libvirt/src/qemu/MIGRATION.txt

    QEMU Migration Phases
    =====================

Qemu supports only migration protocols 2 and 3 (1 was lacking too many
steps).  Repeating the protocol sequences from libvirt.c:

Sequence v2:

  Src: DumpXML
    - Generate XML to pass to dst

  Dst: Prepare
    - Get ready to accept incoming VM
    - Generate optional cookie to pass to src

  Src: Perform
    - Start migration and wait for send completion
    - Kill off VM if successful, resume if failed

  Dst: Finish
    - Wait for recv completion and check status
    - Kill off VM if unsuccessful

Sequence v3:

  Src: Begin
    - Generate XML to pass to dst
    - Generate optional cookie to pass to dst

  Dst: Prepare
    - Get ready to accept incoming VM
    - Generate optional cookie to pass to src

  Src: Perform
    - Start migration and wait for send completion
    - Generate optional cookie to pass to dst

  Dst: Finish
    - Wait for recv completion and check status
    - Kill off VM if failed, resume if success
    - Generate optional cookie to pass to src

  Src: Confirm
    - Kill off VM if success, resume if failed

    QEMU Migration Locking Rules
    ============================

Migration is a complicated beast which may span across several APIs on both
source and destination side and we need to keep the domain we are migrating in
a consistent state during the whole process.

To avoid anyone from changing the domain in the middle of migration we need to
keep MIGRATION_OUT job active during migration from Begin to Confirm on the
source side and MIGRATION_IN job has to be active from Prepare to Finish on
the destination side.

For this purpose we introduce several helper methods to deal with locking
primitives (described in THREADS.txt) in the right way:

* qemuMigrationJobStart

* qemuMigrationJobContinue

* qemuMigrationJobStartPhase

* qemuMigrationJobSetPhase

* qemuMigrationJobFinish

The sequence of calling qemuMigrationJob* helper methods is as follows:

- The first API of a migration protocol (Prepare or Perform/Begin depending on
  migration type and version) has to start migration job and keep it active:

      qemuMigrationJobStart(driver, vm, QEMU_JOB_MIGRATION_{IN,OUT});
      qemuMigrationJobSetPhase(driver, vm, QEMU_MIGRATION_PHASE_*);
      ...do work...
      qemuMigrationJobContinue(vm);

- All consequent phases except for the last one have to keep the job active:

      if (!qemuMigrationJobIsActive(vm, QEMU_JOB_MIGRATION_{IN,OUT}))
          return;
      qemuMigrationJobStartPhase(driver, vm, QEMU_MIGRATION_PHASE_*);
      ...do work...
      qemuMigrationJobContinue(vm);

- The last migration phase finally finishes the migration job:

      if (!qemuMigrationJobIsActive(vm, QEMU_JOB_MIGRATION_{IN,OUT}))
          return;
      qemuMigrationJobStartPhase(driver, vm, QEMU_MIGRATION_PHASE_*);
      ...do work...
      qemuMigrationJobFinish(driver, vm);

While migration job is running (i.e., after qemuMigrationJobStart* but before
qemuMigrationJob{Continue,Finish}), migration phase can be advanced using

      qemuMigrationJobSetPhase(driver, vm, QEMU_MIGRATION_PHASE_*);