qemu doesn't recognize block device file

Question

I have a working QEMU image emulating an ARM vexpress-a9 and I run it like so:

sudo qemu-system-arm -m 512M -M vexpress-a9 -D qemu.log -d unimp -kernel buildroot-2019.02.5/output/images/zImage -dtb buildroot-2019.02.5/output/images/vexpress-v2p-ca9.dtb -append "console=ttyAMA0,115200 kgdboc=kbd,ttyAMA0,115200 ip=dhcp nokaslr" -initrd buildroot-2019.02.5/output/images/rootfs.cpio -nographic -net nic -net bridge,br=mybridge -s

I would now like to add a hard disk for persistent storage and then transfer control from busybox initrd based rootfs over to the full fledged version offered with Linux. So I add it to the command line

sudo qemu-system-arm -m 1024M -M vexpress-a9 -D qemu.log -drive if=none,format=raw,file=disk.img -kernel buildroot-2019.02.5/output/images/zImage -dtb buildroot-2019.02.5/output/images/vexpress-v2p-ca9.dtb -append "console=ttyAMA0,115200 kgdboc=kbd,ttyAMA0,115200 ip=dhcp nokaslr" -initrd buildroot-2019.02.5/output/images/rootfs.cpio -nographic -net nic -net bridge,br=mybridge -s

of course I first create a disk image and format it as ext2: qemu-img create disk.img 10G && mkfs.ext2 -F disk.img

From the log messages I see that it has not been able to detect this at all. I think I need to understand how block devices work with Qemu. I know the older -hda has been changed to a newer -drive option can combines the cumbersome specification of the front and back ends separately. But I don't know the basics and why I am getting this problem.

I am basically looking to switch_root from initrd to the full fledged Linux rootfs but this is only the first step.

Answer 1

From the log messages I see that it has not been able to detect this at all.

That's because you haven't created QEMU device connected to that drive.

I think I need to understand how block devices work with Qemu.

You have front-ends that represent some kind of hardware to the guest and you have back-ends that interact with backing storage on the host. You create a front-end with -device option and block back-end with -drive option. You give the drive an id and refer to that id from the device. E.g. this is how I attach virtio-blk-pci device to a disk image on my virt machine: -device virtio-blk-pci,drive=vd0 -drive file=rootfs.ext2,format=raw,id=vd0.

qemu-system-arm -device help will give you the list of supported device types and qemu-system-arm -device <specific-device-type>,help will show detailed help for specific-device-type properties.