This post contains components for getting up and running with contributing to the AArch32 backend of LLVM JITLink project. Especially if you cannot get a hands on a Raspberry Pi, you can still go ahead (albeit, with some limitations) by emulating the RPi in QEMU.

Running RPi in QEMU

This repository contains information about emulation. There are two ways to emulate RPi in QEMU.

Using pre-built kernels and dtb (device tree) files

devicetree is a mechanism to provide a way to describe non-discoverable hardware. The repository consists of various pre-built kernels and device tree files which can directly boot an RPi image. Though a pretty straightforward way to emulate, it has a caveat due to an error in the provided devicetree files. This issue explains how this error limits CPU usage to 1 core and limits RAM to 256MB. This may be a serious bottleneck for you.

To avoid this limitation, you can use native emulation, which is a direct backend support from QEMU to emulate raspi2b/3b (and few other boards).

Native emulation

Because ARM machines are generally designed as system-on-chip, many peripheral details may vary across different machines. QEMU system emulator for ARM provides a number of implemented machines. These can be listed via qemu-system-aarch64 -machine help.

It also provides support for certain RPi boards. This backend is used along with the pre-built kernel and dtb file. And the image is then run by the QEMU backend. Some of the board backends provide upto 4 cores and 1GB of RAM. The latest image (bookworm) will not work for native emulation, use the legacy image (bullseye).

Note: You can cross-compile your own kernel and boot that instead (in either of the two ways above). But you should use correct devicetree file for corresponding kernel.

On a sidenote, if you want to edit the raspios image file, you can mount it as a loop device. I needed it for once changing the default password and locale. The image contains two partitions, first contains the bootloader, second is the Linux filesystem. The bootloader contains kernel and various copies of devicetree files.

$ fdisk -l 2023-05-03-raspios-bullseye-armhf-lite.img
Device                                      Boot  Start     End Sectors  Size Id Type
2023-05-03-raspios-bullseye-armhf-lite.img1        8192  532479  524288  256M  c W95 FA
2023-05-03-raspios-bullseye-armhf-lite.img2      532480 3842047 3309568  1.6G 83 Linux

$ sudo losetup -f --show -P <img-file>
/dev/loopN
$ sudo mount /dev/loopNp1 /mnt/     # mounts FAT32
$ sudo mount /dev/loopNp2 /mnt/     # mounts the linux fs
... changes to the image ...
$ sudo umount /mnt/
$ sudo losetup -d /dev/loopN        # unmount and detach loop device

If you are facing issues with a network connection during native-emulation, check if a network card is available or not via $ ip a. If there is no network card, you must tell QEMU to emulate it.

-device usb-net,netdev=net0 -netdev user,id=net0,hostfwd=tcp::5555-:22

The QEMU command will be:

git clone https://github.com/dhruvvyas90/qemu-rpi-kernel.git
cd qemu-rpi-kernel
cp native-emulation/5.4.51 kernels/kernel8.img .
cp native-emulation/dtbs/bcm2710-rpi-3-b.dtb .

curl -LO https://downloads.raspberrypi.com/raspios_oldstable_armhf/images/raspios_oldstable_armhf-2023-10-10/2023-05-03-raspios-bullseye-armhf.img.xz
unxz 2023-05-03-raspios-bullseye-armhf.img.xz

qemu-img resize 2023-05-03-raspios-bullseye-armhf.img 8G

qemu-system-aarch64 -m 1024 -M raspi3b -nographic 
    -kernel kernel8.img -dtb bcm2710-rpi-3-b.dtb 
    -sd 2023-05-03-raspios-bullseye-armhf.img 
    -append "console=ttyAMA0 root=/dev/mmcblk0p2 rw rootwait rootfstype=ext4" 
    -device usb-net,netdev=net0 -netdev user,id=net0,hostfwd=tcp::5555-:22

Cross-compiling JITLink for AArch32 (on Fedora)

Another way to reduce build efforts can be to cross-compile the llvm-jitlink and llvm-jitlink-executor binaries on your host machine. To cross-compile JITLink, we require arm-linux-gnueabihf-gcc compiler and its corresponding glibc, and although Fedora officially provides one, that compiler is only able to build kernels and no userspace programs can be built[1].

But fortunately, a copr package is available which provides a latest build of the toolchain.

You need to enable it and then you can install the compiler.

$ sudo dnf copr enable lantw44/arm-linux-gnueabihf-toolchain
$ sudo dnf install arm-linux-gnueabihf-{gcc,glibc}

You would also require a host build of llvm-tblgen. You can cross compile JITLink using this cmake command:

cmake -G Ninja -DCMAKE_BUILD_TYPE=Debug 
  -DCMAKE_CROSSCOMPILING=1 
  -DCMAKE_C_COMPILER=/usr/bin/arm-linux-gnueabihf-gcc 
  -DCMAKE_CXX_COMPILER=/usr/bin/arm-linux-gnueabihf-g++ 
  -DLLVM_HOST_TRIPLE=arm-unknown-linux-gnueabihf 
  -DLLVM_TABLEGEN=<path/to/host/build>/bin/llvm-tblgen 
  -DLLVM_ENABLE_PIC=OFF 
  -DLLVM_BUILD_STATIC=ON 
  -DLLVM_ENABLE_LIBXML2=OFF 
  <path/to/llvm>

You must verify that the gcc and glibc version on host machine matches on the user RPi machine (either the emulated RPi or an actual one). If not, then you will have to build appropriate version of glibc which can be another hassle.

Once built, you can copy these binaries over to the RPi via scp or similar tools.

I am thankful to Stefan Gränitz (Github) for proofreading this blogpost.

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