[v3,30/30] docs: ntsync: Add documentation for the ntsync uAPI.

Add an overall explanation of the driver architecture, and complete and precise
specification for its intended behaviour.

Signed-off-by: Elizabeth Figura <zfigura@codeweavers.com>
---
 Documentation/userspace-api/index.rst  |   1 +
 Documentation/userspace-api/ntsync.rst | 399 +++++++++++++++++++++++++
 2 files changed, 400 insertions(+)
 create mode 100644 Documentation/userspace-api/ntsync.rst

On Thu, Mar 28, 2024 at 07:06:21PM -0500, Elizabeth Figura wrote:
> diff --git a/Documentation/userspace-api/ntsync.rst b/Documentation/userspace-api/ntsync.rst
> new file mode 100644
> index 000000000000..202c2350d3af
> --- /dev/null
> +++ b/Documentation/userspace-api/ntsync.rst
> @@ -0,0 +1,399 @@
> +===================================
> +NT synchronization primitive driver
> +===================================
> +
> +This page documents the user-space API for the ntsync driver.
> +
> +ntsync is a support driver for emulation of NT synchronization
> +primitives by user-space NT emulators. It exists because implementation
> +in user-space, using existing tools, cannot match Windows performance
> +while offering accurate semantics. It is implemented entirely in
> +software, and does not drive any hardware device.
> +
> +This interface is meant as a compatibility tool only, and should not
> +be used for general synchronization. Instead use generic, versatile
> +interfaces such as futex(2) and poll(2).
> +
> +Synchronization primitives
> +==========================
> +
> +The ntsync driver exposes three types of synchronization primitives:
> +semaphores, mutexes, and events.
> +
> +A semaphore holds a single volatile 32-bit counter, and a static 32-bit
> +integer denoting the maximum value. It is considered signaled when the
> +counter is nonzero. The counter is decremented by one when a wait is
> +satisfied. Both the initial and maximum count are established when the
> +semaphore is created.
> +
> +A mutex holds a volatile 32-bit recursion count, and a volatile 32-bit
> +identifier denoting its owner. A mutex is considered signaled when its
> +owner is zero (indicating that it is not owned). The recursion count is
> +incremented when a wait is satisfied, and ownership is set to the given
> +identifier.
> +
> +A mutex also holds an internal flag denoting whether its previous owner
> +has died; such a mutex is said to be abandoned. Owner death is not
> +tracked automatically based on thread death, but rather must be
> +communicated using ``NTSYNC_IOC_MUTEX_KILL``. An abandoned mutex is
> +inherently considered unowned.
> +
> +Except for the "unowned" semantics of zero, the actual value of the
> +owner identifier is not interpreted by the ntsync driver at all. The
> +intended use is to store a thread identifier; however, the ntsync
> +driver does not actually validate that a calling thread provides
> +consistent or unique identifiers.
> +
> +An event holds a volatile boolean state denoting whether it is signaled
> +or not. There are two types of events, auto-reset and manual-reset. An
> +auto-reset event is designaled when a wait is satisfied; a manual-reset
> +event is not. The event type is specified when the event is created.
> +
> +Unless specified otherwise, all operations on an object are atomic and
> +totally ordered with respect to other operations on the same object.
> +
> +Objects are represented by files. When all file descriptors to an
> +object are closed, that object is deleted.
> +
> +Char device
> +===========
> +
> +The ntsync driver creates a single char device /dev/ntsync. Each file
> +description opened on the device represents a unique instance intended
> +to back an individual NT virtual machine. Objects created by one ntsync
> +instance may only be used with other objects created by the same
> +instance.
> +
> +ioctl reference
> +===============
> +
> +All operations on the device are done through ioctls. There are four
> +structures used in ioctl calls::
> +
> +   struct ntsync_sem_args {
> +   	__u32 sem;
> +   	__u32 count;
> +   	__u32 max;
> +   };
> +
> +   struct ntsync_mutex_args {
> +   	__u32 mutex;
> +   	__u32 owner;
> +   	__u32 count;
> +   };
> +
> +   struct ntsync_event_args {
> +   	__u32 event;
> +   	__u32 signaled;
> +   	__u32 manual;
> +   };
> +
> +   struct ntsync_wait_args {
> +   	__u64 timeout;
> +   	__u64 objs;
> +   	__u32 count;
> +   	__u32 owner;
> +   	__u32 index;
> +   	__u32 alert;
> +   	__u32 flags;
> +   	__u32 pad;
> +   };
> +
> +Depending on the ioctl, members of the structure may be used as input,
> +output, or not at all. All ioctls return 0 on success.
> +
> +The ioctls on the device file are as follows:
> +
> +.. c:macro:: NTSYNC_IOC_CREATE_SEM
> +
> +  Create a semaphore object. Takes a pointer to struct
> +  :c:type:`ntsync_sem_args`, which is used as follows:
> +
> +  .. list-table::
> +
> +     * - ``sem``
> +       - On output, contains a file descriptor to the created semaphore.
> +     * - ``count``
> +       - Initial count of the semaphore.
> +     * - ``max``
> +       - Maximum count of the semaphore.
> +
> +  Fails with ``EINVAL`` if ``count`` is greater than ``max``.
> +
> +.. c:macro:: NTSYNC_IOC_CREATE_MUTEX
> +
> +  Create a mutex object. Takes a pointer to struct
> +  :c:type:`ntsync_mutex_args`, which is used as follows:
> +
> +  .. list-table::
> +
> +     * - ``mutex``
> +       - On output, contains a file descriptor to the created mutex.
> +     * - ``count``
> +       - Initial recursion count of the mutex.
> +     * - ``owner``
> +       - Initial owner of the mutex.
> +
> +  If ``owner`` is nonzero and ``count`` is zero, or if ``owner`` is
> +  zero and ``count`` is nonzero, the function fails with ``EINVAL``.
> +
> +.. c:macro:: NTSYNC_IOC_CREATE_EVENT
> +
> +  Create an event object. Takes a pointer to struct
> +  :c:type:`ntsync_event_args`, which is used as follows:
> +
> +  .. list-table::
> +
> +     * - ``event``
> +       - On output, contains a file descriptor to the created event.
> +     * - ``signaled``
> +       - If nonzero, the event is initially signaled, otherwise
> +         nonsignaled.
> +     * - ``manual``
> +       - If nonzero, the event is a manual-reset event, otherwise
> +         auto-reset.
> +
> +The ioctls on the individual objects are as follows:
> +
> +.. c:macro:: NTSYNC_IOC_SEM_POST
> +
> +  Post to a semaphore object. Takes a pointer to a 32-bit integer,
> +  which on input holds the count to be added to the semaphore, and on
> +  output contains its previous count.
> +
> +  If adding to the semaphore's current count would raise the latter
> +  past the semaphore's maximum count, the ioctl fails with
> +  ``EOVERFLOW`` and the semaphore is not affected. If raising the
> +  semaphore's count causes it to become signaled, eligible threads
> +  waiting on this semaphore will be woken and the semaphore's count
> +  decremented appropriately.
> +
> +.. c:macro:: NTSYNC_IOC_MUTEX_UNLOCK
> +
> +  Release a mutex object. Takes a pointer to struct
> +  :c:type:`ntsync_mutex_args`, which is used as follows:
> +
> +  .. list-table::
> +
> +     * - ``mutex``
> +       - Ignored.
> +     * - ``owner``
> +       - Specifies the owner trying to release this mutex.
> +     * - ``count``
> +       - On output, contains the previous recursion count.
> +
> +  If ``owner`` is zero, the ioctl fails with ``EINVAL``. If ``owner``
> +  is not the current owner of the mutex, the ioctl fails with
> +  ``EPERM``.
> +
> +  The mutex's count will be decremented by one. If decrementing the
> +  mutex's count causes it to become zero, the mutex is marked as
> +  unowned and signaled, and eligible threads waiting on it will be
> +  woken as appropriate.
> +
> +.. c:macro:: NTSYNC_IOC_SET_EVENT
> +
> +  Signal an event object. Takes a pointer to a 32-bit integer, which on
> +  output contains the previous state of the event.
> +
> +  Eligible threads will be woken, and auto-reset events will be
> +  designaled appropriately.
> +
> +.. c:macro:: NTSYNC_IOC_RESET_EVENT
> +
> +  Designal an event object. Takes a pointer to a 32-bit integer, which
> +  on output contains the previous state of the event.
> +
> +.. c:macro:: NTSYNC_IOC_PULSE_EVENT
> +
> +  Wake threads waiting on an event object while leaving it in an
> +  unsignaled state. Takes a pointer to a 32-bit integer, which on
> +  output contains the previous state of the event.
> +
> +  A pulse operation can be thought of as a set followed by a reset,
> +  performed as a single atomic operation. If two threads are waiting on
> +  an auto-reset event which is pulsed, only one will be woken. If two
> +  threads are waiting a manual-reset event which is pulsed, both will
> +  be woken. However, in both cases, the event will be unsignaled
> +  afterwards, and a simultaneous read operation will always report the
> +  event as unsignaled.
> +
> +.. c:macro:: NTSYNC_IOC_READ_SEM
> +
> +  Read the current state of a semaphore object. Takes a pointer to
> +  struct :c:type:`ntsync_sem_args`, which is used as follows:
> +
> +  .. list-table::
> +
> +     * - ``sem``
> +       - Ignored.
> +     * - ``count``
> +       - On output, contains the current count of the semaphore.
> +     * - ``max``
> +       - On output, contains the maximum count of the semaphore.
> +
> +.. c:macro:: NTSYNC_IOC_READ_MUTEX
> +
> +  Read the current state of a mutex object. Takes a pointer to struct
> +  :c:type:`ntsync_mutex_args`, which is used as follows:
> +
> +  .. list-table::
> +
> +     * - ``mutex``
> +       - Ignored.
> +     * - ``owner``
> +       - On output, contains the current owner of the mutex, or zero
> +         if the mutex is not currently owned.
> +     * - ``count``
> +       - On output, contains the current recursion count of the mutex.
> +
> +  If the mutex is marked as abandoned, the function fails with
> +  ``EOWNERDEAD``. In this case, ``count`` and ``owner`` are set to
> +  zero.
> +
> +.. c:macro:: NTSYNC_IOC_READ_EVENT
> +
> +  Read the current state of an event object. Takes a pointer to struct
> +  :c:type:`ntsync_event_args`, which is used as follows:
> +
> +  .. list-table::
> +
> +     * - ``event``
> +       - Ignored.
> +     * - ``signaled``
> +       - On output, contains the current state of the event.
> +     * - ``manual``
> +       - On output, contains 1 if the event is a manual-reset event,
> +         and 0 otherwise.
> +
> +.. c:macro:: NTSYNC_IOC_KILL_OWNER
> +
> +  Mark a mutex as unowned and abandoned if it is owned by the given
> +  owner. Takes an input-only pointer to a 32-bit integer denoting the
> +  owner. If the owner is zero, the ioctl fails with ``EINVAL``. If the
> +  owner does not own the mutex, the function fails with ``EPERM``.
> +
> +  Eligible threads waiting on the mutex will be woken as appropriate
> +  (and such waits will fail with ``EOWNERDEAD``, as described below).
> +
> +.. c:macro:: NTSYNC_IOC_WAIT_ANY
> +
> +  Poll on any of a list of objects, atomically acquiring at most one.
> +  Takes a pointer to struct :c:type:`ntsync_wait_args`, which is
> +  used as follows:
> +
> +  .. list-table::
> +
> +     * - ``timeout``
> +       - Absolute timeout in nanoseconds. If ``NTSYNC_WAIT_REALTIME``
> +         is set, the timeout is measured against the REALTIME clock;
> +         otherwise it is measured against the MONOTONIC clock. If the
> +         timeout is equal to or earlier than the current time, the
> +         function returns immediately without sleeping. If ``timeout``
> +         is U64_MAX, the function will sleep until an object is
> +         signaled, and will not fail with ``ETIMEDOUT``.
> +     * - ``objs``
> +       - Pointer to an array of ``count`` file descriptors
> +         (specified as an integer so that the structure has the same
> +         size regardless of architecture). If any object is
> +         invalid, the function fails with ``EINVAL``.
> +     * - ``count``
> +       - Number of objects specified in the ``objs`` array.
> +         If greater than ``NTSYNC_MAX_WAIT_COUNT``, the function fails
> +         with ``EINVAL``.
> +     * - ``owner``
> +       - Mutex owner identifier. If any object in ``objs`` is a mutex,
> +         the ioctl will attempt to acquire that mutex on behalf of
> +         ``owner``. If ``owner`` is zero, the ioctl fails with
> +         ``EINVAL``.
> +     * - ``index``
> +       - On success, contains the index (into ``objs``) of the object
> +         which was signaled. If ``alert`` was signaled instead,
> +         this contains ``count``.
> +     * - ``alert``
> +       - Optional event object file descriptor. If nonzero, this
> +         specifies an "alert" event object which, if signaled, will
> +         terminate the wait. If nonzero, the identifier must point to a
> +         valid event.
> +     * - ``flags``
> +       - Zero or more flags. Currently the only flag is
> +         ``NTSYNC_WAIT_REALTIME``, which causes the timeout to be
> +         measured against the REALTIME clock instead of MONOTONIC.
> +     * - ``pad``
> +       - Unused, must be set to zero.
> +
> +  This function attempts to acquire one of the given objects. If unable
> +  to do so, it sleeps until an object becomes signaled, subsequently
> +  acquiring it, or the timeout expires. In the latter case the ioctl
> +  fails with ``ETIMEDOUT``. The function only acquires one object, even
> +  if multiple objects are signaled.
> +
> +  A semaphore is considered to be signaled if its count is nonzero, and
> +  is acquired by decrementing its count by one. A mutex is considered
> +  to be signaled if it is unowned or if its owner matches the ``owner``
> +  argument, and is acquired by incrementing its recursion count by one
> +  and setting its owner to the ``owner`` argument. An auto-reset event
> +  is acquired by designaling it; a manual-reset event is not affected
> +  by acquisition.
> +
> +  Acquisition is atomic and totally ordered with respect to other
> +  operations on the same object. If two wait operations (with different
> +  ``owner`` identifiers) are queued on the same mutex, only one is
> +  signaled. If two wait operations are queued on the same semaphore,
> +  and a value of one is posted to it, only one is signaled. The order
> +  in which threads are signaled is not specified.
> +
> +  If an abandoned mutex is acquired, the ioctl fails with
> +  ``EOWNERDEAD``. Although this is a failure return, the function may
> +  otherwise be considered successful. The mutex is marked as owned by
> +  the given owner (with a recursion count of 1) and as no longer
> +  abandoned, and ``index`` is still set to the index of the mutex.
> +
> +  The ``alert`` argument is an "extra" event which can terminate the
> +  wait, independently of all other objects. If members of ``objs`` and
> +  ``alert`` are both simultaneously signaled, a member of ``objs`` will
> +  always be given priority and acquired first.
> +
> +  It is valid to pass the same object more than once, including by
> +  passing the same event in the ``objs`` array and in ``alert``. If a
> +  wakeup occurs due to that object being signaled, ``index`` is set to
> +  the lowest index corresponding to that object.
> +
> +  The function may fail with ``EINTR`` if a signal is received.
> +
> +.. c:macro:: NTSYNC_IOC_WAIT_ALL
> +
> +  Poll on a list of objects, atomically acquiring all of them. Takes a
> +  pointer to struct :c:type:`ntsync_wait_args`, which is used
> +  identically to ``NTSYNC_IOC_WAIT_ANY``, except that ``index`` is
> +  always filled with zero on success if not woken via alert.
> +
> +  This function attempts to simultaneously acquire all of the given
> +  objects. If unable to do so, it sleeps until all objects become
> +  simultaneously signaled, subsequently acquiring them, or the timeout
> +  expires. In the latter case the ioctl fails with ``ETIMEDOUT`` and no
> +  objects are modified.
> +
> +  Objects may become signaled and subsequently designaled (through
> +  acquisition by other threads) while this thread is sleeping. Only
> +  once all objects are simultaneously signaled does the ioctl acquire
> +  them and return. The entire acquisition is atomic and totally ordered
> +  with respect to other operations on any of the given objects.
> +
> +  If an abandoned mutex is acquired, the ioctl fails with
> +  ``EOWNERDEAD``. Similarly to ``NTSYNC_IOC_WAIT_ANY``, all objects are
> +  nevertheless marked as acquired. Note that if multiple mutex objects
> +  are specified, there is no way to know which were marked as
> +  abandoned.
> +
> +  As with "any" waits, the ``alert`` argument is an "extra" event which
> +  can terminate the wait. Critically, however, an "all" wait will
> +  succeed if all members in ``objs`` are signaled, *or* if ``alert`` is
> +  signaled. In the latter case ``index`` will be set to ``count``. As
> +  with "any" waits, if both conditions are filled, the former takes
> +  priority, and objects in ``objs`` will be acquired.
> +
> +  Unlike ``NTSYNC_IOC_WAIT_ANY``, it is not valid to pass the same
> +  object more than once, nor is it valid to pass the same object in
> +  ``objs`` and in ``alert``. If this is attempted, the function fails
> +  with ``EINVAL``.

The doc LGTM, thanks!

Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>