Kernel Development with QEMU Virtual Machines (part 2)

What if you want to test a USB drive kernel module in a Virtualized environment?

In this post, I will show you how to pass-through a USB device to the Virtual Machine, which is fundamental when working with USB drivers, and I will give a little bonus about installing the Kernel modules into the VM using QEMU tools.

USB pass-through

There is a fast way and a better way to allow USB pass-through, and this “better” way still does not work for me when using the --kernel flag 😄 and it was blocking this blog post for weeks 😠. So I will tell you what it is, but consider that right now I made it to work only if not using a custom kernel.

The easy way is to use the -device switch (see this) to add a USB controller to the Virtual machine in a way that the Guest OS will pass the USB device as soon as it is connected.

There are countless of flags and filters, for example you can pass the entire USB bus and so all the devices connected, but I prefer to be specific and pass only the single device selected via it’s Vendor and Product ID, adding the following line to the QEMU argument list

-usb -device usb-host,vendorid=0x1bc7,productid=1066

but you also need to run QEMU with superuser privileges¹, so something like this

sudo qemu-system-x86_64 \
    -pidfile /tmp/qemu.pid \
    -enable-kvm \
    -drive file=$DISK \
    -m $RAM \
    -kernel $KERNEL \
    -append "root=/dev/sda3 console=ttyS0 rw" \
    -serial mon:stdio \
    -display none \
    -usb -device usb-host,vendorid=0x1bc7,productid=1066   

A better way is to use Spice

Spice is an open remote computing solution, providing client access to remote displays and devices (e.g. keyboard, mouse, audio). The main use case is to get remote access to virtual machines, although other use cases are possible and in various development stage.

Install the dependencies first

sudo apt install spice-gtk spice-vdagent

To make it work we need to use the QXL/SPICE display method (-vda qxl) and to enable the spice server in qemu-kvm (-spice port=5900,addr=127.0.0.1,disable-ticketing=on).

Finally we will instruct QEMU to emulate the USB via the XHCI controller (-device qemu-xhci,id=spicepass) and configure some channels for USB redirection. The following will create 1 channel

-chardev spicevmc,id=usbredirchardev1,name=usbredir \
    -device usb-redir,chardev=usbredirchardev1,id=usbredirdev1

as a bonus we can enable clipboard sharing between host and guest adding the following line

-device virtio-serial-pci \
-device virtserialport,chardev=spicechannel0,name=com.redhat.spice.0 \
-chardev spicevmc,id=spicechannel0,name=vdagent

Packing it all together it looks like this

qemu-system-x86_64 \
    -pidfile /tmp/qemu.pid \
    -enable-kvm \
    -m 4G -smp 4 -cpu host \
    -drive file=${DISK},if=virtio \
    -vga qxl -spice port=5900,addr=127.0.0.1,disable-ticketing=on \
    -device qemu-xhci,id=spicepass \
    -chardev spicevmc,id=usbredirchardev1,name=usbredir \
        -device usb-redir,chardev=usbredirchardev1,id=usbredirdev1 \
    -chardev spicevmc,id=usbredirchardev2,name=usbredir \
        -device usb-redir,chardev=usbredirchardev2,id=usbredirdev2 \
    -chardev spicevmc,id=usbredirchardev3,name=usbredir \
        -device usb-redir,chardev=usbredirchardev3,id=usbredirdev3 \
    -device virtio-serial-pci \
        -device virtserialport,chardev=spicechannel0,name=com.redhat.spice.0 \
        -chardev spicevmc,id=spicechannel0,name=vdagent

This will spawn the VM in a vnc session, so kick off a spice client (apt install spice-client-gtk) to connect to it

spicy -h 127.0.0.1 -p 5900

As said before, this will work, but there is no --kernel option passed, so we can only test the kernel already installed into the provided disk image, which can still be our kernel, but for this we should build the (in case of Debian/Ubuntu) the related debian packages, which takes much longer.

I will update the series when I will find the solution 😉

Bonus: QEMU-nbd to mount the QEMU image

In the previous post, I suggested guestmount to mount the VM image and install the Kernel modules, however the QEMU project already has a tool for such purpose: Qemu-nbd, which comes with the qemu-utils package.

Qemu-nbd exports a QEMU disk image as Network Block Device (from which NBD²), fulfilling my simple goal with the following two passages:

1. Load the kernel module and connect the NBD device to the image

   $ sudo modprobe nbd max_part=3
   $ sudo qemu-nbd --connect=/dev/nbd0 /path/to/image
   

   max_part is the option that specifies the maximum number of partitions we want nbd to manage and since the machine I installed in the previous post had the filesystem on sda3, max_part=3 is enough.

2. Mount the created /dev/nbd0pX (where X is the partition number) device.

   $ sudo mount /dev/nbd0p3 /mnt/tmp
   

Once the kernel modules are installed into the image, we can unmount the device.

$ sudo umount /mnt
$ sudo qemu-nbd --disconnect /dev/nbd0

Cool!