QEMU

There are two modes in QEMU:

• user mode: execute simple application code in different architecutre. qemu-xxx
• system mode: need a kernel img file including the whole os system. qemu-system-xxx

User mode

Compile a 32-bits i386 program：gcc -m32 hello.c -o hello

run in qemu by user mode: qemu-i386 ./hello

System mode

In system mode we need to provide kernel, rootfs and other arguments to qemu.

qemu-system-x86_64 -kernel xxx -initrd xxx -append xxx

Compile Kernel

After download linux source code, execute make under root directory. When compile finished, vmlinux is generated which is the raw kernel including debug info, and arch/x86/boot/bzImage which is the compressed kernel。

Make RootFS

There are two ways, for convenience, only a simple program is written to initrd and is used as the init program. Alternatively, you can use busybox as the init program in initrd.

prepare a simple c program init.c

#include <stdio.h>

int main()
{
	printf("hello world\n");
	while(1);
	return 0;
}

Then compile the code with -static linking.
gcc -staic -o init init.c

Then use the cpio program to package.
echo init | cpio -o --format=newc > initrd_rootfs.img

Thus, an initrd-based rootfs is created.

Busybox

Debug the kernel in QEMU

When the kernel and rootfs are ready, we can start the QEMU with

qemu-system-x86_64 \
  -kernel ./linux/arch/x86/boot/bzImage \
  -initrd initrd_rootfs.img \
  -append "root=/dev/ram rdinit=/init"