Development:Setting up FEX

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Build dependencies

Debian/Ubuntu dependencies

  • git
  • cmake
  • ninja-build
  • pkgconf
  • ccache (Recommended if you're rebuilding constantly)
  • clang
  • llvm
  • lld (For ThinLTO)
  • binfmt-support (For AArch64 binfmt_misc support)
  • libsdl2-dev (For GUI)
  • libepoxy-dev (For GUI)
  • libssl-dev
  • python-setuptools (For build scripts)
  • g++-x86-64-linux-gnu (For building thunks)
  • libgcc-XX-dev-i386-cross (For building thunks, version should match your g++-x86-64-linux-gnu package)
  • libgcc-XX-dev-amd64-cross (For building thunks, version should match your g++-x86-64-linux-gnu package)
  • nasm (only if building tests)
  • python3-clang (for struct verifier)
  • libstdc++-10-dev-i386-cross (for struct verifier)
  • libstdc++-10-dev-amd64-cross (for struct verifier)
  • libstdc++-10-dev-arm64-cross (for struct verifier)
  • squashfs-tools (For squashfs rootfs support)
  • squashfuse (For mounting the squashfs)
  • libc-bin or glibc-tools (For unittests, needs catchsegv from this)
  • libc6-dev-i386-amd64-cross (For FEXLinuxTests)
  • lib32stdc++-XX-dev-amd64-cross (For FEXLinuxTests, version should match your g++-x86-64-linux-gnu package)

Gentoo dependencies

  • dev-vcs/git
  • cmake
  • ninja
  • ccache
  • clang
  • lld
  • libsdl2
  • libepoxy
  • nasm

Use gubbins

  • >=media-libs/libglvnd-1.3.4 X

Fedora dependencies

  • git
  • cmake
  • ninja-build
  • pkg-config
  • ccache
  • clang
  • lld
  • llvm
  • llvm-devel
  • SDL2-devel
  • libepoxy-devel
  • openssl-devel
  • nasm
  • python3-clang
  • python3-setuptools
  • squashfs-tools
  • squashfuse
  • erofs-fuse
  • erofs-utils

Build Configuration

  • Ensure release mode is enabled
  • Disable test building
  • Enable LTO in a release build
  • Enable LLD to make LTO builds faster
 git clone --recurse-submodules
 cd FEX
 mkdir Build
 cd Build


 sudo ninja install

You can install a binfmt_misc handler for both 32bit and 64bit x86 execution directly from the environment. If you already have box86's 32bit binfmt_misc handler installed then make sure to uninstall their's first. Make sure to have run install prior to this, otherwise binfmt_misc will install an old handler even if the executable has been updated.

 sudo ninja binfmt_misc_32
 sudo ninja binfmt_misc_64


binfmt_misc problems

  • Double check that binfmt_misc has worked
    • `ls /usr/share/binfmts/FEX*` should return files
    • `ls /var/lib/binfmts/FEX*` should return files
    • `/proc/sys/fs/binfmt_misc/status` should exist
      • If it doesn't exist then make sure your kernel has `CONFIG_BINFMT_MISC=y`
      • If it still doesn't exist then make sure systemd isn't disabling it `systemctl enable binfmt-support`

DESTDIR quirks

  • When using DESTDIR, make sure the path is absolute instead of relative
    • FEX's thunks use subprojects because it needs to cross-compile
    • If DESTDIR is relative then the install paths of the subprojects go to the wrong location
    • Using $(pwd)/install works well

Runtime Configuration

FEXConfig is the application to use for runtime configuration setup.

CTLR+S saves the configuration.

Things to ensure to set

On AArch64 host, a rootfs path is mandatory, while it is optional on x86-64 host

  • Core: JIT
  • RootFS
  • Silent Logging (Useful to get FEX information spam out of the way)
  • Block Size (500 is a good default. Optimization passes may break things though)

More Details at: Follow the instructions here

RootFS generation

AArch64 hosts require a rootfs for running applications. FEX provides various x86 images that can be installed through FEXRootFSFetcher

For more detailed instructions see Development:Setting_up_RootFS

Installing Thunks

Only if you want

Follow the instructions here

Additional details here:


  • FEXLoader
    • This is useful for development purposes but is generally recommended against using.
    • Allows passing arguments to FEX for configuration purposes, which can break under some execve situations
  • FEXInterpreter
    • This is a hardlink to FEXLoader which refuses to treat arguments as configuration options
    • Is recommended to use over FEXLoader; Using FEXConfig to set configuration
    • Requires being a hardlink or executable copy, execve logic sees through a softlink
    • Is the executable used for binfmt_misc for interpreters
  • FEXConfig
    • Used for setting configuration options from a GUI
    • Written using Dear ImGui
    • Can load/save global configuration and application configuration files
    • Application configuration files are compared against the executable filename
  • FEXBash
    • Can be useful for starting a bash instance that is running under emulation
    • It's not a chroot! Don't use sudo inside of this environment!
  • FEXMountDaemon
    • This is a mount daemon that will run in the background when you are using a squashfs based rootfs
    • Usually one one instance open at any given moment
    • It usually closes when the FEXInterpreter exits, but not always. See the problems section in setting up a rootfs
      • Has a ten second timeout window in case a another instance of FEX opens and uses it.

Debug binaries

  • TestHarnessRunner
    • Used for running FEX's ASM unit tests
    • Runs some binary in a host x86_64 host environment or FEX and checks results
    • Useful for quickly creating ASM based unit tests
  • IRLoader
    • Used for running FEX's IR based unit tests
    • Only runs through FEX and checks the results
    • Minimal number of tests
  • UnitTestGenerator
    • This is supposed to be an x86-64 instruction generator
    • Has an understanding of x86-64 instruction encoding format
    • Generates hundreds of thousands of instructions
    • Used to be used with a lockstep runner to ensure correct behaviour
    • Largely unsupported, avoiding undefined behaviour is difficult and FEX doesn't need to match undefined behaviour
    • Lockstep hostrunner wasn't ever made to be bug free, hard to support
    • Doesn't ensure correct behaviour on AArch64 device