On Fri, Dec 13, 2024 at 01:34:41PM -0600, Elizabeth Figura wrote:

This patch series implements a new char misc driver, /dev/ntsync, which is used
to implement Windows NT synchronization primitives.

NT synchronization primitives are unique in that the wait functions both are
vectored, operate on multiple types of object with different behaviour (mutex,
semaphore, event), and affect the state of the objects they wait on. This model
is not compatible with existing kernel synchronization objects or interfaces,
and therefore the ntsync driver implements its own wait queues and locking.

This patch series is rebased against the "char-misc-next" branch of
gregkh/char-misc.git.

== Background ==

The Wine project emulates the Windows API in user space. One particular part of
that API, namely the NT synchronization primitives, have historically been
implemented via RPC to a dedicated "kernel" process. However, more recent
applications use these APIs more strenuously, and the overhead of RPC has become
a bottleneck.

The NT synchronization APIs are too complex to implement on top of existing
primitives without sacrificing correctness. Certain operations, such as
NtPulseEvent() or the "wait-for-all" mode of NtWaitForMultipleObjects(), require
direct control over the underlying wait queue, and implementing a wait queue
sufficiently robust for Wine in user space is not possible. This proposed
driver, therefore, implements the problematic interfaces directly in the Linux
kernel.

This driver was presented at Linux Plumbers Conference 2023. For those further
interested in the history of synchronization in Wine and past attempts to solve
this problem in user space, a recording of the presentation can be viewed here:

    https://www.youtube.com/watch?v=NjU4nyWyhU8


== Performance ==

The performance measurements described below are copied from earlier versions of
the patch set. While some of the code has changed, I do not currently anticipate
that it has changed drastically enough to affect those measurements.

The gain in performance varies wildly depending on the application in question
and the user's hardware. For some games NT synchronization is not a bottleneck
and no change can be observed, but for others frame rate improvements of 50 to
150 percent are not atypical. The following table lists frame rate measurements
from a variety of games on a variety of hardware, taken by users Dmitry
Skvortsov, FuzzyQuils, OnMars, and myself:

Game                            Upstream        ntsync          improvement
===========================================================================
Anger Foot                       69              99              43%
Call of Juarez                   99.8           224.1           125%
Dirt 3                          110.6           860.7           678%
Forza Horizon 5                 108             160              48%
Lara Croft: Temple of Osiris    141             326             131%
Metro 2033                      164.4           199.2            21%
Resident Evil 2                  26              77             196%
The Crew                         26              51              96%
Tiny Tina's Wonderlands         130             360             177%
Total War Saga: Troy            109             146              34%
===========================================================================


== Patches ==

The intended semantics of the patches are broadly intended to match those of the
corresponding Windows functions. For those not already familiar with the Windows
functions (or their undocumented behaviour), patch 27/28 provides a detailed
specification, and individual patches also include a brief description of the
API they are implementing.

The patches making use of this driver in Wine can be retrieved or browsed here:

    https://repo.or.cz/wine/zf.git/shortlog/refs/heads/ntsync7

Given a lack of complaints, I've now applied this to my testing tree.
Thanks for sticking with it!

greg k-h