WinFsp on ARM64

This document summarizes the changes made to WinFsp to support Windows on ARM64. With these changes WinFsp runs natively in the ARM64 environment, while maintaining backwards compatibility.

Build System

  • Visual Studio 2019 or later is required to build WinFsp. Visual Studio 2015 builds are no longer possible because of lack of ARM64 support.

    • The script tools\gensrc\remove-build-arm64.bat can be used to remove the ARM64 configuration from the project files. This allows WinFsp to be built with Visual Studio 2015, but without ARM64 support. This is a hack to support AppVeyor builds on Server 2012 images that have only Visual Studio 2015 installed.

  • Build configuration is now done by the file build.common.props located in build\VStudio.

  • Product configuration (MyProductName, etc.) is done by the file build.version.props located in build\VStudio. This file was previously named

  • Project files under build\VStudio have been updated to support ARM64 builds.

Include Files

  • The macros FSP_FSCTL_PRODUCT_FULL_REGKEY and FSP_FSCTL_PRODUCT_FILE_ARCH have been added to header file inc\winfsp\fsctl.h:

    • FSP_FSCTL_PRODUCT_FULL_REGKEY: The full registry path for WinFsp.

    • FSP_FSCTL_PRODUCT_FILE_ARCH: The expected computer architecture suffix to use for file names (a64 for ARM64, x64 for x64, x86 for x86).

File System Driver

  • The IoInitializeTimer, IoStartTimer and IoStopTimer DDI’s are missing from Windows on ARM64 (link). The same functionality is now implemented within WinFsp in file src\sys\devtimer.c

  • The FSD (and other WinFsp components) used a number of memory barriers primarily for the purpose of lock-free access of certain variables. Most of these memory barriers have been removed and replaced with "interlocked" loads/stores so that any such accesses would continue to be correct under ARM64, which has weaker consistency guarantees than x86/x64. The new FspInterlocked* calls are implemented in inc/winfsp/fsctl.h and are used by both kernel and user mode code.


  • The sample project files have been updated to support ARM64 builds.


  • The installer has been retrofitted to support native installation on ARM64, x64 and x86 environments.

  • The installer remains a single MSI file that supports all architectures.

    • NOTE: The current technique for detecting OS architecture relies on reading the registry value PROCESSOR_ARCHITECTURE from key HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Environment. This may be fragile and a better technique may be required.

  • Files are installed in \Program Files (x86)\WinFsp on ARM64 and x64 Windows and \Program Files on x86 Windows.

  • Registry settings are placed in HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\WinFsp on ARM64 and x64 Windows and HKEY_LOCAL_MACHINE\SOFTWARE\WinFsp on x86 Windows.

  • Files for all architectures are always installed regardless of OS architecture. For example, all of winfsp-a64.dll, winfsp-x64.dll and winfsp-x86.dll are found in the bin subdirectory on ARM64, x64 and x86 Windows.

  • WinFsp supports x86 file systems on x86 Windows, x86 and x64 file systems on x64 Windows and x86, x64 and ARM64 file systems on ARM64 Windows.


  • The special exception for FLOSS to GPLv3 has been updated to mention that linking to winfsp-a64.dll is also allowed.