. When is getppages first called and why? Think about how that will have to differ in an implementation that needs to talk to the coremap to find free pages.

  • Have a look at ram_stealmem in kern/arch/mips/mips/ram.c. Understand how it works and what firstpaddr and lastpaddr are doing.

  • Have a look at kern/lib/kheap.c and kmalloc. Understand how kmalloc figures out which physical page frame to give to the caller (when a new frame is needed) and how the physical address of that frame is turned into a virtual address that the caller/kernel uses.

  • Be sure you understand how the MIPS translates a kernel virtual address to a physical address and what that means as it relates to allocating free page frames.

  • Have a look at kern/arch/mips/mips/ram.c and the function ram_bootstrap(). You should figure out what is in physical memory at this point, where it is in physical memory and why.

  • Looking at the comments and information in kern/arch/mips/mips/start.S should help quite a bit.
    Remember that the kernel is also a MIPS executable (ELF file). You may find that you can better understand what lives in memory after booting the kernel if you examine the contents of it's executable file (ELF headers).
    # Dump the contents of the ELF file into readelf.out
    # Now you can look at the contents of the readelf.out
    # file to learn more about the kernel.
    cs350-readelf -a kernel-ASST3 > readelf.out

  • Think about how to mark the frames that are already occupied as used in your coremap. Note that there will be a bit of a chicken and an egg problem. In order to mark frames as used you will need to allocate a coremap data structure (using kmalloc). But kmalloc may be needed to find a free page frame to allocate.