[PATCH V3 16/16] migration: cpr-transfer documentation

[Steve Sistare <steven.sistare@oracle.com>]

Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
---
 docs/devel/migration/CPR.rst | 144 ++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 142 insertions(+), 2 deletions(-)

diff --git a/docs/devel/migration/CPR.rst b/docs/devel/migration/CPR.rst
index 63c3647..732d5a6 100644
--- a/docs/devel/migration/CPR.rst
+++ b/docs/devel/migration/CPR.rst
@@ -5,7 +5,7 @@ CPR is the umbrella name for a set of migration modes in which the
 VM is migrated to a new QEMU instance on the same host.  It is
 intended for use when the goal is to update host software components
 that run the VM, such as QEMU or even the host kernel.  At this time,
-cpr-reboot is the only available mode.
+the cpr-reboot and cpr-transfer modes are available.
 
 Because QEMU is restarted on the same host, with access to the same
 local devices, CPR is allowed in certain cases where normal migration
@@ -53,7 +53,7 @@ RAM is copied to the migration URI.
 Outgoing:
   * Set the migration mode parameter to ``cpr-reboot``.
   * Set the ``x-ignore-shared`` capability if desired.
-  * Issue the ``migrate`` command.  It is recommended the the URI be a
+  * Issue the ``migrate`` command.  It is recommended the URI be a
     ``file`` type, but one can use other types such as ``exec``,
     provided the command captures all the data from the outgoing side,
     and provides all the data to the incoming side.
@@ -145,3 +145,143 @@ Caveats
 
 cpr-reboot mode may not be used with postcopy, background-snapshot,
 or COLO.
+
+cpr-transfer mode
+-----------------
+
+This mode allows the user to transfer a guest to a new QEMU instance
+on the same host with minimal guest pause time, by preserving guest
+RAM in place, albeit with new virtual addresses in new QEMU.
+
+The user starts new QEMU on the same host as old QEMU, with the
+same arguments as old QEMU, plus the ``-incoming option``.  The user
+issues the migrate command to old QEMU, which stops the VM, saves
+state to the migration channels, and enters the postmigrate state.
+Execution resumes in new QEMU.
+
+This mode requires a second migration channel, specified by the
+``cpr-uri`` migration property on the outgoing side, and by the
+``cpr-uri`` QEMU command-line option on the incoming side.  The
+channel must be a type, such as unix socket, that supports SCM_RIGHTS.
+
+Usage
+^^^^^
+
+Memory backend objects must have the ``share=on`` attribute.
+
+The VM must be started with the ``-machine anon-alloc=memfd``
+option.  This causes implicit RAM blocks (those not described by
+a memory-backend object) to be allocated by mmap'ing a memfd.
+Examples include VGA and ROM.
+
+Outgoing:
+  * Set the migration mode parameter to ``cpr-transfer``.
+  * Set the ``cpr-uri`` parameter.  It must be a ``unix`` type.
+  * Issue the ``migrate`` command.
+
+Incoming:
+  * Start new QEMU with the ``-incoming`` and ``-cpr-uri`` options.
+  * If the VM was running when the outgoing ``migrate`` command was
+    issued, then QEMU automatically resumes VM execution.
+
+Caveats
+^^^^^^^
+
+cpr-transfer mode may not be used with postcopy, background-snapshot,
+or COLO.
+
+memory-backend-epc and memory-backend-ram are not supported.
+
+The incoming migration channel cannot be a file type.
+
+If the incoming migration channel is a tcp type, then the port cannot
+be 0 (meaning dynamically choose a port).
+
+When using ``-incoming defer``, you must issue the migrate command to
+old QEMU before issuing any monitor commands to new QEMU, because new
+QEMU blocks waiting to read from cpr-uri before starting its monitor,
+and old QEMU does not write to cpr-uri until the migrate command is
+issued.  However, new QEMU does not open and read the migration stream
+until you issue the migrate incoming command.
+
+Example 1: incoming channel
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+In these examples, we simply restart the same version of QEMU, but
+in a real scenario one would start new QEMU on the incoming side.
+Note that new QEMU does not print the monitor prompt until old QEMU
+has issued the migrate command.
+
+::
+
+  Outgoing:                             Incoming:
+
+  # qemu-kvm -monitor stdio
+  -object memory-backend-file,id=ram0,size=4G,
+  mem-path=/dev/shm/ram0,share=on -m 4G
+  -machine anon-alloc=memfd'
+  ...
+                                        # qemu-kvm -incoming tcp:0:44444
+                                        -cpr-uri unix:cpr.sock
+                                        ...
+  QEMU 9.2.50 monitor
+  (qemu) info status
+  VM status: running
+  (qemu) migrate_set_parameter mode cpr-transfer
+  (qemu) migrate_set_parameter cpr-uri unix:cpr.sock
+  (qemu) migrate -d tcp:0:44444
+
+                                        QEMU 9.2.50 monitor
+                                        (qemu) info status
+                                        VM status: running
+
+  (qemu) info status
+  VM status: paused (postmigrate)
+
+
+Example 2: incoming defer
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+This example uses ``-incoming defer`` to hot plug a device before
+accepting the migration stream.  Again note you must issue the
+migrate command to old QEMU before you can issue any monitor
+commands to new QEMU.
+
+
+::
+
+  Outgoing:                             Incoming:
+
+  # qemu-kvm -monitor stdio
+  -object memory-backend-file,id=ram0,size=4G,
+  mem-path=/dev/shm/ram0,share=on -m 4G
+  -machine anon-alloc=memfd'
+  ...
+                                        # qemu-kvm -incoming defer
+                                        -cpr-uri unix:cpr.sock
+                                        ...
+  QEMU 9.2.50 monitor
+  (qemu) device_add pcie-root-port
+  (qemu) migrate_set_parameter mode cpr-transfer
+  (qemu) migrate_set_parameter cpr-uri unix:cpr.sock
+  (qemu) migrate -d tcp:0:44444
+
+                                        QEMU 9.2.50 monitor
+                                        (qemu) info status
+                                        VM status: paused (inmigrate)
+                                        (qemu) device_add pcie-root-port
+                                        (qemu) migrate_incoming tcp:0:44444
+                                        (qemu) info status
+                                        VM status: running
+
+  (qemu) info status
+  VM status: paused (postmigrate)
+
+Futures
+^^^^^^^
+
+cpr-transfer mode is based on a capability to transfer open file
+descriptors from old to new QEMU.  In the future, descriptors for
+vfio, iommufd, vhost, and char devices could be transferred,
+preserving those devices and their kernel state without interruption,
+even if they do not explicitly support live migration.
-- 
1.8.3.1