Lab: page tables

In this lab you will explore page tables and modify them to speed up certain system calls and to detect which pages have been accessed.

Before you start coding, read Chapter 3 of the xv6 book, and related files:

It may also help to consult the RISC-V privileged architecture manual.

To start the lab, switch to the pgtbl branch:

  $ git fetch
  $ git checkout pgtbl
  $ make clean

Speed up system calls

Some operating systems (e.g., Linux) speed up certain system calls by sharing data in a read-only region between userspace and the kernel. This eliminates the need for kernel crossings when performing these system calls. To help you learn how to insert mappings into a page table, your first task is to implement this optimization for the getpid() system call in xv6.

When each process is created, map one read-only page at USYSCALL (a virtual address defined in memlayout.h). At the start of this page, store a struct usyscall (also defined in memlayout.h), and initialize it to store the PID of the current process. For this lab, ugetpid() has been provided on the userspace side and will automatically use the USYSCALL mapping. You will receive full credit for this part of the lab if the ugetpid test case passes when running pgtbltest.

Some hints:

Which other xv6 system call(s) could be made faster using this shared page? Explain how.

Print a page table

To help you visualize RISC-V page tables, and perhaps to aid future debugging, your second task is to write a function that prints the contents of a page table.

Define a function called vmprint(). It should take a pagetable_t argument, and print that pagetable in the format described below. Insert if(p->pid==1) vmprint(p->pagetable) in exec.c just before the return argc, to print the first process's page table. You receive full credit for this part of the lab if you pass the pte printout test of make grade.

Now when you start xv6 it should print output like this, describing the page table of the first process at the point when it has just finished exec()ing init:

page table 0x0000000087f6b000
 ..0: pte 0x0000000021fd9c01 pa 0x0000000087f67000
 .. ..0: pte 0x0000000021fd9801 pa 0x0000000087f66000
 .. .. ..0: pte 0x0000000021fda01b pa 0x0000000087f68000
 .. .. ..1: pte 0x0000000021fd9417 pa 0x0000000087f65000
 .. .. ..2: pte 0x0000000021fd9007 pa 0x0000000087f64000
 .. .. ..3: pte 0x0000000021fd8c17 pa 0x0000000087f63000
 ..255: pte 0x0000000021fda801 pa 0x0000000087f6a000
 .. ..511: pte 0x0000000021fda401 pa 0x0000000087f69000
 .. .. ..509: pte 0x0000000021fdcc13 pa 0x0000000087f73000
 .. .. ..510: pte 0x0000000021fdd007 pa 0x0000000087f74000
 .. .. ..511: pte 0x0000000020001c0b pa 0x0000000080007000
init: starting sh
The first line displays the argument to vmprint. After that there is a line for each PTE, including PTEs that refer to page-table pages deeper in the tree. Each PTE line is indented by a number of " .." that indicates its depth in the tree. Each PTE line shows the PTE index in its page-table page, the pte bits, and the physical address extracted from the PTE. Don't print PTEs that are not valid. In the above example, the top-level page-table page has mappings for entries 0 and 255. The next level down for entry 0 has only index 0 mapped, and the bottom-level for that index 0 has entries 0, 1, and 2 mapped.

Your code might emit different physical addresses than those shown above. The number of entries and the virtual addresses should be the same.

Some hints:

For every leaf page in the vmprint output, explain what it logically contains and what its permission bits are. Figure 3.4 in the xv6 book might be helpful, although note that the figure might have a slightly different set of pages than the init process that's being inspected here.

Detect which pages have been accessed

Some garbage collectors (a form of automatic memory management) can benefit from information about which pages have been accessed (read or write). In this part of the lab, you will add a new feature to xv6 that detects and reports this information to userspace by inspecting the access bits in the RISC-V page table. The RISC-V hardware page walker marks these bits in the PTE whenever it resolves a TLB miss.

Your job is to implement pgaccess(), a system call that reports which pages have been accessed. The system call takes three arguments. First, it takes the starting virtual address of the first user page to check. Second, it takes the number of pages to check. Finally, it takes a user address to a buffer to store the results into a bitmask (a datastructure that uses one bit per page and where the first page corresponds to the least significant bit). You will receive full credit for this part of the lab if the pgaccess test case passes when running pgtbltest.

Some hints:

Submit the lab

Time spent

Create a new file, time.txt, and put in a single integer, the number of hours you spent on the lab. git add and git commit the file.


If this lab had questions, write up your answers in answers-*.txt. git add and git commit these files.


Assignment submissions are handled by Gradescope. You will need an MIT gradescope account. See Piazza for the entry code to join the class. Use this link if you need more help joining.

When you're ready to submit, run make zipball, which will generate Upload this zip file to the corresponding Gradescope assignment.

If you run make zipball and you have either uncomitted changes or untracked files, you will see output similar to the following:

 M hello.c
?? bar.c
?? foo.pyc
Untracked files will not be handed in.  Continue? [y/N]
Inspect the above lines and make sure all files that your lab solution needs are tracked, i.e., not listed in a line that begins with ??. You can cause git to track a new file that you create using git add {filename}.

Optional challenge exercises