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Based on:

Creating working folder and getting resources

mkdir riscv-disk
mkdir riscv-disk/initdir # the cpio of this folder + devNodes.cpio will be initramfs
wget https://raw.githubusercontent.com/firesim/FireMarshal/master/wlutil/initramfs/makeDevNodes.sh
chmod +x makeDevNodes.sh
./makeDevNodes.sh # this command will create the devNodes.cpio file

Note: to check the content of a cpio file, you can use lsinitramfs or binwalk.

Compile gem5 m5

git clone https://gem5.googlesource.com/public/gem5
cd gem5/util/m5
scons build/riscv/out/m5

Note: the default cross-compiler is riscv64-unknown-linux-gnu-. To change the cross-compiler, you can set the cross-compiler using the scons sticky variable riscv.CROSS_COMPILE. For example,

scons riscv.CROSS_COMPILE=riscv64-linux-gnu- build/riscv/out/m5

Compiling RISCV Toolchain

git clone https://github.com/riscv/riscv-gnu-toolchain
cd riscv-gnu-toolchain
./configure --prefix=$HOME/opt/riscv
make linux -j$(nproc)
export PATH=$PATH:$HOME/opt/riscv/bin

UCanLinux

This contains a working sample of Linux Kernel configuration and a working sample of Busybox configuration.

cd riscv-disk
git clone https://github.com/UCanLinux/riscv64-sample

Busybox

cd riscv-disk
git clone git://busybox.net/busybox.git
cd busybox
git checkout 1_32_stable
cp ../riscv64-sample/busybox.config .config
make CROSS_COMPILE=riscv64-linux-gnu- all -j$(nproc)
make CROSS_COMPILE=riscv64-linux-gnu- install

Copying content to initdir

cd riscv-disk/initdir
cp -a ../riscv64-sample/skeleton/* .
cp -a ../busybox/_install/* .
cd ../linux
make ARCH=riscv CROSS_COMPILE=riscv64-linux-gnu- INSTALL_MOD_PATH=../initdir modules_install
cd -
cp -a $HOME/opt/riscv/sysroot/lib .
mkdir dev home mnt proc sys tmp var
cd etc/network
mkdir if-down.d  if-post-down.d  if-pre-up.d  if-up.d

Compiling Linux Kernel (1st time) to get the cpio of initdir

cd riscv-disk
git clone --depth 1 --branch v5.10 https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
cd linux
cp ../riscv64-sample/kernel.config .config
make ARCH=riscv CROSS_COMPILE=riscv64-linux-gnu- menuconfig
# Go to "General setup"
#  -> check on "Initial RAM filesystem and RAM disk (initramfs/initrd) support"
#  -> change "Initramfs source file(s)" to "/abs/path/to/riscv-disk/initdir"
make ARCH=riscv CROSS_COMPILE=riscv64-linux-gnu- all -j$(nproc)
cp usr/initramfs_data.cpio ../

Merging initramfs_data.cpio and devNodes.cpio

cd riscv-disk
cat initramfs_data.cpio devNodes.cpio > init.cpio

Compiling Linux Kernel (2nd time) to get the desired initramfs cpio

cd riscv-disk
make ARCH=riscv CROSS_COMPILE=riscv64-linux-gnu- menuconfig
# Go to "General setup"
#  -> check on "Initial RAM filesystem and RAM disk (initramfs/initrd) support"
#  -> change "Initramfs source file(s)" to "/abs/path/to/riscv-disk/init.cpio"
make ARCH=riscv CROSS_COMPILE=riscv64-linux-gnu- all -j$(nproc)

Bootloader stuff (bbl)

cd riscv-disk
git clone https://github.com/riscv/riscv-pk.git
cd riscv-pk
mkdir build
cd build
../configure --host=riscv64-linux-gnu --with-payload=../../linux/arch/riscv/boot/Image --prefix=$HOME/opt/riscv
make -j$(nproc)
riscv64-linux-gnu-strip bbl

Output

The desired bootloader is located at riscv-disk/riscv-pk/build/bbl.