Deliverables

1. A coded solution to the basic assignment as specified below (C, geometrically weighted at 70%).

• This part of the assignment can be done in a group or individually

2. A recorded video overview of your solution (V, geometrically weighted at 30%).

• This part of the assignment must be done as an individual.

3. An optional coded solution to the advanced assignment specified below (B, potential bonus of 3 marks)

• This part of the assignment is assumed to be done in a group or individually the same as for deliverable 1.

Final Mark Computation

The assignment mark will be determined from a weighted geometric mean of the two assessed components, plus any bonus, as follows:

Mark = min(20, 20 * (C/14)^0.7 * (V/6)^0.3 + min(3, B))

The maximum achievable mark including any bonuses is 20.

The System/161 TLB

This section provides a summary of the R3000 (System/161) virtual memory mechanisms. Further info is provided in the lectures and in the R3000 Reference Manual and Hardware Guide on the course website.

The R3000 TLB entry includes a 20-bit virtual page number and a 20-bit physical frame number as well as the following five fields:

• global: 1 bit; if set, ignore the PID bits in the TLB.
• valid: 1 bit; set if the TLB entry contains a valid translation.
• dirty: 1 bit; enables writing to the page referenced by the entry; if this bit is 0, the page is only accessible for reading.
• nocache: 1 bit; unused in System/161. In a real processor, indicates that the hardware cache will be disabled when accessing this page.
• asid: 6 bits; a context or address space ID that can be used to allow entries to remain in the TLB after a context switch.

All these bits/values are maintained by the operating system (i.e. your code). When the valid bit is set, the TLB entry contains a valid translation. This implies that the virtual page is present in physical memory. A TLB miss occurs when no TLB entry can be found with a matching virtual page and address space ID (unless the global bit is set in which case the address space ID is ignored) and a valid bit that is set.

For this assignment, you may largely ignore the ASID field and set it to zero in your TLB entries. Note: In OS/161, as_activate() is called whenever a new address space becomes active in the TLB, so as_activate() is typically programmed to flush the TLB (why?).

The System/161 Virtual Address Space Map

The MIPS divides its address space into several regions that have hardwired properties. These are:

• kseg2, TLB-mapped cacheable kernel space
• kseg1, direct-mapped uncached kernel space
• kseg0, direct-mapped cached kernel space
• kuseg, TLB-mapped cacheable user space

Both direct-mapped segments map to the first 512 megabytes of the physical address space.

The top of kuseg is 0x80000000. The top of kseg0 is 0xa0000000, and the top of kseg1 is 0xc0000000.

The memory map thus looks like this:

Configure OS/161 for Assignment 3

Remember to set your PATH environment variable as in previous assignments (or run the 3231 command).

Before proceeding further, configure your new sources, and build and install the user-level libraries and binaries.

% cd ~/cs3231/asst3-src
% ./configure
% bmake
% bmake install

You have to reconfigure your kernel before you can use the framework provided to do this assignment. The procedure for configuring a kernel is the same as before, except you will use the ASST3 configuration file:

% cd ~/cs3231/asst3-src/kern/conf
% ./config ASST3

You should now see an ASST3 directory in the compile directory.

% cd ../compile/ASST3
% bmake depend
% bmake
% bmake install

OS/161 kernel [? for menu]: p /bin/true
Running program /bin/true failed: Function not implemented
Program (pid 2) exited with status 1
Operation took 0.173469806 seconds
OS/161 kernel [? for menu]:

% cd ~/cs3231/root
% wget http://cgi.cse.unsw.edu.au/~cs3231/24T1/assignments/asst3/sys161.conf -O sys161.conf

Address Space Management

OS/161 has an address space data type that encapsulates the book-keeping needed to describe an address space: the struct addrspace. To enable OS/161 to interact with your VM implementation, you will need to implement the functions in kern/vm/addrspace.c and potentialy modify the data type. The semantics of these functions is documented in kern/include/addrspace.h.

Note: You may use a fixed-size stack region (say 16 pages) for each process.

Address Translation

The main goal for this assignment is to provide virtual memory translation for user programs. To do this, you will need add a extend OS/161 address spaces with a page table, and implement a TLB refill handler for the page table.

For this assignment, you will implement a 2-level hierarchical page table. The first level of the page table is to be indexed using the 11 most significant bits of the page number, the second-level nodes of the page tabe are to be indexed using the 9 least significant bits of the page number. Thus the first-level node will have 2048 (2^11) entries and the second-level nodes will have 512 (2^9) entries.

Note that a hierarchical page table is a lazy data-structure. This means that the contents of the page table, including the second-level nodes in the hierarchy, are only allocated when they are needed. You may find allocating the required pages at load time helps you start your assignment, however, your final solution should allocate pages (and level-2 nodes if needed) only when a page-fault occurs.

The following questions may assist you in designing the contents of your page table

• What information do you need to store for each page?
• How does the page table get populated?
• Is the data structure global or a per-process data structure?

Note: Applications expect pages to contain zeros when first used. This implies that newly allocated frames that are used to back pages should be zero-filled prior to mapping

Testing and Debugging Your Assignment

To test this assignment, you should run a process that requires more virtual memory than the TLB can map at any one time. You should also ensure that touching memory not in a valid region will raise an exception. The huge and faulter tests in testbin may be useful. See the Wiki for more options.

Apart from GDB, you may also find the trace161 command useful. trace161 will run the simulator with tracing, for example

% trace161 -t t -f outfile kernel

will record all TLB accesses in outfile.

Don't use kprintf() for vm_fault() debugging. See the Wiki for more info.

Hints

One approach to implementing the assignment is in the following order:

• Review how the specified page table works from the lectures, and understand its relationship with the TLB.
• Review the assignment specification and its relationship with the supplied code.
  • dumbvm is not longer compiled into the OS/161 kernel for this assignment (kern/arch/mips/vm/dumbvm.c), but you can review it as an example implementation within the interface/framework you will be working within.
    • The only candidate for code re-use is the TLB flush in as_activate().
    • Note: Your implementation of TLB refill in vm_fault() should use tlb_random().
• Work out a basic design for your page table implementation.
• Modify kern/vm/vm.c to insert , lookup, and update page table entries in your page table structure.
• Implement the TLB exception handler vm_fault() in vm.c to refill the TLB and keep it consistent with your page table.
• Implement the functions in kern/vm/addrspace.c that are required for basic functionality (e.g. as_create(), as_prepare_load(), etc.). Allocating user pages in as_define_region() may also simplify your assignment, however good solution allocate pages in vm_fault().
  • E.g. as_create() should initialise your page table, as_destroy() should clean it up.
• Test and debug this. Use the debugger or trace161!

Note: Interrupts should be disabled when writing to the TLB, see dumbvm for an example. Otherwise, unexpected concurrency issues can occur.

as_activate() and as_deactivate() can be copied from dumbvm.

FAQ, Gotchas and Video

Don't forget to look at https://wiki.cse.unsw.edu.au/cs3231cgi/2024t1/Asst3 for an up to date list of potential issues you might encounter.

There is also an overview video on the assignment available on the lectures page in the course account https://cgi.cse.unsw.edu.au/~cs3231/lectures.php.

Basic Assignment Submission

The submission instructions are available on the Wiki. Like previous assignments, you will be submitting the git repository bundle via CSE's give system. For ASST3, the submission system will do a test build and run a simple test to confirm your bundle at least compiles.

Warning! Don't ignore the submission system! If your submission fails the submission process, you may not receive any marks.

Warning! Don't forget to commit your changes prior to generating your bundle.

To submit your bundle:

% cd ~
% give cs3231 asst3 asst3.bundle

Even though the generated bundle should represent all the changes you have made to the supplied code, occasionally students do something "ingenious". So always keep your git repository so that you may recover your assignment should something go wrong.

Advanced Assignment Submission

Submission for the advanced assignment is similar to the basic assignment, except the advance component is given to a separate assignment name: asst3_adv. Again, you need to generate a bundle based on your repository. Note: Our marking scripts will switch to the asst3_adv branch prior to testing the advanced assignment.

Submit your solution

% cd ~
% give cs3231 asst3_adv asst3_adv.bundle

You're now done.