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.