[PATCH V4 8/8] Documentation: document ublk user recovery feature

[ZiyangZhang <ZiyangZhang@linux.alibaba.com>]

Add documentation for user recovery feature of ublk subsystem.

Signed-off-by: ZiyangZhang <ZiyangZhang@linux.alibaba.com>
---
 Documentation/block/ublk.rst | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/Documentation/block/ublk.rst b/Documentation/block/ublk.rst
index 2122d1a4a541..3f1bfd51898b 100644
--- a/Documentation/block/ublk.rst
+++ b/Documentation/block/ublk.rst
@@ -144,6 +144,31 @@ managing and controlling ublk devices with help of several control commands:
   For retrieving device info via ``ublksrv_ctrl_dev_info``. It is the server's
   responsibility to save IO target specific info in userspace.
 
+- ``UBLK_CMD_RESTART_DEV``
+
+  This command is valid if ``UBLK_F_USER_RECOVERY`` feature is enabled. The old
+  process has exited and ublk device is quiesced. Then, user should start a new
+  process which opens ``/dev/ublkc*`` and gets all ublk queues be ready. Finally
+  user should send this command. When this command returns, ublk device is
+  unquiesced and new I/O requests are passed to the new process.
+
+- user recovery feature description
+
+  Two new features are added for user recovery: ``UBLK_F_USER_RECOVERY`` and
+  ``UBLK_F_USER_RECOVERY_REISSUE``.
+
+  With ``UBLK_F_USER_RECOVERY`` set, after one ubq_daemon(ublksrv io handler) is
+  dying, ublk does not release ``/dev/ublkc*`` or ``/dev/ublkb*`` but requeues all
+  inflight requests which have not been issued to userspace. Requests which have
+  been issued to userspace are aborted.
+
+  With ``UBLK_F_USER_RECOVERY_REISSUE`` set, after one ubq_daemon(ublksrv io
+  handler) is dying, 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_RESTART_DEV``. ``UBLK_F_USER_RECOVERY_REISSUE`` is designed
+  for backends who tolerate double-write since the driver may issue the same
+  I/O request twice. It might be useful to a read-only FS or a VM backend.
+
 Data plane
 ----------
 
-- 
2.27.0