docs: Move pr-manager.rst into the system manual

Move the pr-manager documentation into the system manual.
Some of it (the documentation of the pr-manager-helper tool)
should be in tools, but we will split it up after moving it.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

docs/system/pr-manager.rst

@@ -0,0 +1,111 @@
+======================================
+Persistent reservation managers
+======================================
+
+SCSI persistent Reservations allow restricting access to block devices
+to specific initiators in a shared storage setup.  When implementing
+clustering of virtual machines, it is a common requirement for virtual
+machines to send persistent reservation SCSI commands.  However,
+the operating system restricts sending these commands to unprivileged
+programs because incorrect usage can disrupt regular operation of the
+storage fabric.
+
+For this reason, QEMU's SCSI passthrough devices, ``scsi-block``
+and ``scsi-generic`` (both are only available on Linux) can delegate
+implementation of persistent reservations to a separate object,
+the "persistent reservation manager".  Only PERSISTENT RESERVE OUT and
+PERSISTENT RESERVE IN commands are passed to the persistent reservation
+manager object; other commands are processed by QEMU as usual.
+
+-----------------------------------------
+Defining a persistent reservation manager
+-----------------------------------------
+
+A persistent reservation manager is an instance of a subclass of the
+"pr-manager" QOM class.
+
+Right now only one subclass is defined, ``pr-manager-helper``, which
+forwards the commands to an external privileged helper program
+over Unix sockets.  The helper program only allows sending persistent
+reservation commands to devices for which QEMU has a file descriptor,
+so that QEMU will not be able to effect persistent reservations
+unless it has access to both the socket and the device.
+
+``pr-manager-helper`` has a single string property, ``path``, which
+accepts the path to the helper program's Unix socket.  For example,
+the following command line defines a ``pr-manager-helper`` object and
+attaches it to a SCSI passthrough device::
+
+      $ qemu-system-x86_64
+          -device virtio-scsi \
+          -object pr-manager-helper,id=helper0,path=/var/run/qemu-pr-helper.sock
+          -drive if=none,id=hd,driver=raw,file.filename=/dev/sdb,file.pr-manager=helper0
+          -device scsi-block,drive=hd
+
+Alternatively, using ``-blockdev``::
+
+      $ qemu-system-x86_64
+          -device virtio-scsi \
+          -object pr-manager-helper,id=helper0,path=/var/run/qemu-pr-helper.sock
+          -blockdev node-name=hd,driver=raw,file.driver=host_device,file.filename=/dev/sdb,file.pr-manager=helper0
+          -device scsi-block,drive=hd
+
+----------------------------------
+Invoking :program:`qemu-pr-helper`
+----------------------------------
+
+QEMU provides an implementation of the persistent reservation helper,
+called :program:`qemu-pr-helper`.  The helper should be started as a
+system service and supports the following option:
+
+-d, --daemon              run in the background
+-q, --quiet               decrease verbosity
+-v, --verbose             increase verbosity
+-f, --pidfile=path        PID file when running as a daemon
+-k, --socket=path         path to the socket
+-T, --trace=trace-opts    tracing options
+
+By default, the socket and PID file are placed in the runtime state
+directory, for example :file:`/var/run/qemu-pr-helper.sock` and
+:file:`/var/run/qemu-pr-helper.pid`.  The PID file is not created
+unless :option:`-d` is passed too.
+
+:program:`qemu-pr-helper` can also use the systemd socket activation
+protocol.  In this case, the systemd socket unit should specify a
+Unix stream socket, like this::
+
+    [Socket]
+    ListenStream=/var/run/qemu-pr-helper.sock
+
+After connecting to the socket, :program:`qemu-pr-helper`` can optionally drop
+root privileges, except for those capabilities that are needed for
+its operation.  To do this, add the following options:
+
+-u, --user=user           user to drop privileges to
+-g, --group=group         group to drop privileges to
+
+---------------------------------------------
+Multipath devices and persistent reservations
+---------------------------------------------
+
+Proper support of persistent reservation for multipath devices requires
+communication with the multipath daemon, so that the reservation is
+registered and applied when a path is newly discovered or becomes online
+again.  :command:`qemu-pr-helper` can do this if the ``libmpathpersist``
+library was available on the system at build time.
+
+As of August 2017, a reservation key must be specified in ``multipath.conf``
+for ``multipathd`` to check for persistent reservation for newly
+discovered paths or reinstated paths.  The attribute can be added
+to the ``defaults`` section or the ``multipaths`` section; for example::
+
+    multipaths {
+        multipath {
+            wwid   XXXXXXXXXXXXXXXX
+            alias      yellow
+            reservation_key  0x123abc
+        }
+    }
+
+Linking :program:`qemu-pr-helper` to ``libmpathpersist`` does not impede
+its usage on regular SCSI devices.