Running qemu with a custom kernel on ARM

So I was porting sysdig to ARM, and needed a target device to test the progress. Sysdig uses syscall tracepoints, which were added to Linux relatively recently, in version 3.7. Thus the ARM devices I had lying around were too old, and thus weren't suitable to test on (I could forward port the kernel patches that make them work, but this would be too much of a tangential effort).

The solution I settled on was emulation. Qemu can run in system mode to emulate a full machine. Specific instructions on how to run a vanilla-ish Debian system with a custom kernel were hard to find, so I'm documenting them here. Aurélien Jarno has disk images of a fresh Debian install and kernel images for Qemu emulation here: http://people.debian.org/~aurel32/qemu/armel/. This is the armel images, but he has the other arches as well.

Those images work fine. I just need to run a custom kernel I build myself. Aurélien provides the kernel image and an init ramdisk. It's not immediately clear how to build this initrd image (and my various attempts weren't fruitful). It was also not obvious how to run without initrd. The solution that worked for me in the end was a monolithic kernel (all necessary drivers compiled in) and a particular set of qemu options to workwithout a ramdisk.

I built a vanilla Linux 3.14 kernel zImage. The multiarch cross-toolchain isn't in Debian proper yet, but packages are available from http://people.debian.org/~wookey/tools/debian/.

The kernel config was based off the vanilla versatile_defconfig, with a few drivers and things built in. As usual, the kernel can be built with something like this:

cd linux
git reset --hard v3.14
cp /tmp/versatile.config .config
make ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- -j4 zImage

This produces an image in arch/arm/boot/zImage. Everything I care about is built-in, so I don't care about shipping modules, or building a ramdisk. A qemu command to use this:

qemu-system-arm -M versatilepb -kernel ~/linux/arch/arm/boot/zImage -hda debian_wheezy_armel_standard.qcow2 -append "noinitrd root=/dev/sda1 rw"

This appears to boot successfully, mounting everything, bringing up the network, etc. There is a benign warning about not being able to talk to the modules from the original kernel, but I obviously don't care. By default eth0 is at 10.0.2.15 with the host machine reachable at 10.0.2.2.