How to build the Linux kernel and test changes locally in qemu

HOWTO_LinuxKernelQemu.md

This is the process I followed on my Fedora 23 host machine to build a small/minimal vanilla Linux kernel and test in Qemu (based on this blog post). This will provide a safe sandbox in which to test kernel changes, and is generally faster than developing natively on the host machine. Qemu will boot the kernel image directly in the emulated system.

Install required build tools on host machine

sudo dnf install ncurses-devel kernel-devel kernel-headers gcc gcc-c++ git qemu openssl-devel glibc-static

Prepare a working space for kernel development

cd $HOME
mkdir kdev
TOP=$HOME/kdev

Download source code

Download stable busybox, which will provide a basic set of tools for the kernel-under-test:

wget http://busybox.net/downloads/busybox-1.24.2.tar.bz2
tar xvf busybox-1.24.2.tar.bz2
rm busybox-1.24.2.tar.bz2

Clone Linux kernel:

git clone https://github.com/torvalds/linux.git

Configure busybox

cd $TOP/busybox-1.24.2
mkdir -p $TOP/build/busybox-x86
make O=$TOP/build/busybox-x86 defconfig
make O=$TOP/build/busybox-x86 menuconfig

Enable building BusyBox as a static binary:

-> Busybox Settings
  -> Build Options
[*] Build BusyBox as a static binary (no shared libs)

Build and install busybox (local install, no sudo required)

cd $TOP/build/busybox-x86
make -j2
make install

Create minimal filesystem

mkdir -p $TOP/build/initramfs/busybox-x86
cd $TOP/build/initramfs/busybox-x86
mkdir -pv {bin,sbin,etc,proc,sys,usr/{bin,sbin}}
cp -av $TOP/build/busybox-x86/_install/* .

Create simple init script

Create a file called init with the following contents:

#!/bin/sh
 
mount -t proc none /proc
mount -t sysfs none /sys
 
echo -e "\nBoot took $(cut -d' ' -f1 /proc/uptime) seconds\n"
 
exec /bin/sh

Make it executable:

chmod +x init

Generate initramfs

find . -print0 | cpio --null -ov --format=newc | gzip -9 > $TOP/build/initramfs-busybox-x86.cpio.gz

Configure Linux using simple defconfig and kvm options

cd $TOP/linux
make O=$TOP/build/linux-x86-basic x86_64_defconfig
make O=$TOP/build/linux-x86-basic kvmconfig

Build Linux

make O=$TOP/build/linux-x86-basic -j2

Boot with Qemu in seconds!

cd $TOP
qemu-system-x86_64 \
  -kernel build/linux-x86-basic/arch/x86_64/boot/bzImage \
  -initrd build/initramfs-busybox-x86.cpio.gz \
  -nographic -append "console=ttyS0" \
  -enable-kvm

References

• Linux Insides: Linux Kernel Development
• Stefan Hajnoczi's Blog
• Mitchel Humpherys' Blog

after booting with qemu its showing error like:
1.521955] Kernel Offset: 0x33c00000 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffffbfffffff)
[ 1.524625] ---[ end Kernel panic - not syncing: No working init found. Try passing init= option to kernel. See Linux Documentation/admin-guide/init.rst for guidance. ]---

make kvmconfig is deprecated: use kvm_guest.config.

Thank you for this!