[PATCH v4 00/30] NT synchronization primitive driver

From: Elizabeth Figura
Date: Mon Apr 15 2024 - 21:15:36 EST

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.

Hence I would like to request review from someone familiar with locking to make
sure that the usage of low-level kernel primitives is correct and that the wait
queues work as intended, and to that end I've CC'd the locking maintainers.

== 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 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 27/27 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/ntsync5


== Implementation ==

Some aspects of the implementation may deserve particular comment:

* In the interest of performance, each object is governed only by a single
spinlock. However, NTSYNC_IOC_WAIT_ALL requires that the state of multiple
objects be changed as a single atomic operation. In order to achieve this, we
first take a device-wide lock ("wait_all_lock") any time we are going to lock
more than one object at a time.

The maximum number of objects that can be used in a vectored wait, and
therefore the maximum that can be locked simultaneously, is 64. This number is
NT's own limit.

The acquisition of multiple spinlocks will degrade performance. This is a
conscious choice, however. Wait-for-all is known to be a very rare operation
in practice, especially with counts that approach the maximum, and it is the
intent of the ntsync driver to optimize wait-for-any at the expense of
wait-for-all as much as possible.

* NT mutexes are tied to their threads on an OS level, and the kernel includes
builtin support for "robust" mutexes. In order to keep the ntsync driver
self-contained and avoid touching more code than necessary, it does not hook
into task exit nor use pids.

Instead, the user space emulator is expected to manage thread IDs and pass
them as an argument to any relevant functions; this is the "owner" field of
ntsync_wait_args and ntsync_mutex_args.

When the emulator detects that a thread dies, it should therefore call
NTSYNC_IOC_MUTEX_KILL on any open mutexes.

* ntsync is module-capable mostly because there was nothing preventing it, and
because it aided development. It is not a hard requirement, though.


== Previous versions ==

Changes from v3:

* Add .gitignore and use KHDR_INCLUDES in selftest build files, per Muhammad
Usama Anjum.

* Try to explain why we are rolling our own primitives a little better, per Greg
Kroah-Hartman.

* Link to v3: https://lore.kernel.org/lkml/20240329000621.148791-1-zfigura@xxxxxxxxxxxxxxx/
* Link to v2: https://lore.kernel.org/lkml/20240219223833.95710-1-zfigura@xxxxxxxxxxxxxxx/
* Link to v1: https://lore.kernel.org/lkml/20240214233645.9273-1-zfigura@xxxxxxxxxxxxxxx/
* Link to RFC v2: https://lore.kernel.org/lkml/20240131021356.10322-1-zfigura@xxxxxxxxxxxxxxx/
* Link to RFC v1: https://lore.kernel.org/lkml/20240124004028.16826-1-zfigura@xxxxxxxxxxxxxxx/

Elizabeth Figura (27):
ntsync: Introduce NTSYNC_IOC_WAIT_ANY.
ntsync: Introduce NTSYNC_IOC_WAIT_ALL.
ntsync: Introduce NTSYNC_IOC_CREATE_MUTEX.
ntsync: Introduce NTSYNC_IOC_MUTEX_UNLOCK.
ntsync: Introduce NTSYNC_IOC_MUTEX_KILL.
ntsync: Introduce NTSYNC_IOC_CREATE_EVENT.
ntsync: Introduce NTSYNC_IOC_EVENT_SET.
ntsync: Introduce NTSYNC_IOC_EVENT_RESET.
ntsync: Introduce NTSYNC_IOC_EVENT_PULSE.
ntsync: Introduce NTSYNC_IOC_SEM_READ.
ntsync: Introduce NTSYNC_IOC_MUTEX_READ.
ntsync: Introduce NTSYNC_IOC_EVENT_READ.
ntsync: Introduce alertable waits.
selftests: ntsync: Add some tests for semaphore state.
selftests: ntsync: Add some tests for mutex state.
selftests: ntsync: Add some tests for NTSYNC_IOC_WAIT_ANY.
selftests: ntsync: Add some tests for NTSYNC_IOC_WAIT_ALL.
selftests: ntsync: Add some tests for wakeup signaling with
WINESYNC_IOC_WAIT_ANY.
selftests: ntsync: Add some tests for wakeup signaling with
WINESYNC_IOC_WAIT_ALL.
selftests: ntsync: Add some tests for manual-reset event state.
selftests: ntsync: Add some tests for auto-reset event state.
selftests: ntsync: Add some tests for wakeup signaling with events.
selftests: ntsync: Add tests for alertable waits.
selftests: ntsync: Add some tests for wakeup signaling via alerts.
selftests: ntsync: Add a stress test for contended waits.
maintainers: Add an entry for ntsync.
docs: ntsync: Add documentation for the ntsync uAPI.

Documentation/userspace-api/index.rst | 1 +
Documentation/userspace-api/ntsync.rst | 399 +++++
MAINTAINERS | 9 +
drivers/misc/ntsync.c | 925 ++++++++++-
include/uapi/linux/ntsync.h | 39 +
tools/testing/selftests/Makefile | 1 +
.../selftests/drivers/ntsync/.gitignore | 1 +
.../testing/selftests/drivers/ntsync/Makefile | 7 +
tools/testing/selftests/drivers/ntsync/config | 1 +
.../testing/selftests/drivers/ntsync/ntsync.c | 1407 +++++++++++++++++
10 files changed, 2786 insertions(+), 4 deletions(-)
create mode 100644 Documentation/userspace-api/ntsync.rst
create mode 100644 tools/testing/selftests/drivers/ntsync/.gitignore
create mode 100644 tools/testing/selftests/drivers/ntsync/Makefile
create mode 100644 tools/testing/selftests/drivers/ntsync/config
create mode 100644 tools/testing/selftests/drivers/ntsync/ntsync.c


base-commit: ebbc1a4789c666846b9854ef845a37a64879e5f9
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2.43.0