= Migration = QEMU has code to load/save the state of the guest that it is running. These are two complementary operations. Saving the state just does that, saves the state for each device that the guest is running. Restoring a guest is just the opposite operation: we need to load the state of each device. For this to work, QEMU has to be launched with the same arguments the two times. I.e. it can only restore the state in one guest that has the same devices that the one it was saved (this last requirement can be relaxed a bit, but for now we can consider that configuration has to be exactly the same). Once that we are able to save/restore a guest, a new functionality is requested: migration. This means that QEMU is able to start in one machine and being "migrated" to another machine. I.e. being moved to another machine. Next was the "live migration" functionality. This is important because some guests run with a lot of state (specially RAM), and it can take a while to move all state from one machine to another. Live migration allows the guest to continue running while the state is transferred. Only while the last part of the state is transferred has the guest to be stopped. Typically the time that the guest is unresponsive during live migration is the low hundred of milliseconds (notice that this depends on a lot of things). === Types of migration === Now that we have talked about live migration, there are several ways to do migration: - tcp migration: do the migration using tcp sockets - unix migration: do the migration using unix sockets - exec migration: do the migration using the stdin/stdout through a process. - fd migration: do the migration using an file descriptor that is passed to QEMU. QEMU doesn't care how this file descriptor is opened. All these four migration protocols use the same infrastructure to save/restore state devices. This infrastructure is shared with the savevm/loadvm functionality. === State Live Migration === This is used for RAM and block devices. It is not yet ported to vmstate. === What is the common infrastructure === QEMU uses a QEMUFile abstraction to be able to do migration. Any type of migration that wants to use QEMU infrastructure has to create a QEMUFile with: QEMUFile *qemu_fopen_ops(void *opaque, QEMUFilePutBufferFunc *put_buffer, QEMUFileGetBufferFunc *get_buffer, QEMUFileCloseFunc *close); The functions have the following functionality: This function writes a chunk of data to a file at the given position. The pos argument can be ignored if the file is only used for streaming. The handler should try to write all of the data it can. typedef int (QEMUFilePutBufferFunc)(void *opaque, const uint8_t *buf, int64_t pos, int size); Read a chunk of data from a file at the given position. The pos argument can be ignored if the file is only be used for streaming. The number of bytes actually read should be returned. typedef int (QEMUFileGetBufferFunc)(void *opaque, uint8_t *buf, int64_t pos, int size); Close a file and return an error code. typedef int (QEMUFileCloseFunc)(void *opaque); You can use any internal state that you need using the opaque void * pointer that is passed to all functions. The important functions for us are put_buffer()/get_buffer() that allow to write/read a buffer into the QEMUFile. === How to save the state of one device === The state of a device is saved using intermediate buffers. There are some helper functions to assist this saving. There is a new concept that we have to explain here: device state version. When we migrate a device, we save/load the state as a series of fields. Some times, due to bugs or new functionality, we need to change the state to store more/different information. We use the version to identify each time that we do a change. Each version is associated with a series of fields saved. The save_state always saves the state as the newer version. But load_state sometimes is able to load state from an older version. === Legacy way === This way is going to disappear as soon as all current users are ported to VMSTATE. Each device has to register two functions, one to save the state and another to load the state back. int register_savevm(DeviceState *dev, const char *idstr, int instance_id,