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, 8the cpr-reboot and cpr-transfer modes are available. 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 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 149cpr-transfer mode 150----------------- 151 152This mode allows the user to transfer a guest to a new QEMU instance 153on the same host with minimal guest pause time, by preserving guest 154RAM in place, albeit with new virtual addresses in new QEMU. Devices 155and their pinned memory pages are also preserved for VFIO and IOMMUFD. 156 157The user starts new QEMU on the same host as old QEMU, with command- 158line arguments to create the same machine, plus the ``-incoming`` 159option for the main migration channel, like normal live migration. 160In addition, the user adds a second -incoming option with channel 161type ``cpr``. This CPR channel must support file descriptor transfer 162with SCM_RIGHTS, i.e. it must be a UNIX domain socket. 163 164To initiate CPR, the user issues a migrate command to old QEMU, 165adding a second migration channel of type ``cpr`` in the channels 166argument. Old QEMU stops the VM, saves state to the migration 167channels, and enters the postmigrate state. Execution resumes in 168new QEMU. 169 170New QEMU reads the CPR channel before opening a monitor, hence 171the CPR channel cannot be specified in the list of channels for a 172migrate-incoming command. It may only be specified on the command 173line. 174 175Usage 176^^^^^ 177 178Memory backend objects must have the ``share=on`` attribute. 179 180The VM must be started with the ``-machine aux-ram-share=on`` 181option. This causes implicit RAM blocks (those not described by 182a memory-backend object) to be allocated by mmap'ing a memfd. 183Examples include VGA and ROM. 184 185Outgoing: 186 * Set the migration mode parameter to ``cpr-transfer``. 187 * Issue the ``migrate`` command, containing a main channel and 188 a cpr channel. 189 190Incoming: 191 * Start new QEMU with two ``-incoming`` options. 192 * If the VM was running when the outgoing ``migrate`` command was 193 issued, then QEMU automatically resumes VM execution. 194 195Caveats 196^^^^^^^ 197 198cpr-transfer mode may not be used with postcopy, background-snapshot, 199or COLO. 200 201memory-backend-epc is not supported. 202 203The main incoming migration channel address cannot be a file type. 204 205If the main incoming channel address is an inet socket, then the port 206cannot be 0 (meaning dynamically choose a port). 207 208When using ``-incoming defer``, you must issue the migrate command to 209old QEMU before issuing any monitor commands to new QEMU, because new 210QEMU blocks waiting to read from the cpr channel before starting its 211monitor, and old QEMU does not write to the channel until the migrate 212command is issued. However, new QEMU does not open and read the 213main migration channel until you issue the migrate incoming command. 214 215Example 1: incoming channel 216^^^^^^^^^^^^^^^^^^^^^^^^^^^ 217 218In these examples, we simply restart the same version of QEMU, but 219in a real scenario one would start new QEMU on the incoming side. 220Note that new QEMU does not print the monitor prompt until old QEMU 221has issued the migrate command. The outgoing side uses QMP because 222HMP cannot specify a CPR channel. Some QMP responses are omitted for 223brevity. 224 225:: 226 227 Outgoing: Incoming: 228 229 # qemu-kvm -qmp stdio 230 -object memory-backend-file,id=ram0,size=4G, 231 mem-path=/dev/shm/ram0,share=on -m 4G 232 -machine memory-backend=ram0 233 -machine aux-ram-share=on 234 ... 235 # qemu-kvm -monitor stdio 236 -incoming tcp:0:44444 237 -incoming '{"channel-type": "cpr", 238 "addr": { "transport": "socket", 239 "type": "unix", "path": "cpr.sock"}}' 240 ... 241 {"execute":"qmp_capabilities"} 242 243 {"execute": "query-status"} 244 {"return": {"status": "running", 245 "running": true}} 246 247 {"execute":"migrate-set-parameters", 248 "arguments":{"mode":"cpr-transfer"}} 249 250 {"execute": "migrate", "arguments": { "channels": [ 251 {"channel-type": "main", 252 "addr": { "transport": "socket", "type": "inet", 253 "host": "0", "port": "44444" }}, 254 {"channel-type": "cpr", 255 "addr": { "transport": "socket", "type": "unix", 256 "path": "cpr.sock" }}]}} 257 258 QEMU 10.0.50 monitor 259 (qemu) info status 260 VM status: running 261 262 {"execute": "query-status"} 263 {"return": {"status": "postmigrate", 264 "running": false}} 265 266Example 2: incoming defer 267^^^^^^^^^^^^^^^^^^^^^^^^^ 268 269This example uses ``-incoming defer`` to hot plug a device before 270accepting the main migration channel. Again note you must issue the 271migrate command to old QEMU before you can issue any monitor 272commands to new QEMU. 273 274 275:: 276 277 Outgoing: Incoming: 278 279 # qemu-kvm -monitor stdio 280 -object memory-backend-file,id=ram0,size=4G, 281 mem-path=/dev/shm/ram0,share=on -m 4G 282 -machine memory-backend=ram0 283 -machine aux-ram-share=on 284 ... 285 # qemu-kvm -monitor stdio 286 -incoming defer 287 -incoming '{"channel-type": "cpr", 288 "addr": { "transport": "socket", 289 "type": "unix", "path": "cpr.sock"}}' 290 ... 291 {"execute":"qmp_capabilities"} 292 293 {"execute": "device_add", 294 "arguments": {"driver": "pcie-root-port"}} 295 296 {"execute":"migrate-set-parameters", 297 "arguments":{"mode":"cpr-transfer"}} 298 299 {"execute": "migrate", "arguments": { "channels": [ 300 {"channel-type": "main", 301 "addr": { "transport": "socket", "type": "inet", 302 "host": "0", "port": "44444" }}, 303 {"channel-type": "cpr", 304 "addr": { "transport": "socket", "type": "unix", 305 "path": "cpr.sock" }}]}} 306 307 QEMU 10.0.50 monitor 308 (qemu) info status 309 VM status: paused (inmigrate) 310 (qemu) device_add pcie-root-port 311 (qemu) migrate_incoming tcp:0:44444 312 (qemu) info status 313 VM status: running 314 315 {"execute": "query-status"} 316 {"return": {"status": "postmigrate", 317 "running": false}} 318 319Futures 320^^^^^^^ 321 322cpr-transfer mode is based on a capability to transfer open file 323descriptors from old to new QEMU. In the future, descriptors for 324vhost, and char devices could be transferred, 325preserving those devices and their kernel state without interruption, 326even if they do not explicitly support live migration. 327