migration: cpr-reboot documentation

Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/1710338119-330923-1-git-send-email-steven.sistare@oracle.com
Signed-off-by: Peter Xu <peterx@redhat.com>

docs/devel/migration/CPR.rst

@@ -0,0 +1,147 @@
+CheckPoint and Restart (CPR)
+============================
+
+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.
+
+Because QEMU is restarted on the same host, with access to the same
+local devices, CPR is allowed in certain cases where normal migration
+would be blocked.  However, the user must not modify the contents of
+guest block devices between quitting old QEMU and starting new QEMU.
+
+CPR unconditionally stops VM execution before memory is saved, and
+thus does not depend on any form of dirty page tracking.
+
+cpr-reboot mode
+---------------
+
+In this mode, QEMU stops the VM, and writes VM state to the migration
+URI, which will typically be a file.  After quitting QEMU, the user
+resumes by running QEMU with the ``-incoming`` option.  Because the
+old and new QEMU instances are not active concurrently, the URI cannot
+be a type that streams data from one instance to the other.
+
+Guest RAM can be saved in place if backed by shared memory, or can be
+copied to a file.  The former is more efficient and is therefore
+preferred.
+
+After state and memory are saved, the user may update userland host
+software before restarting QEMU and resuming the VM.  Further, if
+the RAM is backed by persistent shared memory, such as a DAX device,
+then the user may reboot to a new host kernel before restarting QEMU.
+
+This mode supports VFIO devices provided the user first puts the
+guest in the suspended runstate, such as by issuing the
+``guest-suspend-ram`` command to the QEMU guest agent.  The agent
+must be pre-installed in the guest, and the guest must support
+suspend to RAM.  Beware that suspension can take a few seconds, so
+the user should poll to see the suspended state before proceeding
+with the CPR operation.
+
+Usage
+^^^^^
+
+It is recommended that guest RAM be backed with some type of shared
+memory, such as ``memory-backend-file,share=on``, and that the
+``x-ignore-shared`` capability be set.  This combination allows memory
+to be saved in place.  Otherwise, after QEMU stops the VM, all guest
+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
+    ``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.
+  * Quit when QEMU reaches the postmigrate state.
+
+Incoming:
+  * Start QEMU with the ``-incoming defer`` option.
+  * Set the migration mode parameter to ``cpr-reboot``.
+  * Set the ``x-ignore-shared`` capability if desired.
+  * Issue the ``migrate-incoming`` command.
+  * If the VM was running when the outgoing ``migrate`` command was
+    issued, then QEMU automatically resumes VM execution.
+
+Example 1
+^^^^^^^^^
+::
+
+  # qemu-kvm -monitor stdio
+  -object memory-backend-file,id=ram0,size=4G,mem-path=/dev/dax0.0,align=2M,share=on -m 4G
+  ...
+
+  (qemu) info status
+  VM status: running
+  (qemu) migrate_set_parameter mode cpr-reboot
+  (qemu) migrate_set_capability x-ignore-shared on
+  (qemu) migrate -d file:vm.state
+  (qemu) info status
+  VM status: paused (postmigrate)
+  (qemu) quit
+
+  ### optionally update kernel and reboot
+  # systemctl kexec
+  kexec_core: Starting new kernel
+  ...
+
+  # qemu-kvm ... -incoming defer
+  (qemu) info status
+  VM status: paused (inmigrate)
+  (qemu) migrate_set_parameter mode cpr-reboot
+  (qemu) migrate_set_capability x-ignore-shared on
+  (qemu) migrate_incoming file:vm.state
+  (qemu) info status
+  VM status: running
+
+Example 2: VFIO
+^^^^^^^^^^^^^^^
+::
+
+  # qemu-kvm -monitor stdio
+  -object memory-backend-file,id=ram0,size=4G,mem-path=/dev/dax0.0,align=2M,share=on -m 4G
+  -device vfio-pci, ...
+  -chardev socket,id=qga0,path=qga.sock,server=on,wait=off
+  -device virtserialport,chardev=qga0,name=org.qemu.guest_agent.0
+  ...
+
+  (qemu) info status
+  VM status: running
+
+  # echo '{"execute":"guest-suspend-ram"}' | ncat --send-only -U qga.sock
+
+  (qemu) info status
+  VM status: paused (suspended)
+  (qemu) migrate_set_parameter mode cpr-reboot
+  (qemu) migrate_set_capability x-ignore-shared on
+  (qemu) migrate -d file:vm.state
+  (qemu) info status
+  VM status: paused (postmigrate)
+  (qemu) quit
+
+  ### optionally update kernel and reboot
+  # systemctl kexec
+  kexec_core: Starting new kernel
+  ...
+
+  # qemu-kvm ... -incoming defer
+  (qemu) info status
+  VM status: paused (inmigrate)
+  (qemu) migrate_set_parameter mode cpr-reboot
+  (qemu) migrate_set_capability x-ignore-shared on
+  (qemu) migrate_incoming file:vm.state
+  (qemu) info status
+  VM status: paused (suspended)
+  (qemu) system_wakeup
+  (qemu) info status
+  VM status: running
+
+Caveats
+^^^^^^^
+
+cpr-reboot mode may not be used with postcopy, background-snapshot,
+or COLO.

docs/devel/migration/features.rst

@@ -11,3 +11,4 @@ Migration has plenty of features to support different use cases.
    vfio
    virtio
    mapped-ram
+   CPR