virtio-win

Overview

Virtiofs is a shared file system that lets virtual machines access a directory tree on the host. More information on the underlying approach is available at virtio-fs.gitlab.io. Virtiofs for Windows is a user mode file system, implemented using WinFsp framework. Virtiofs consists of VirtIO-powered driver and user-space service based on WinFsp.

Status

Virtiofs is at an early stage of development and should be considered as a “Tech Preview” feature. To see what may not work, please check known limitations.

Setup

Host

This section shows how to setup virtiofs device on the host side. Two options are described: libvirt and QEMU.

libvirt

Following XML should be added to your libvirt VM descrition:

<domain>
  ...
  <memoryBacking>
    <source type='memfd'/>
    <access mode='shared'/>
  </memoryBacking>
  ...
  <devices>
    <filesystem type="mount" accessmode="passthrough">
      <driver type="virtiofs" queue="1024"/>
      <source dir="/home/user/viofs"/>
      <target dir="mount_tag"/>
      <address type="pci" domain="0x0000" bus="0x06" slot="0x00" function="0x0"/>
    </filesystem>
  </devices>
  ...
</domain>

The <memoryBacking> is necessary. Element <source dir="/home/user/viofs"/> describes host directory to share.

More information on libvirt virtiofs options is provided in libvirt docs.

QEMU

Run virtiofsd daemon:

/usr/libexec/virtiofsd --socket-path=/tmp/virtiofs_socket -o source=/home/user/viofs

Adjust following QEMU command-line parameters:

Instantiate the character device for socket communication between QEMU and virtiofsd: -chardev socket,id=char0,path=/tmp/virtiofs_socket
Instantiate the virtiofs PCI device: -device vhost-user-fs-pci,queue-size=1024,chardev=char0,tag=my_virtiofs
Force use of memory sharing with virtiofsd (replace 4G with your desired VM RAM size): -m 4G -object memory-backend-file,id=mem,size=4G,mem-path=/dev/shm,share=on -numa node,memdev=mem

virtiofsd

Modern virtiofsd is written in Rust and supported here: https://gitlab.com/virtio-fs/virtiofsd

To build virtiofsd:

cargo build

Guest

Setup with installer

Download and install WinFSP with at least “Core” feature enabled.
Install virtiofs driver and service from VirtIO-Win package.

Manual setup (for development purposes)

Download and install WinFSP with at least “Core” feature enabled. If you plan to make changes to virtiofs driver or service then enable “Core”, “Developer” and “Kernel Developer” features in the installer.
Install virtiofs driver with Device Manager or pnputil.exe.
Setup virtiofs service by running sc create VirtioFsSvc binPath="<path to the binary>\virtiofs.exe" start=auto depend=VirtioFsDrv. Don’t forget to appropriately set binPath.
You can immediately start the service by running sc start VirtioFsSvc. The service will start automatically on boot. Virtiofs uses the first available drive letter starting with Z: unless no mount-point is specified.

Options

Virtiofs service can parse following settings from command-line:

  -d DebugFlags       [-1: enable all debug logs]
  -D DebugLogFile     [file path; use - for stderr]
  -i                  [case insensitive file system]
  -F FileSystemName   [file system name for OS]
  -m MountPoint       [X:|* (required if no UNC prefix)]
  -t Tag              [mount tag; max 36 symbols]
  -o UID:GID          [host owner UID:GID]

Since command-line arguments can’t be assigned to Windows service permanently, virtiofs can parse them from the registry. When command-line arguments are absent the service looks up for the following parameters under HKLM\Software\virtiofs:

DebugFlags (DWORD)
DebugLogFile (String)
MountPoint (String)
CaseInsensitive (DWORD)
Owner (String)
FileSystemName (String)

For example, registry values depicted below correspond to virtiofs.exe -d -1 -D C:\viofs_debug.log -m X:. Please note that -1 corresponds to 0xffffffff in DWORD value.

Also, parameters named OverflowUid and OverflowGid are always parsed from the registry. Normally, these parameters only affect if the host daemon is running inside a Linux user namespace. They denote UID and GID perceived as nobody on the host, so they should be in sync with corresponding host values. If the service finds out shared folder root owner UID/GID becomes nobody, it will try previous UID/GID as owner for new files and folders. They assumed to be 65534 if no such parameters are found in the registry.

Case insensitivity

At the moment, the case insensitivity feature is under development and may not work properly in some cases. Case insensitivity can be turned on by -i command-line parameter of CaseInsensitive registry key. In this situation, when processing a request, virtiofs first tries to access the file or directory by its exact name, as in case-sensitive mode. Second, if there is no such node, virtiofs finds a node with a matching name in the parent directory, ignoring case.

File system name

When running an executable as administrator, the Windows OS seems to require that the name of the file system that is housing the executable is “NTFS”. This can be done by -F command-line parameter or FileSystemName registry key.

Multiple virtiofs instances

Setup

Support for multiple virtiofs instances is made by WinFSP.Launcher service, so virtiofs own service should not be running:

sc stop VirtioFsSvc
sc config VirtioFsSvc start=demand

The virtiofs service is now stopped and will not start even after reboot.

Virtiofs configuration for WinFsp.Launcher:

"C:\Program Files (x86)\WinFsp\bin\fsreg.bat" virtiofs "<path to the binary>\virtiofs.exe" "-t %1 -m %2"

Corresponding data is now available to view and edit under HKLM\SOFTWARE\WOW6432Node\WinFsp\Services\virtiofs registry key.

Mount

Mount virtiofs with tag mount_tag0 to Y:\:

"C:\Program Files (x86)\WinFsp\bin\launchctl-x64.exe" start virtiofs viofsY mount_tag0 Y:

Mount virtiofs with tag mount_tag1 to Z:\:

"C:\Program Files (x86)\WinFsp\bin\launchctl-x64.exe" start virtiofs viofsZ mount_tag1 Z:

Here viofsY and viofsZ are instance names for WinFsp.Launcher. They are selected arbitrary, but must differ between instances.

Unmount

Unmount is done by the instance name:

"C:\Program Files (x86)\WinFsp\bin\launchctl-x64.exe" stop virtiofs viofsY
"C:\Program Files (x86)\WinFsp\bin\launchctl-x64.exe" stop virtiofs viofsZ

Known limitations

Case insensitivity may not work properly

Testing

WinFsp Tests

The following command line is used to execute WinFsp’s test suite:

winfsp-tests-x64.exe --external --resilient -create_fileattr_test -create_readonlydir_test -create_allocation_test -create_notraverse_test -getfileattr_test -getfileinfo_test -getfileinfo_name_test -setfileinfo_test -setsecurity_test -reparse_guid_test -reparse_nfs_test -stream_*

Virtiofs for Windows does not currently pass all available tests so some of them are skipped for now:

Test Name	Failure Description
create_fileattr_test	Can’t set FILE_ATTRIBUTE_SYSTEM.
create_readonlydir_test	Can’t create a file on a read-only directory.
create_allocation_test	Can’t set a file allocation to zero.
create_notraverse_test
getfileattr_test	Not all Windows’ file attributes exists.
getfileinfo_test	File name length is longer than expected. File system is mounted as network drive?
getfileinfo_name_test
setfileinfo_test	Can’t remove FILE_ATTRIBUTE_ARCHIVE as it is hard-coded.
setsecurity_test	Not all Window’s security descriptors are implemented.
reparse_guid_test
reparse_nfs_test
stream_*	NTFS-like file streams are not supported. Need to decide where and how the information is stored.