What? This patch series adds the live migration cpr-transfer mode, which 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, and by preserving device file descriptors.
The new user-visible interfaces are: * cpr-transfer (MigMode migration parameter) * cpr-uri (migration parameter) * cpr-uri (command-line argument) The user sets the mode parameter before invoking the migrate command. In this mode, the user starts new QEMU on the same host as old QEMU, with the same arguments as old QEMU, plus the -incoming and the -cpr-uri options. The user issues the migrate command to old QEMU, which stops the VM, saves state to the migration channels, and enters the postmigrate state. Execution resumes in new QEMU. Memory-backend objects must have the share=on attribute, but memory-backend-epc is not supported. The VM must be started with the '-machine anon-alloc=memfd' option, which allows anonymous memory to be transferred in place to the new process. This mode requires a second migration channel, specified by the cpr-uri migration property on the outgoing side, and by the cpr-uri QEMU command-line option on the incoming side. The channel must be a type, such as unix socket, that supports SCM_RIGHTS. Why? This mode has less impact on the guest than any other method of updating in place. The pause time is much lower, because devices need not be torn down and recreated, DMA does not need to be drained and quiesced, and minimal state is copied to new QEMU. Further, there are no constraints on the guest. By contrast, cpr-reboot mode requires the guest to support S3 suspend-to-ram, and suspending plus resuming vfio devices adds multiple seconds to the guest pause time. Lastly, there is no loss of connectivity to the guest, because chardev descriptors remain open and connected. These benefits all derive from the core design principle of this mode, which is preserving open descriptors. This approach is very general and can be used to support a wide variety of devices that do not have hardware support for live migration, including but not limited to: vfio, chardev, vhost, vdpa, and iommufd. Some devices need new kernel software interfaces to allow a descriptor to be used in a process that did not originally open it. How? All memory that is mapped by the guest is preserved in place. Indeed, it must be, because it may be the target of DMA requests, which are not quiesced during cpr-transfer. All such memory must be mmap'able in new QEMU. This is easy for named memory-backend objects, as long as they are mapped shared, because they are visible in the file system in both old and new QEMU. Anonymous memory must be allocated using memfd_create rather than MAP_ANON, so the memfd's can be sent to new QEMU. Pages that were locked in memory for DMA in old QEMU remain locked in new QEMU, because the descriptor of the device that locked them remains open. cpr-transfer preserves descriptors by sending them to new QEMU via the cpr-uri, which must support SCM_RIGHTS, and by sending the unique name and value of each descriptor to new QEMU via CPR state. For device descriptors, new QEMU reuses the descriptor when creating the device, rather than opening it again. For memfd descriptors, new QEMU mmap's the preserved memfd when a ramblock is created. CPR state cannot be sent over the normal migration channel, because devices and backends are created prior to reading the channel, so this mode sends CPR state over a second migration channel, specified by cpr-uri. New QEMU reads the second channel prior to creating devices or backends. Example: In this example, we simply restart the same version of QEMU, but in a real scenario one would use a new QEMU binary path in terminal 2. Terminal 1: start old QEMU # qemu-kvm -monitor stdio -object memory-backend-file,id=ram0,size=4G,mem-path=/dev/shm/ram0,share=on -m 4G -machine anon-alloc=memfd ... Terminal 2: start new QEMU # qemu-kvm ... -incoming unix:vm.sock -cpr-uri unix:cpr.sock Terminal 1: QEMU 9.1.50 monitor - type 'help' for more information (qemu) info status VM status: running (qemu) migrate_set_parameter mode cpr-transfer (qemu) migrate_set_parameter cpr-uri unix:cpr.sock (qemu) migrate -d unix:vm.sock (qemu) info status VM status: paused (postmigrate) Terminal 2: QEMU 9.1.50 monitor - type 'help' for more information (qemu) info status VM status: running This patch series implements a minimal version of cpr-transfer. Additional series are ready to be posted to deliver the complete vision described above, including * vfio * chardev * vhost and tap * blockers * migration-test cases * cpr-exec mode Works in progress include: * vdpa * iommufd Changes in V2: * cpr-transfer is the first new mode proposed, and cpr-exec is deferred * anon-alloc does not apply to memory-backend-object * replaced hack with proper synchronization between source and target * defined QEMU_CPR_FILE_MAGIC * addressed misc review comments The first 6 patches below are foundational and are needed for both cpr-transfer mode and the proposed cpr-exec mode. The last 7 patches are specific to cpr-transfer and implement the mechanisms for sharing state across a socket using SCM_RIGHTS. Steve Sistare (13): machine: alloc-anon option migration: cpr-state migration: save cpr mode migration: stop vm earlier for cpr physmem: preserve ram blocks for cpr hostmem-memfd: preserve for cpr migration: SCM_RIGHTS for QEMUFile migration: VMSTATE_FD migration: cpr-transfer save and load migration: cpr-uri parameter migration: cpr-uri option migration: split qmp_migrate migration: cpr-transfer mode backends/hostmem-memfd.c | 12 +- hw/core/machine.c | 19 +++ include/hw/boards.h | 1 + include/migration/cpr.h | 38 ++++++ include/migration/vmstate.h | 9 ++ migration/cpr-transfer.c | 81 +++++++++++++ migration/cpr.c | 269 +++++++++++++++++++++++++++++++++++++++++ migration/meson.build | 2 + migration/migration-hmp-cmds.c | 10 ++ migration/migration.c | 116 ++++++++++++++++-- migration/migration.h | 2 + migration/options.c | 37 +++++- migration/options.h | 1 + migration/qemu-file.c | 83 ++++++++++++- migration/qemu-file.h | 2 + migration/ram.c | 2 + migration/trace-events | 7 ++ migration/vmstate-types.c | 33 +++++ qapi/machine.json | 14 +++ qapi/migration.json | 45 ++++++- qemu-options.hx | 19 +++ stubs/vmstate.c | 7 ++ system/physmem.c | 58 +++++++++ system/trace-events | 3 + system/vl.c | 10 ++ 25 files changed, 857 insertions(+), 23 deletions(-) create mode 100644 include/migration/cpr.h create mode 100644 migration/cpr-transfer.c create mode 100644 migration/cpr.c -- 1.8.3.1
