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, 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 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 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.

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. Devices and their pinned memory pages will also be preserved in a future QEMU release.

The user starts new QEMU on the same host as old QEMU, with command- line arguments to create the same machine, plus the -incoming option for the main migration channel, like normal live migration. In addition, the user adds a second -incoming option with channel type cpr. This CPR channel must support file descriptor transfer with SCM_RIGHTS, i.e. it must be a UNIX domain socket.

To initiate CPR, the user issues a migrate command to old QEMU, adding a second migration channel of type cpr in the channels argument. Old QEMU stops the VM, saves state to the migration channels, and enters the postmigrate state. Execution resumes in new QEMU.

New QEMU reads the CPR channel before opening a monitor, hence the CPR channel cannot be specified in the list of channels for a migrate-incoming command. It may only be specified on the command line.

Usage

Memory backend objects must have the share=on attribute.

The VM must be started with the -machine aux-ram-share=on 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.

  • Issue the migrate command, containing a main channel and a cpr channel.

Incoming:

  • Start new QEMU with two -incoming 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 is not supported.

The main incoming migration channel address cannot be a file type.

If the main incoming channel address is an inet socket, 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 the cpr channel before starting its monitor, and old QEMU does not write to the channel until the migrate command is issued. However, new QEMU does not open and read the main migration channel 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. The outgoing side uses QMP because HMP cannot specify a CPR channel. Some QMP responses are omitted for brevity.

Outgoing:                             Incoming:

# qemu-kvm -qmp stdio
-object memory-backend-file,id=ram0,size=4G,
mem-path=/dev/shm/ram0,share=on -m 4G
-machine memory-backend=ram0
-machine aux-ram-share=on
...
                                      # qemu-kvm -monitor stdio
                                      -incoming tcp:0:44444
                                      -incoming '{"channel-type": "cpr",
                                        "addr": { "transport": "socket",
                                        "type": "unix", "path": "cpr.sock"}}'
                                      ...
{"execute":"qmp_capabilities"}

{"execute": "query-status"}
{"return": {"status": "running",
            "running": true}}

{"execute":"migrate-set-parameters",
 "arguments":{"mode":"cpr-transfer"}}

{"execute": "migrate", "arguments": { "channels": [
  {"channel-type": "main",
   "addr": { "transport": "socket", "type": "inet",
             "host": "0", "port": "44444" }},
  {"channel-type": "cpr",
   "addr": { "transport": "socket", "type": "unix",
             "path": "cpr.sock" }}]}}

                                      QEMU 10.0.50 monitor
                                      (qemu) info status
                                      VM status: running

{"execute": "query-status"}
{"return": {"status": "postmigrate",
            "running": false}}

Example 2: incoming defer

This example uses -incoming defer to hot plug a device before accepting the main migration channel. 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 memory-backend=ram0
-machine aux-ram-share=on
...
                                      # qemu-kvm -monitor stdio
                                      -incoming defer
                                      -incoming '{"channel-type": "cpr",
                                        "addr": { "transport": "socket",
                                        "type": "unix", "path": "cpr.sock"}}'
                                      ...
{"execute":"qmp_capabilities"}

{"execute": "device_add",
 "arguments": {"driver": "pcie-root-port"}}

{"execute":"migrate-set-parameters",
 "arguments":{"mode":"cpr-transfer"}}

{"execute": "migrate", "arguments": { "channels": [
  {"channel-type": "main",
   "addr": { "transport": "socket", "type": "inet",
             "host": "0", "port": "44444" }},
  {"channel-type": "cpr",
   "addr": { "transport": "socket", "type": "unix",
             "path": "cpr.sock" }}]}}

                                      QEMU 10.0.50 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

{"execute": "query-status"}
{"return": {"status": "postmigrate",
            "running": false}}

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.