Moez Bouhlel @lejenome

Update: A new version of the articale with better solution is available here.

Even Linux is my main system, I frequently need to use Windows for school homework when Mono does not support desired features. Running Windows inside a virtual machine is always enough expect few cases. The best solution was to setup a single windows system accessible from both the virtual machine and from the physical dual boot.

For the virtual machine, we will use Qemu/KVM with Virtio devices and a disk partition for Windows.

First, you need to install Windows on your PC to get the boot entry with right configurations (I couldn't find a work around BCD boot entry issue other than this one). Then reboot to your Linux system to reinstall Windows on the same partition but using the virtual machine this time.

Before we start, check that loop and linear modules are loaded:

sudo modprobe loop
sudo modprobe linear

Because the virtual machine need whole disk with both the Windows partition and the EFI partition, We need to create a virtual RAID. Let's create a small file to hold the EFI partition:

dd if=/dev/zero of=efi count=200000

Now you have efi file of 100 MB (200000 * 512 Bytes). Next, we create a loopback device from the file:

sudo losetup -f efi

It will assign the first available loopback device to the file. We will assume that the assigned device is /dev/loop0. To check the assigned device:

losetup -a

Next, we will merge the loopback device and the real Windows partition into a single linear RAID disk image (We will assume that the windows partition is /dev/sda2):

sudo mdadm --build --verbose /dev/md0 --chunk=16 --level=linear --raid-devices=2 /dev/loop0 /dev/sda2

Time to create the partitions table of the new RAID disk with reusing the same physical Windows partition. For this step, we will use _parted_ utility. You can use other tools on your own. We need to get the size of real Windows partition on sectors.

sudo parted /dev/sda unit s print

Partition your virtual RAID disk:

sudo parted /dev/md0
(parted) unit s
(parted) mktable gpt
(parted) mkpart primary ntfs -WINDOW_PARITION_SIZE -1
(parted) mkpart primary fat32 0 -WINDOW_PARITION_SIZE
(parted) quit

Your final layout will have 2 partitions; Windows partition /dev/md0p1 and EFI partition /dev/md0p2. You may get few warning messages when creating partitions, ignore them. The new partitions need to be formatted.

sudo mkfs.ntfs -f -L Windows -C /dev/md0p1
sudo mkfs.msdos -F 32 -n EFI /dev/md0p2

Now, you are ready to launch the virtual machine and reinstall Windows. Change /dev/md0 owner to the same user _Qemu_ is running as and install _ovmf_ EFI bios for _Qemu_, in my case, it will be available at /usr/share/ovmf/ovmf_x64.bin.

qemu-system-x86_64 \
    -enable-kvm \
    -bios /usr/share/ovmf/ovmf_x64.bin \
    -drive file=/dev/md0,media=disk,format=raw \
    -netdev user,id=windowsnic,hostname=windowshost \
    -device virtio-net,netdev=windowsnic \
    -cpu host \
    -m 2G \
    -vga qxl \
    -usbdevice tablet

Adapt _Qemu_ script to your use case. You may need to download and install Virtio drivers on your guest machine. After each reboot, you need to create the loopback device, merge the two partitions into the RAID disk and change the owner of the device.

Credit to Arch Linux Wiki. Enjoy!