1CheckPoint and Restart (CPR) 2============================ 3 4CPR is the umbrella name for a set of migration modes in which the 5VM is migrated to a new QEMU instance on the same host. It is 6intended for use when the goal is to update host software components 7that run the VM, such as QEMU or even the host kernel. At this time, 8cpr-reboot is the only available mode. 9 10Because QEMU is restarted on the same host, with access to the same 11local devices, CPR is allowed in certain cases where normal migration 12would be blocked. However, the user must not modify the contents of 13guest block devices between quitting old QEMU and starting new QEMU. 14 15CPR unconditionally stops VM execution before memory is saved, and 16thus does not depend on any form of dirty page tracking. 17 18cpr-reboot mode 19--------------- 20 21In this mode, QEMU stops the VM, and writes VM state to the migration 22URI, which will typically be a file. After quitting QEMU, the user 23resumes by running QEMU with the ``-incoming`` option. Because the 24old and new QEMU instances are not active concurrently, the URI cannot 25be a type that streams data from one instance to the other. 26 27Guest RAM can be saved in place if backed by shared memory, or can be 28copied to a file. The former is more efficient and is therefore 29preferred. 30 31After state and memory are saved, the user may update userland host 32software before restarting QEMU and resuming the VM. Further, if 33the RAM is backed by persistent shared memory, such as a DAX device, 34then the user may reboot to a new host kernel before restarting QEMU. 35 36This mode supports VFIO devices provided the user first puts the 37guest in the suspended runstate, such as by issuing the 38``guest-suspend-ram`` command to the QEMU guest agent. The agent 39must be pre-installed in the guest, and the guest must support 40suspend to RAM. Beware that suspension can take a few seconds, so 41the user should poll to see the suspended state before proceeding 42with the CPR operation. 43 44Usage 45^^^^^ 46 47It is recommended that guest RAM be backed with some type of shared 48memory, such as ``memory-backend-file,share=on``, and that the 49``x-ignore-shared`` capability be set. This combination allows memory 50to be saved in place. Otherwise, after QEMU stops the VM, all guest 51RAM is copied to the migration URI. 52 53Outgoing: 54 * Set the migration mode parameter to ``cpr-reboot``. 55 * Set the ``x-ignore-shared`` capability if desired. 56 * Issue the ``migrate`` command. It is recommended the the URI be a 57 ``file`` type, but one can use other types such as ``exec``, 58 provided the command captures all the data from the outgoing side, 59 and provides all the data to the incoming side. 60 * Quit when QEMU reaches the postmigrate state. 61 62Incoming: 63 * Start QEMU with the ``-incoming defer`` option. 64 * Set the migration mode parameter to ``cpr-reboot``. 65 * Set the ``x-ignore-shared`` capability if desired. 66 * Issue the ``migrate-incoming`` command. 67 * If the VM was running when the outgoing ``migrate`` command was 68 issued, then QEMU automatically resumes VM execution. 69 70Example 1 71^^^^^^^^^ 72:: 73 74 # qemu-kvm -monitor stdio 75 -object memory-backend-file,id=ram0,size=4G,mem-path=/dev/dax0.0,align=2M,share=on -m 4G 76 ... 77 78 (qemu) info status 79 VM status: running 80 (qemu) migrate_set_parameter mode cpr-reboot 81 (qemu) migrate_set_capability x-ignore-shared on 82 (qemu) migrate -d file:vm.state 83 (qemu) info status 84 VM status: paused (postmigrate) 85 (qemu) quit 86 87 ### optionally update kernel and reboot 88 # systemctl kexec 89 kexec_core: Starting new kernel 90 ... 91 92 # qemu-kvm ... -incoming defer 93 (qemu) info status 94 VM status: paused (inmigrate) 95 (qemu) migrate_set_parameter mode cpr-reboot 96 (qemu) migrate_set_capability x-ignore-shared on 97 (qemu) migrate_incoming file:vm.state 98 (qemu) info status 99 VM status: running 100 101Example 2: VFIO 102^^^^^^^^^^^^^^^ 103:: 104 105 # qemu-kvm -monitor stdio 106 -object memory-backend-file,id=ram0,size=4G,mem-path=/dev/dax0.0,align=2M,share=on -m 4G 107 -device vfio-pci, ... 108 -chardev socket,id=qga0,path=qga.sock,server=on,wait=off 109 -device virtserialport,chardev=qga0,name=org.qemu.guest_agent.0 110 ... 111 112 (qemu) info status 113 VM status: running 114 115 # echo '{"execute":"guest-suspend-ram"}' | ncat --send-only -U qga.sock 116 117 (qemu) info status 118 VM status: paused (suspended) 119 (qemu) migrate_set_parameter mode cpr-reboot 120 (qemu) migrate_set_capability x-ignore-shared on 121 (qemu) migrate -d file:vm.state 122 (qemu) info status 123 VM status: paused (postmigrate) 124 (qemu) quit 125 126 ### optionally update kernel and reboot 127 # systemctl kexec 128 kexec_core: Starting new kernel 129 ... 130 131 # qemu-kvm ... -incoming defer 132 (qemu) info status 133 VM status: paused (inmigrate) 134 (qemu) migrate_set_parameter mode cpr-reboot 135 (qemu) migrate_set_capability x-ignore-shared on 136 (qemu) migrate_incoming file:vm.state 137 (qemu) info status 138 VM status: paused (suspended) 139 (qemu) system_wakeup 140 (qemu) info status 141 VM status: running 142 143Caveats 144^^^^^^^ 145 146cpr-reboot mode may not be used with postcopy, background-snapshot, 147or COLO. 148