Running Raw Linux Kernel in QEMU

In last post we see how to run a packed Linux distribution in QEMU. This time let’s check out how to run a raw Linux kernel in QEMU.

Initial ramdisk: a very short introduction

Many Linux distributions ship a small, generic kernel image. The device drivers are included as loadable kernel modules and stored in file system, it is just not practical to bake all the device drivers into the kernel image. On my machine, the size of vmlinuz-linux is only 6.3 megabytes:

$ ls -lh /boot/vmlinuz-linux
-rw-r--r-- 1 root root 6.3M Mar 15 21:30 /boot/vmlinuz-linux

This raises the problem of detecting and loading the modules necessary to mount the root file system at boot time. It’s a chicken-and-egg problem. To further complicate the situation, the root file system may require special preparations to mount (for instance, it is on a encrypted partition).

Now comes the initial ramdisk as a temporary, ram-based root file system. It contains user-space utilities which detect hardwares, descover devices, load necessary modules, and mount the real root file system. Once it is loaded into memory, a simple but sufficient environment is set up for the Linux kernel to complete the boot process. This environment is often called “early user space”.

On my machine, the initial ramdisk image sits in the boot partition with the name initramfs-linux.img. It is larger than the Linux kernel image:

$ ls -lh /boot/initramfs-linux.img
-rw-r--r-- 1 root root 9.4M Mar 15 21:30 /boot/initramfs-linux.img

We can inspect contents of the image by using command lsinitcpio /boot/initramfs-linux.img. Indeed, it is a simplified root system with a bunch of helper tools:

$ lsinitcpio /boot/initramfs-linux.img

Making an initial ramdisk

The initial ramdisk creation command in Arch Linux is mkinitcpio, and may defer in other distributions.

Quoting from man page of mkinitcpio:

(mkinitcpio) creates an initial ramdisk environment for booting the Linux kernel. The initial ramdisk is in essence a very small environment (early userspace) which loads various kernel modules and sets up necessary things before handing over control to init. This makes it possible to have, for example, encrypted root filesystems and root filesystems on a software RAID array. mkinitcpio allows for easy extension with custom hooks, has autodetection at runtime, and many other features.

Let’s go ahead and create the initial ramdisk.

$ mkinitcpio -g initramfs.img
==> Starting build: 5.5.9-arch1-2
  -> Running build hook: [base]
  -> Running build hook: [udev]
  -> Running build hook: [autodetect]
  -> Running build hook: [modconf]
  -> Running build hook: [block]
  -> Running build hook: [filesystems]
  -> Running build hook: [keyboard]
  -> Running build hook: [fsck]
==> Generating module dependencies
==> Creating gzip-compressed initcpio image: /home/yuankun/qemu-test/initramfs.img
==> Image generation successful

Configuring and building the Linux kernel

Clone the Linux source code, and configure the Linux kernel with make ARCH=x86_64 menuconfig.


Save the configurations and exit the configuration interface, now let’s compile the kernel image. The compiled kernel image will be located at arch/x86/boot/bzImage.

$ make -j8
Setup is 13820 bytes (padded to 13824 bytes).
System is 8801 kB
CRC 52c54fbc
Kernel: arch/x86/boot/bzImage is ready  (#2)

The -j option specifies the number of jobs (commands) to run simultaneously. It’s a reasonable choice to match it with your available logical CPU cores.

Booting the Linux kernel in QEMU

Now that we have both the Kernel image and the initial ramdisk, it’s time to boot the Linux kernel in QEMU.

$ qemu-system-x86_64 \
    -enable-kvm \
    -kernel bzImage \
    -smp cores=1,threads=2 \
    -m 1024 \
    -append "console=ttyS0" \
    -initrd initramfs.img \
    -serial stdio \
    -display none
[    0.000000] Linux version 5.6.0-rc6+ ([email protected]) (gcc version 9.3.0 (Arch Linux 9.3.0-1)) #2 SMP Tue Mar 17 17:42:13 +08 2020
[    0.000000] Command line: console=ttyS0
[    0.000000] x86/fpu: x87 FPU will use FXSAVE
[    0.000000] BIOS-provided physical RAM map:
[    0.000000] BIOS-e820: [mem 0x0000000000000000-0x000000000009fbff] usable
[    0.000000] BIOS-e820: [mem 0x000000000009fc00-0x000000000009ffff] reserved
[    0.000000] BIOS-e820: [mem 0x00000000000f0000-0x00000000000fffff] reserved
[    0.000000] BIOS-e820: [mem 0x0000000000100000-0x000000007ffdffff] usable
[    0.000000] BIOS-e820: [mem 0x000000007ffe0000-0x000000007fffffff] reserved
[    0.000000] BIOS-e820: [mem 0x00000000feffc000-0x00000000feffffff] reserved
[    0.000000] BIOS-e820: [mem 0x00000000fffc0000-0x00000000ffffffff] reserved
[    0.000000] NX (Execute Disable) protection: active
[    0.000000] SMBIOS 2.8 present.
[rootfs ]#

Soon you will find the [rootfs] # prompt appearing, and cheers we are now in the environment provided by the initial ramdisk.