Documentation: ublk: document UBLK_F_PER_IO_DAEMON

Explain the restrictions imposed on ublk servers in two cases:
1. When UBLK_F_PER_IO_DAEMON is set (current ublk_drv)
2. When UBLK_F_PER_IO_DAEMON is not set (legacy)

Remove most references to per-queue daemons, as the new
UBLK_F_PER_IO_DAEMON feature renders that concept obsolete.

Signed-off-by: Uday Shankar <ushankar@purestorage.com>
Reviewed-by: Caleb Sander Mateos <csander@purestorage.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250529-ublk_task_per_io-v8-9-e9d3b119336a@purestorage.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: ps-ushankar

Documentation/block/ublk.rst

@@ -115,15 +115,15 @@ managing and controlling ublk devices with help of several control commands:
 
 - ``UBLK_CMD_START_DEV``
 
-  After the server prepares userspace resources (such as creating per-queue
-  pthread & io_uring for handling ublk IO), this command is sent to the
+  After the server prepares userspace resources (such as creating I/O handler
+  threads & io_uring for handling ublk IO), this command is sent to the
   driver for allocating & exposing ``/dev/ublkb*``. Parameters set via
   ``UBLK_CMD_SET_PARAMS`` are applied for creating the device.
 
 - ``UBLK_CMD_STOP_DEV``
 
   Halt IO on ``/dev/ublkb*`` and remove the device. When this command returns,
-  ublk server will release resources (such as destroying per-queue pthread &
+  ublk server will release resources (such as destroying I/O handler threads &
   io_uring).
 
 - ``UBLK_CMD_DEL_DEV``
@@ -208,15 +208,15 @@ managing and controlling ublk devices with help of several control commands:
   modify how I/O is handled while the ublk server is dying/dead (this is called
   the ``nosrv`` case in the driver code).
 
-  With just ``UBLK_F_USER_RECOVERY`` set, after one ubq_daemon(ublk server's io
-  handler) is dying, ublk does not delete ``/dev/ublkb*`` during the whole
+  With just ``UBLK_F_USER_RECOVERY`` set, after the ublk server exits,
+  ublk does not delete ``/dev/ublkb*`` during the whole
   recovery stage and ublk device ID is kept. It is ublk server's
   responsibility to recover the device context by its own knowledge.
   Requests which have not been issued to userspace are requeued. Requests
   which have been issued to userspace are aborted.
 
-  With ``UBLK_F_USER_RECOVERY_REISSUE`` additionally set, after one ubq_daemon
-  (ublk server's io handler) is dying, contrary to ``UBLK_F_USER_RECOVERY``,
+  With ``UBLK_F_USER_RECOVERY_REISSUE`` additionally set, after the ublk server
+  exits, contrary to ``UBLK_F_USER_RECOVERY``,
   requests which have been issued to userspace are requeued and will be
   re-issued to the new process after handling ``UBLK_CMD_END_USER_RECOVERY``.
   ``UBLK_F_USER_RECOVERY_REISSUE`` is designed for backends who tolerate
@@ -241,10 +241,11 @@ can be controlled/accessed just inside this container.
 Data plane
 ----------
 
-ublk server needs to create per-queue IO pthread & io_uring for handling IO
-commands via io_uring passthrough. The per-queue IO pthread
-focuses on IO handling and shouldn't handle any control & management
-tasks.
+The ublk server should create dedicated threads for handling I/O. Each
+thread should have its own io_uring through which it is notified of new
+I/O, and through which it can complete I/O. These dedicated threads
+should focus on IO handling and shouldn't handle any control &
+management tasks.
 
 The's IO is assigned by a unique tag, which is 1:1 mapping with IO
 request of ``/dev/ublkb*``.
@@ -265,6 +266,18 @@ with specified IO tag in the command data:
   destined to ``/dev/ublkb*``. This command is sent only once from the server
   IO pthread for ublk driver to setup IO forward environment.
 
+  Once a thread issues this command against a given (qid,tag) pair, the thread
+  registers itself as that I/O's daemon. In the future, only that I/O's daemon
+  is allowed to issue commands against the I/O. If any other thread attempts
+  to issue a command against a (qid,tag) pair for which the thread is not the
+  daemon, the command will fail. Daemons can be reset only be going through
+  recovery.
+
+  The ability for every (qid,tag) pair to have its own independent daemon task
+  is indicated by the ``UBLK_F_PER_IO_DAEMON`` feature. If this feature is not
+  supported by the driver, daemons must be per-queue instead - i.e. all I/Os
+  associated to a single qid must be handled by the same task.
+
 - ``UBLK_IO_COMMIT_AND_FETCH_REQ``
 
   When an IO request is destined to ``/dev/ublkb*``, the driver stores