tree: 4336dae5b7b427dd0e8d2e858803e202609601bb [path history] [tgz]

title: RISC-V full system with no disk tags: - riscv - fullsystem - nodisk layout: default permalink: resources/riscv-fs-nodisk shortdoc: > Resources to build a riscv bootloader containing a linux kernel and a workload expected to run at early userspace. author: [“Hoa Nguyen”]

This resource provides the possibility of conducting a RISC-V full system simulation without a block device by leveraging [Linux's userspace support] (


This document provides instructions to create a RISCV bootloader (berkeley bootloader (bbl)) and also points to the associated gem5 scripts to run riscv Linux full system simulations without using a disk image. The bootloader bbl is compiled with a Linux kernel, a device tree, and a workload. Similar to the riscv-fs resource, we'll also rely on BusyBox for basic Linux utilities, on UCanLinux for the configuration of the Linux kernel and the configuration of BusyBox, and on riscv-pk for building a proxy kernel.

  |___ gem5/                                   # gem5 source code (to be cloned here)
  |___ riscv-gnu-toolchain/                    # riscv tool chain for cross compilation
  |___ riscv64-sample/                         # UCanLinux source
  |___ linux/                                  # linux source
  |___ busybox/                                # busybox source
  |___ riscv-pk/                               # riscv proxy kernel source (bbl)
  |___ cpio/                                   # contains the .cpio files
  |___ initdir/                                # contains the structure of initramfs
  |___ configs/
  |      |___ system                           # gem5 system config files
  |      |___                     # gem5 run script
  |___                               # This README file

How does it work?

When Linux kernel booting process takes place, initramfs, a root filesystem embedded into the kernel, will be loaded to memory. When initramfs is loaded, the kernel will try to execute one of the following scripts located in that filesystem, {/init, /sbin/init, /etc/init, /bin/init, /bin/sh}. Instead of using the default /init script, we will use our version of /init to execute the desired workload right after the early userspace is loaded.

Note: Since the initramfs decompressing process takes place while Linux kernel is booting (which means it will happen during the full system simulation), we'll try to minimize the size of the initramfs.

Building the resource

Step 1. Building the riscv-gnu-toolchain

In this step, we'll use GNU toolchain for RISC-V.

This step is necessary if you do not have basic libraries built for RISCV or if you're cross-compiling RISCV.

cd riscv-fs-nodisk/
git clone --recursive
cd riscv-gnu-toolchain
git checkout 1a36b5dc44d71ab6a583db5f4f0062c2a4ad963b
# --prefix parameter specifying the installation location
./configure --prefix=/opt/riscv
make linux -j $(nproc)

To update the PATH environment variable so that the RISCV compilers can be found,

export PATH=$PATH:/opt/riscv/bin/

Step 2. Getting the UCanLinux Source

This repo contains a Linux configuration for RISCV at riscv64-sample/kernel.config and a BusyBox configuration at riscv64-sample/busybox.config.

# going back to base riscv-fs directory
cd riscv-fs-nodisk/
git clone

Step 3. Getting and Building busybox

More information about Busybox is here.

cd riscv-fs-nodisk/
git clone git://
cd busybox
git checkout 1_34_stable  # checkout the a stable branch
cp ../riscv64-sample/busybox.config .config
yes "" | make CROSS_COMPILE=riscv64-unknown-linux-gnu- oldconfig
make CROSS_COMPILE=riscv64-unknown-linux-gnu- all -j$(nproc)
make CROSS_COMPILE=riscv64-unknown-linux-gnu- install

The files of interest are in busybox/_install/bin.

Step 4. Getting and Compiling the Linux kernel

We'll compiling the Linux kernel to get the linux/usr/gen_init_cpio, which would be used later.

cd riscv-fs-nodisk/
git clone --depth 1 --branch v5.10
cd linux
cp ../riscv64-sample/kernel.config .config
yes "" | make ARCH=riscv CROSS_COMPILE=riscv64-unknown-linux-gnu- oldconfig
make ARCH=riscv CROSS_COMPILE=riscv64-unknown-linux-gnu- menuconfig
# Go to "General setup --->"
#   Check on "Initial RAM filesystem and RAM disk (initramfs/initrd) support"
make ARCH=riscv CROSS_COMPILE=riscv64-unknown-linux-gnu- all -j $(nproc)

Step 5. Compiling the Workload (e.g. gem5's m5)

cd riscv-fs-nodisk/
git clone
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

Step 6. Determining the Structure of initramfs

cd riscv-fs-nodisk/
mkdir cpio
mkdir misc
mkdir initdir


We'll use the riscv64-sample/initdir to define the structure of initramfs.

cd riscv-fs-nodisk/initdir
cp -r ../busybox/_install/bin/ .
mkdir lib
cp /opt/riscv/sysroot/lib/ lib/ # busybox' dependency
cp /opt/riscv/sysroot/lib/ lib/ # busybox' dependency
cp /opt/riscv/sysroot/lib/ lib/ # busybox' dependency
cp /opt/riscv/sysroot/lib/ lib/ # busybox' dependency
mkdir proc
mkdir sys
mkdir sbin
cp ../gem5/util/m5/build/riscv/out/m5 sbin/m5 # replace m5 by the desired workload

Create initdir/init script with the following content,

#!/bin/busybox sh

exec /sbin/init # script to execute the workload

Create initdir/sbin/init script with the following content,

#!/bin/busybox sh

/sbin/m5 exit

Make the scripts executable,

chmod +x init
chmod +x sbin/init

To create the cpio file of the initdir folder,

cd riscv-fs-nodisk/linux
usr/ -o ../cpio/disk.cpio ../initdir/
lsinitramfs ../cpio/disk.cpio # checking the file structure of the created cpio file

/dev/ folder

By default, initramfs would have a /dev/console and /dev/tty. Without these devices, we cannot see what is written to stdout and stderr.

The following commands will build a .cpio file with /dev/console and /dev/tty,

cd riscv-fs-nodisk/misc
mkdir dev
fakeroot -- mknod -m 622 dev/console c 5 1
fakeroot -- mknod -m 622 dev/tty c 5 0
fakeroot -- mknod -m 622 dev/ttyprintk c 5 3
fakeroot -- mknod -m 622 dev/null c 1 3
fakeroot -- find . -print0 | cpio --owner root:root --null -o --format=newc > ../cpio/dev.cpio
cd ../
rm -r misc

Note: mknod -m 622 /dev/tty c 5 0 means we're creating /dev/tty with permission of 622. c means a character device being created, 5 is the major number, and 0 is the minor number. More information about the major/minor numbering is available at (

Merging .cpio files to a single .cpio file

cd riscv-fs-nodisk/cpio
cat disk.cpio dev.cpio > init.cpio

Step 7. Compiling Linux Kernel with a customized initramfs

cd riscv-fs-nodisk/linux
make ARCH=riscv CROSS_COMPILE=riscv64-unknown-linux-gnu- menuconfig
# Go to "General setup --->"
#   Check on "Initial RAM filesystem and RAM disk (initramfs/initrd) support"
#   Change "Initramfs source file(s)" to the absoblute path of riscv-fs-nodisk/cpio/init.cpio
make ARCH=riscv CROSS_COMPILE=riscv64-unknown-linux-gnu- all -j $(nproc)

The file of interest is at arch/riscv/boot/Image.

Step 8. Compiling bbl with the Linux kernel as the payload

cd riscv-fs-nodisk/
git clone
cd riscv-pk
mkdir build
cd build

# configure bbl build
../configure --host=riscv64-unknown-linux-gnu --with-payload=../../linux/arch/riscv/boot/Image --prefix=/opt/riscv/

make -j$(nproc)

chmod 755 bbl

riscv64-unknown-linux-gnu-strip bbl
cp bbl bbl-m5-exit

The desired bootloader is file is at riscv-fs-nodisk/riscv-pk/build/bbl or riscv-fs-nodisk/riscv-pk/build/bbl-m5-exit.


This resource is used in gem5/SST integration. For instructions to run the integeration, please refer to ext/sst/ of the gem5 repo.