Moez Bouhlel @lejenome

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Recently, I needed to set up a new Windows system for use in an Internship project. Like the old setup which was described in an older post, I needed to have access to the same Windows from inside Qemu and from dual-boot for performance reasons. But instead of requiring to install Windows two times (once directly on PC as host and the other inside Qemu) which caused some issues including slow filesystem operations when booting as the host system, I did an update to setup instructions this time.

For the virtual machine, we will use Qemu/KVM with Ovmf EFI bios.

First, Install Windows directly on your PC then reboot back to Linux. Check that loop and linear modules are loaded:

sudo modprobe loop
sudo modprobe linear

Now, we need to create a virtual RAID disk for Qemu that will hold our physical Windows partition. As we will use GPT partitions table schema, we will allocate few megabytes efi1 for GPT metadata (34 Sectors) and for EFI partition before Windows partition and one megabyte efi2 after Windows partition for GPT secondary metadata (even we need just 34 Sectors, we will use one megabyte for best RAID performance)

dd if=/dev/zero of=efi1 bs=1M count=100
dd if=/dev/zero of=efi2 bs=1M count=1

Now that we have efi1 file of 100 MB (100 * 1M) and efi2 of 1 MB. Next, we create loopback devices from both files:

sudo losetup -f efi1
sudo losetup -f efi2

We will assume that the assigned devices are /dev/loop0 and /dev/loop1 for efi1 and efi2 respectively. To check the assigned devices, type:

losetup -a

Next, we will merge the loopback devices 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=512 --level=linear --raid-devices=3 /dev/loop0 /dev/sda2 /dev/loop1

Time to create the partitions table of the new RAID disk reusing the same physical Windows partition. For this step, we will use parted utility. You can use another tool of your choice.

Partition your virtual RAID disk:

sudo parted /dev/md0
(parted) unit s
(parted) mktable gpt
(parted) mkpart primary fat32 2048 204799    # depends on size of efi1 file
(parted) mkpart primary ntfs 204800 -2049    # depends on size of efi1 and efi2 files
(parted) set 1 boot on
(parted) set 1 esp on
(parted) set 2 msftdata on
(parted) name 1 EFI
(parted) name 2 Windows
(parted) quit

Your final layout will have 2 partitions; Windows partition /dev/md0p2 and EFI partition /dev/md0p1. The EFI partition needs to be formatted.

sudo mkfs.msdos -F 32 -n EFI /dev/md0p1

Now let add Windows boot entry to the virtual RAID disk.

Boot to Windows live DVD from inside Qemu virtual machine with /dev/md0 as disk after changing /dev/md0 owner to the current user and installing Ovmf EFI bios, which in my case, it is available at /usr/share/ovmf/ovmf_x64.bin.

chown $USER:$USER /dev/md0  # We need to change the owner of md0
qemu-system-x86_64 \
    -bios /usr/share/ovmf/ovmf_x64.bin \
    -drive file=/dev/md0,media=disk,format=raw \
    -cpu host -enable-kvm -m 2G \
    -cdrom /path/to/windows.iso

Press Shift+F10 when Windows installer starts to open the terminal. We need to assign a letter to EFI volume (partition).

diskpart
DISKPART> list disk
DISKPART> select disk 0    # Select the disk
DISKPART> list volume      # Find EFI volume (partition) number
DISKPART> select volume 2  # Select EFI volume
DISKPART> assign letter=B  # Assign B: to EFI volume
DISKPART> exit

Finally, we create BCD boot entry for Windows partition on the same terminal.

bcdboot C:\Windows /s B: /f ALL

Now, you are ready to boot to Windows from inside Qemu.

qemu-system-x86_64 \
    -bios /usr/share/ovmf/ovmf_x64.bin \
    -drive file=/dev/md0,media=disk,format=raw \
    -cpu host -enable-kvm -m 2G

After each Linux system reboot, you need to create the loopback devices, merge the partitions into the RAID disk and change the owner of the device before launching the virtual machine. You can find the script I use as a reference.

Credit to Arch Linux Wiki. Enjoy!