Setting up a virtual Big Endian environment

I mentioned previously that it is a good idea to be able to actually test your code works on a Big-Endian architecture. I recently updated CryptLib and made it compatible for Big-Endian. The way I tested it was to use QEMU to emulate a Big-Endian MIPS processor running Linux. QEMU creates Virtual Machines, but unlike VMWare (which virtualises the computer but leaves instructions running on the hardware) it emulates the CPU. This is great because we can now run an operating system for a different chip. It is however extremely slow.

There is pretty much a Linux distribution for every CPU ever made, so you should be able to install a version of Linux for any chosen chip emulation in QEMU. I chose to use MIPS because I know that is big-endian and is also the chips used in a bunch of OPEN-WRT routers, so I figured it would be supported.

Installing the OS from an installation .iso takes a long time, however I was lucky enough to find that someone had made a QEMU image of a Debian MIPS installation available on the Internet:

From that site download the two files:

Install QEMU if you haven’t already got it. Then run the following command

qemu-system-mips -M malta -kernel vmlinux-3.2.0-4-4kc-malta -hda debian\_wheezy\_mips\_standard.qcow2 -append "root=/dev/sda1 console=tty0" -nographic -redir tcp:2222::22 -m 256

This tells QEMU to boot up the image with no graphics terminal, and instead output the console to stdout. This means the output will just appear in your terminal rather than coming up as a separate window.

QEMU will by default give this a NAT network interface. The -redir flag says to redirect the host port 2222 to the virtual machine’s internal 22 (SSH) port. This makes it easy to SSH into the guest from another computer on your network by SSHing to port 2222 of your host machine.

When you run this you will see Linux slowly boot up. After a while you will get a login prompt. This image has been setup with the user accounts:

user (password user) as a standard user account.
root (password root) as root account.

It is a fairly minimal install, but it does have SSH on, which for my purposes was sufficient. as that is enough to copy files onto it (using scp) and then to execute them.

You can add to this image by installing gcc etc, however actually using the VM to build is extremely slow. I did attempt to do it, but it is not worth the effort. For example to install cmake required building it from source. That took 24 hours!

Instead of trying to build on the VM, the best approach is to build on your actual computer using a cross-compiler.

So if you are using Linux then it is easy, simply run

sudo apt-get install gcc-mips-linux-gnu g++-mips-linux-gnu

to install a MIPS cross compiler.

If you use CMake then it is very easy to use a ToolChain file that is setup to use the mips compiler. Here is my toolchain file. I call it mips.cmake

# toolchain for compiling Linux for MIPS

set(CMAKE_C_COMPILER mips-linux-gnu-gcc)
set(CMAKE_CXX_COMPILER mips-linux-gnu-g++)

# Compiler settings
set( CMAKE_C_FLAGS "-Wall -Werror -Wno-deprecated-declarations" CACHE STRING "" )

 message(STATUS "No build type selected, default to Debug")

# Set install location

Most of this is just my other settings I use. The important part for cross compiling is the first block where it sets up the C compiler.

The following cmake command generates a build system for a project and then builds it:

cmake -H. -Bbuild/mips -DCMAKE_TOOLCHAIN_FILE=mips.cmake
cmake --build build/mips --target install

This will generate the build system in the directory build/mips.

One your program is built you can scp it over to the mips VM and run it. I used this to test CryptoLib. The MD5 and SHA1 algorithms were broken and the test vectors failed in Big-Endian originally until I fixed them up.


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