In this lab you will explore page tables and modify them to simplify the functions that copy data from user space to kernel space.
Before you start coding, read Chapter 3 of the xv6 book, and related files:
To start the lab, switch to the pgtbl branch:
$ git fetch $ git checkout pgtbl $ make clean
To help you learn about RISC-V page tables, and perhaps to aid future debugging, your first task is to write a function that prints the contents of a page table.
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 0x0000000087f6e000 ..0: pte 0x0000000021fda801 pa 0x0000000087f6a000 .. ..0: pte 0x0000000021fda401 pa 0x0000000087f69000 .. .. ..0: pte 0x0000000021fdac1f pa 0x0000000087f6b000 .. .. ..1: pte 0x0000000021fda00f pa 0x0000000087f68000 .. .. ..2: pte 0x0000000021fd9c1f pa 0x0000000087f67000 ..255: pte 0x0000000021fdb401 pa 0x0000000087f6d000 .. ..511: pte 0x0000000021fdb001 pa 0x0000000087f6c000 .. .. ..510: pte 0x0000000021fdd807 pa 0x0000000087f76000 .. .. ..511: pte 0x0000000020001c0b pa 0x0000000080007000The 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.
Xv6 has a single kernel page table that's used whenever it executes in the kernel. The kernel page table is a direct mapping to physical addresses, so that kernel virtual address x maps to physical address x. Xv6 also has a separate page table for each process's user address space, containing only mappings for that process's user memory, starting at virtual address zero. Because the kernel page table doesn't contain these mappings, user addresses are not valid in the kernel. Thus, when the kernel needs to use a user pointer passed in a system call (e.g., the buffer pointer passed to write()), the kernel must first translate the pointer to a physical address. The goal of this section and the next is to allow the kernel to directly dereference user pointers.
Read the book chapter and code mentioned at the start of this assignment; it will be easier to modify the virtual memory code correctly with an understanding of how it works. Bugs in page table setup can cause traps due to missing mappings, can cause loads and stores to affect unexpected pages of physical memory, and can cause execution of instructions from incorrect pages of memory.
This scheme relies on the user virtual address range not overlapping the range of virtual addresses that the kernel uses for its own instructions and data. Xv6 uses virtual addresses that start at zero for user address spaces, and luckily the kernel's memory starts at higher addresses. However, this scheme does limit the maximum size of a user process to be less than the kernel's lowest virtual address. After the kernel has booted, that address is 0xC000000 in xv6, the address of the PLIC registers; see kvminit() in kernel/vm.c, kernel/memlayout.h, and Figure 3-4 in the text. You'll need to modify xv6 to prevent user processes from growing larger than the PLIC address.
Linux uses a technique similar to what you have implemented. Until a few years ago many kernels used the same per-process page table in both user and kernel space, with mappings for both user and kernel addresses, to avoid having to switch page tables when switching between user and kernel space. However, that setup allowed side-channel attacks such as Meltdown and Spectre.
Explain why the third test srcva + len < srcva is necessary in copyin_new(): give values for srcva and len for which the first two test fail (i.e., they will not cause to return -1) but for which the third one is true (resulting in returning -1).
This completes the lab. Make sure you pass all of the make
grade tests. If this lab had questions, don't forget to write up your
answers to the questions in answers-lab-name.txt. Commit your changes
(including adding answers-lab-name.txt) and type make handin in the lab
directory to hand in your lab.
Create a new file, time.txt, and put in it a single integer, the
number of hours you spent on the lab. Don't forget to git add and
git commit the file.
Submit the lab
You will turn in your assignments using
website. You need to request once an API key from the submission
website before you can turn in any assignments or labs.
This completes the lab. Make sure you pass all of the make grade tests. If this lab had questions, don't forget to write up your answers to the questions in answers-lab-name.txt. Commit your changes (including adding answers-lab-name.txt) and type make handin in the lab directory to hand in your lab.
Create a new file, time.txt, and put in it a single integer, the number of hours you spent on the lab. Don't forget to git add and git commit the file.
After committing your final changes to the lab, type make handin to submit your lab.
$ git commit -am "ready to submit my lab" [util c2e3c8b] ready to submit my lab 2 files changed, 18 insertions(+), 2 deletions(-) $ make handin tar: Removing leading `/' from member names Get an API key for yourself by visiting https://6828.scripts.mit.edu/2020/handin.py/ Please enter your API key: XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX % Total % Received % Xferd Average Speed Time Time Time Current Dload Upload Total Spent Left Speed 100 79258 100 239 100 79019 853 275k --:--:-- --:--:-- --:--:-- 276k $make handin will store your API key in myapi.key. If you need to change your API key, just remove this file and let make handin generate it again (myapi.key must not include newline characters).
If you run make handin 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.
If make handin does not work properly, try fixing the problem with the curl or Git commands. Or you can run make tarball. This will make a tar file for you, which you can then upload via our web interface.