Instructions for building xv6 on Openlab (circinus) servers

Xv6 is a real operating system kernel, and hence, it needs real hardware to boot. Fortunately, today we can emulate hardware in software. Programs like QEMU can emulate functionality of the real physical CPU in software. I.e., QEMU implements the normal CPU loop similar to the one we discussed in class: fetches an instruction pointed by the instruction pointer register (EIP), decodes it, performs all permission and condition checks, computes the outcome, increments EIP and continues to the next instruction. Like a real PC platform, QEMU emulates hardware boot protocol. QEMU starts by loading the disk sector number 0 into the memory location 0x7c00 and jumping to it. Xv6 takes it from there. At a high level, for xv6 it does not matter if it runs on the real hardware or under QEMU. Of course, emulation is slower than real hardware, but besides that as long as QEMU implements the logic of the CPU correctly we do not see any deviations from a real baremetal execution. Surprisingly, QEMU is reasonably fast, so you as a human barely notice the difference.

SSH into Openlab (circinus-XX)

We will use circinus machines 1 through 48: circinus-1.ics.uci.edu to circinus-48.ics.uci.edu.

Before you begin, you need to select a circinus server for yourself. We are following the following method to select a server just to provide some uniform distribution of students across different servers:

  • Find out your student ID
    (Let's say: 66541280)
  • Evaluate serverNumber = (studentIDNumber mod 47) + 1
    Ex: 66541280 => 44
  • Your server name is : circinus-{serverNumber}.ics.uci.edu
    Ex: circinus-44.ics.uci.edu

To configure your xv6 environment, login to your server 

 
$ ssh UCInetID@circinus-XX.ics.uci.edu
Note: if a specific server is not available, just choose any circinus machine randomly.

Troubleshooting your Circinus ssh connection

If you get something like 
Permission denied (publickey,gssapi-keyex,gssapi-with-mic,password)
message from ssh and you're connecting from outside of campus, you need to need to either use AnyConnect VPN to tunnel in or setup an SSH keypair for passwordless login. Openlab machines do not accept outside password login attempts.

Clone and build Qemu

To run xv6 we need to compile and install a version of the QEMU emulator. Default QEMU's debugging facilities, while powerful, are somewhat immature, so it is highly recommend you use a patched version of Qemu which is maintained by MIT instead of the stock version that may come with your distribution. From inside your home folder create a new directory for this class. I suggest you start in your home folder (unless you know what you're doing this is a good way to go) 

 
cd ~
mkdir cs238p
Change into this new directory 
cd cs238p
Clone the MIT's qemu distribution into the qemu folder 
git clone https://github.com/mit-pdos/6.828-qemu.git qemu
Change into qemu folder and initialize another git submodule 
cd qemu 
git submodule update --init pixman
Configure qemu with minimal settings and the prefix pointing to the install folder 
./configure --disable-kvm --disable-werror --prefix=/home/<YourUCInetID>/cs238p/qemu-install --target-list="i386-softmmu x86_64-softmmu"
For example, for me this becomes: 
./configure --disable-kvm --disable-werror --prefix=/home/aburtsev/cs238p/qemu-install --target-list="i386-softmmu x86_64-softmmu"
Make and install qemu (this will take some time) 
make -j 8
make install
To make qemu accessible from other programs add it to your path. You can either add it every time you log in to the circinus machine by exporting the PATH variable, or you can add this line to .bash_profile file in your home folder. 
export PATH=$PATH:$HOME/cs238p/qemu-install/bin
If you decide to add the PATH permanently to your .bash_profile the line should look something like 
export PATH=$HOME/cs238p/qemu-install/bin:/sbin:/bin:/usr/sbin:/usr/bin:/usr/games:/usr/local/sbin:/usr/local/bin:/usr/X11R6/bin:$HOME/bin:$PATH

Clone, build, and boot xv6

Change back into the cs238p folder and clone xv6 repository. 
cd ~/cs238p
git clone git://github.com/mit-pdos/xv6-public.git
Cloning into xv6...
...
Build xv6 (you only will see a bunch of output from the make command that invokes the gcc compiler on all kernel files, links the kernel with ld, creates the kernel file system with all user-level programs that will be available inside xv6, and so on): 
$ cd xv6-public 
$ make

You're now ready to run xv6. 

$ make qemu-nox ...

You are now running xv6 on top a hardware platform that is emulated by the Qemu emulator. You are now done with the xv6 setup and can continue moving to any homeworks that are currently assigned.

You can find more information about QEMU monitor and GDB debugger here, feel free to explore them.

Troubleshooting your Circinus ssh connection

If you get something like 
 Permission denied
(publickey,gssapi-keyex,gssapi-with-mic,password)
message from ssh and you're connecting from outside of campus, you need to need to either use AnyConnect VPN to tunnel in or setup an SSH keypair for passwordless login. Openlab machines do not accept outside password login attempts.

Alternative ways to install xv6 if you feel like it (not required for this class)

Xv6 on your own Linux system

While we provide instructions for how to use Circinus machines, you are more than welcome to configure and run xv6 on your own laptop, desktop, or VM. If you decide to use your own environment, see the instructions on the xv6 tools page for how to set up xv6. I've successfully built xv6 on my Ubuntu 14.04 LTS and later on Ubuntu 16.04 and 18.04 LTS. I had to install the following packages in order to build QEMU: libz-dev, libtool-bin, libtool, libglib2.0-dev, libpixman-1-dev, libfdt-dev.

XV6 in Docker

In case you want to use run xv6 on your own machine using docker containers, you can try it out as well. I have successfully built XV6 using the grantbot/xv6 image hosted in the docker hub

In my localmachine, I downloaded the XV6 source code as follows : 

localhost$ mkdir XV6_Dev
localhost$ cd XV6_Dev
localhost$ git clone git://github.com/mit-pdos/xv6-public.git
Cloning into xv6...
...

Next, you will need to setup Docker, if you don't have it already on your machine. I followed the instructions from here. You will find similar instructions for other OS as well in the docker website. Once you have the setup ready, download the grantbot/xv6 image using 

docker pull grantbot/xv6

Then you can start the container using 
docker run -v '/{Path to local XV6 folder}/XV6_Dev':/home/a/XV6_Dev/ -i -t grantbot/xv6
Once you have the bash prompt you can type the following to start XV6, 
cd ~/XV6_Dev/
make qemu-nox

Updated: March, 2020