Some Things Virtiofs and Windows

This post hopes to explain how to enable a shared folder with a Windows guest on Libvirt/QEMU using Virtiofs.

An official Windows Guide is provided that generally covers everything needed. This post has snippets that help automate the process.

Virtiofs

A newer approach to shared file systems between virtual machines and hosts. From the main site:

Virtiofs is a shared file system that lets virtual machines access a directory tree on the host. Unlike existing approaches, it is designed to offer local file system semantics and performance.

Virtiofs was started at Red Hat and is being developed in the Linux, QEMU, FUSE, and Kata Containers open source communities.

I liked the idea of a dedicated solution instead of network based ones like NFS or Samba that have historically been repurposed for such things. Plus the boast of better performance.

There are minimum requirements for a host computer to use Virtiofs:

How To Setup Virtiofs on Windows

We’ll install some tools to the Windows guest and then configure the VM to use the shared filesystem.

WinFsp

As explained in Windows Guide, WinFsp is needed inside the guest.

Download and install the application.

PowerShell

PS C:\> Invoke-WebRequest https://github.com/winfsp/winfsp/releases/download/v1.10/winfsp-1.10.22006.msi -OutFile winfsp-1.10.22006.msi
PS C:\> winfsp-1.10.22006.msi /qn /norestart

Ansible

- name: Check services
  win_service:
    name: WinFsp.Launcher
  register: winfspservice_result

- name: Install winfsp
  block:
    - name: Download winfsp
      win_get_url:
        url: "https://github.com/winfsp/winfsp/releases/download/v1.10/winfsp-1.10.22006.msi"
        dest: "{{ ansible_env.TEMP }}\\winfsp-1.10.22006.msi"

    - name: Install winfsp
      win_package:
        path: "{{ ansible_env.TEMP }}\\winfsp-1.10.22006.msi"
        state: present

  when: winfspservice_result.state is not defined or winfspservice_result.name is not defined

Viofs Driver and Service

The virtual filesystem is attached to the guest as device so a proper driver is required.

VirtIO drivers for Windows deserve their own post. I’ll quickly point out the ISO containing everything can be downloaded from the Github page and that it needs to be mounted somewhere so you can grab the viofs drivers within.

Assuming it’s been mounted on the D: drive in the Windows VM, install the driver:

PS C:\> pnputil /install /add-driver D:\viofs\w10\amd64\viofs.inf

Let’s also install the virtiofs.exe service.

PowerShell

PS C:\> cp D:\viofs\w10\amd64\virtiofs.exe C:\Windows\virtiofs.exe

PS C:\> New-Service -Name "VirtioFsSvc" -BinaryPathName "C:\Windows\virtiofs.exe" -DisplayName "Virtio FS Service" -Description "Enables Windows virtual machines to access directories on the host that have been shared with them using virtiofs." -StartupType Automatic -DependsOn "WinFsp.Launcher"

Ansible

- name: Create virtiofs service
  win_service:
    name: VirtioFsSvc
    path: C:\Windows\virtiofs.exe
    description: Enables Windows virtual machines to access directories on the host that have been shared with them using virtiofs.
    display_name: Virtio FS Service
    start_mode: auto
    dependencies:
      - WinFsp.Launcher

Define Synced Folder

As of this writing, it seems you can only share 1 folder at a time. If you define multiple, it appears to use the one is defined last.

Manual method:

• In virt-manager, in the details section for the Windows guest, click Add Hardware.
• Choose Filesystem and supply a Source path and Target path.

The XML snippet should look something like this:

<filesystem type="mount" accessmode="passthrough">
  <driver type="virtiofs"/>
  <binary path="/usr/libexec/virtiofsd"/>
  <source dir="/tmp"/>
  <target dir="tmp"/>
  <address type="pci" domain="0x0000" bus="0x07" slot="0x00" function="0x0"/>
</filesystem>

The virt-manager GUI interface may not offer the ability to set the passthrough as an access mode, or to set driver and binary information for virtiofs. The XML can be manually edited with these settings.

Vagrant (automated) way:

• You’ll need the vagrant-libvirt plugin.

• The virtiofs blurb in the synced folders section of the documentation should work.

  config.vm.provider :libvirt do |l, override|
    l.driver = "kvm"
    ...
  
    l.memorybacking :access, :mode => "shared"
    override.vm.synced_folder ".", "/vagrant", disabled: false, type: "virtiofs"
  
    ...
  end
  

If everything is working, you’ll find a new mapped drive for your shared folder.