Migration framework — QEMU documentation

# Migration framework

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).

## Transports

The migration stream is normally just a byte stream that can be passed over any transport.

- 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 a file descriptor that is passed to QEMU. QEMU doesn’t care how this file descriptor is opened.

- file migration: do the migration using a file that is passed to QEMU by path. A file offset option is supported to allow a management application to add its own metadata to the start of the file without QEMU interference. Note that QEMU does not flush cached file data/metadata at the end of migration.

In addition, support is included for migration using RDMA, which transports the page data using `RDMA`, where the hardware takes care of transporting the pages, and the load on the CPU is much lower. While the internals of RDMA migration are a bit different, this isn’t really visible outside the RAM migration code.

All these migration protocols use the same infrastructure to save/restore state devices. This infrastructure is shared with the savevm/loadvm functionality.

## Common infrastructure

The files, sockets or fd’s that carry the migration stream are abstracted by the `QEMUFile` type (see `migration/qemu-file.h`). In most cases this is connected to a subtype of `QIOChannel` (see `io/`).

## Saving the state of one device

For most devices, the state is saved in a single call to the migration infrastructure; these are non-iterative devices. The data for these devices is sent at the end of precopy migration, when the CPUs are paused. There are also iterative devices, which contain a very large amount of data (e.g. RAM or large tables). See the iterative device section below.

### General advice for device developers

- The migration state saved should reflect the device being modelled rather than the way your implementation works. That way if you change the implementation later the migration stream will stay compatible. That model may include internal state that’s not directly visible in a register.

- When saving a migration stream the device code may walk and check the state of the device. These checks might fail in various ways (e.g. discovering internal state is corrupt or that the guest has done something bad). Consider carefully before asserting/aborting at this point, since the normal response from users is that migration broke their VM since it had apparently been running fine until then. In these error cases, the device should log a message indicating the cause of error, and should consider putting the device into an error state, allowing the rest of the VM to continue execution.

- The migration might happen at an inconvenient point, e.g. right in the middle of the guest reprogramming the device, during guest reboot or shutdown or while the device is waiting for external IO. It’s strongly preferred that migrations do not fail in this situation, since in the cloud environment migrations might happen automatically to VMs that the administrator doesn’t directly control.

- If you do need to fail a migration, ensure that sufficient information is logged to identify what went wrong.

- The destination should treat an incoming migration stream as hostile (which we do to varying degrees in the existing code). Check that offsets into buffers and the like can’t cause overruns. Fail the incoming migration in the case of a corrupted stream like this.

- Take care with internal device state or behaviour that might become migration version dependent. For example, the order of PCI capabilities is required to stay constant across migration. Another example would be that a special case handled by subsections (see below) might become much more common if a default behaviour is changed.

- The state of the source should not be changed or destroyed by the outgoing migration. Migrations timing out or being failed by higher levels of management, or failures of the destination host are not unusual, and in that case the VM is restarted on the source. Note that the management layer can validly revert the migration even though the QEMU level of migration has succeeded as long as it does it before starting execution on the destination.

- Buses and devices should be able to explicitly specify addresses when instantiated, and management tools should use those. For example, when hot adding USB devices it’s important to specify the ports and addresses, since implicit ordering based on the command line order may be different on the destination. This can result in the device state being loaded into the wrong device.

### VMState

Most device data can be described using the `VMSTATE` macros (mostly defined in `include/migration/vmstate.h`).

An example (from hw/input/pckbd.c)

    static const VMStateDescription vmstate_kbd = {
        .name = "pckbd",
        .version_id = 3,
        .minimum_version_id = 3,
        .fields = (const VMStateField[]) {
            VMSTATE_UINT8(write_cmd, KBDState),
            VMSTATE_UINT8(status, KBDState),
            VMSTATE_UINT8(mode, KBDState),
            VMSTATE_UINT8(pending, KBDState),
            VMSTATE_END_OF_LIST()
        }
    };

We are declaring the state with name “pckbd”. The `version_id` is 3, and there are 4 uint8_t fields in the KBDState structure. We registered this `VMSTATEDescription` with one of the following functions. The first one will generate a device `instance_id` different for each registration. Use the second one if you already have an id that is different for each instance of the device:

    vmstate_register_any(NULL, &vmstate_kbd, s);
    vmstate_register(NULL, instance_id, &vmstate_kbd, s);

For devices that are `qdev` based, we can register the device in the class init function:

    dc->vmsd = &vmstate_kbd_isa;

The VMState macros take care of ensuring that the device data section is formatted portably (normally big endian) and make some compile time checks against the types of the fields in the structures.

VMState macros can include other VMStateDescriptions to store substructures (see `VMSTATE_STRUCT_`), arrays (`VMSTATE_ARRAY_`) and variable length arrays (`VMSTATE_VARRAY_`). Various other macros exist for special cases.

Note that the format on the wire is still very raw; i.e. a VMSTATE_UINT32 ends up with a 4 byte bigendian representation on the wire; in the future it might be possible to use a more structured format.

### Legacy way

This way is going to disappear as soon as all current users are ported to VMSTATE; although converting existing code can be tricky, and thus ‘soon’ is relative.

Each device has to register two functions, one to save the state and another to load the state back.

    int register_savevm_live(const char *idstr,
                             int instance_id,
                             int version_id,
                             SaveVMHandlers *ops,
                             void *opaque);

Two functions in the `ops` structure are the `save_state` and `load_state` functions. Notice that `load_state` receives a version_id parameter to know what state format is receiving. `save_state` doesn’t have a version_id parameter because it always uses the latest version.

Note that because the VMState macros still save the data in a raw format, in many cases it’s possible to replace legacy code with a carefully constructed VMState description that matches the byte layout of the existing code.

### Changing migration data structures

When we migrate a device, we save/load the state as a series of fields. Sometimes, due to bugs or new functionality, we need to change the state to store more/different information. Changing the migration state saved for a device can break migration compatibility unless care is taken to use the appropriate techniques. In general QEMU tries to maintain forward migration compatibility (i.e. migrating from QEMU n->n+1) and there are users who benefit from backward compatibility as well.

### Subsections

The most common structure change is adding new data, e.g. when adding a newer form of device, or adding that state that you previously forgot to migrate. This is best solved using a subsection.

A subsection is “like” a device vmstate, but with a particularity, it has a Boolean function that tells if that values are needed to be sent or not. If this functions returns false, the subsection is not sent. Subsections have a unique name, that is looked for on the receiving side.

On the receiving side, if we found a subsection for a device that we don’t understand, we just fail the migration. If we understand all the subsections, then we load the state with success. There’s no check that a subsection is loaded, so a newer QEMU that knows about a subsection can (with care) load a stream from an older QEMU that didn’t send the subsection.

If the new data is only needed in a rare case, then the subsection can be made conditional on that case and the migration will still succeed to older QEMUs in most cases. This is OK for data that’s critical, but in some use cases it’s preferred that the migration should succeed even with the data missing. To support this the subsection can be connected to a device property and from there to a versioned machine type.

The ‘pre_load’ and ‘post_load’ functions on subsections are only called if the subsection is loaded.

One important note is that the outer post_load() function is called “after” loading all subsections, because a newer subsection could change the same value that it uses. A flag, and the combination of outer pre_load and post_load can be used to detect whether a subsection was loaded, and to fall back on default behaviour when the subsection isn’t present.

Example:

    static bool ide_drive_pio_state_needed(void *opaque)
    {
        IDEState *s = opaque;

        return ((s->status & DRQ_STAT) != 0)
            || (s->bus->error_status & BM_STATUS_PIO_RETRY);
    }

    const VMStateDescription vmstate_ide_drive_pio_state = {
        .name = "ide_drive/pio_state",
        .version_id = 1,
        .minimum_version_id = 1,
        .pre_save = ide_drive_pio_pre_save,
        .post_load = ide_drive_pio_post_load,
        .needed = ide_drive_pio_state_needed,
        .fields = (const VMStateField[]) {
            VMSTATE_INT32(req_nb_sectors, IDEState),
            VMSTATE_VARRAY_INT32(io_buffer, IDEState, io_buffer_total_len, 1,
                                 vmstate_info_uint8, uint8_t),
            VMSTATE_INT32(cur_io_buffer_offset, IDEState),
            VMSTATE_INT32(cur_io_buffer_len, IDEState),
            VMSTATE_UINT8(end_transfer_fn_idx, IDEState),
            VMSTATE_INT32(elementary_transfer_size, IDEState),
            VMSTATE_INT32(packet_transfer_size, IDEState),
            VMSTATE_END_OF_LIST()
        }
    };

    const VMStateDescription vmstate_ide_drive = {
        .name = "ide_drive",
        .version_id = 3,
        .minimum_version_id = 0,
        .post_load = ide_drive_post_load,
        .fields = (const VMStateField[]) {
            .... several fields ....
            VMSTATE_END_OF_LIST()
        },
        .subsections = (const VMStateDescription * const []) {
            &vmstate_ide_drive_pio_state,
            NULL
        }
    };

Here we have a subsection for the pio state. We only need to save/send this state when we are in the middle of a pio operation (that is what `ide_drive_pio_state_needed()` checks). If DRQ_STAT is not enabled, the values on that fields are garbage and don’t need to be sent.

### Connecting subsections to properties

Using a condition function that checks a ‘property’ to determine whether to send a subsection allows backward migration compatibility when new subsections are added, especially when combined with versioned machine types.

For example:

> 1. Add a new property using `DEFINE_PROP_BOOL` - e.g. support-foo and default it to true.

> 2. Add an entry to the `hw_compat_` for the previous version that sets the property to false.

> 3. Add a static bool support_foo function that tests the property.

> 4. Add a subsection with a .needed set to the support_foo function

> 5. (potentially) Add an outer pre_load that sets up a default value for ‘foo’ to be used if the subsection isn’t loaded.

Now that subsection will not be generated when using an older machine type and the migration stream will be accepted by older QEMU versions.

### Not sending existing elements

Sometimes members of the VMState are no longer needed:

> - removing them will break migration compatibility

> - making them version dependent and bumping the version will break backward migration compatibility.

Adding a dummy field into the migration stream is normally the best way to preserve compatibility.

If the field really does need to be removed then:

> 1. Add a new property/compatibility/function in the same way for subsections above.

> 2. replace the VMSTATE macro with the _TEST version of the macro, e.g.:

> > `VMSTATE_UINT32(foo, barstruct)`

> > becomes

> > `VMSTATE_UINT32_TEST(foo, barstruct, pre_version_baz)`

> > Sometime in the future when we no longer care about the ancient versions these can be killed off. Note that for backward compatibility it’s important to fill in the structure with data that the destination will understand.

Any difference in the predicates on the source and destination will end up with different fields being enabled and data being loaded into the wrong fields; for this reason conditional fields like this are very fragile.

### Versions

Version numbers are intended for major incompatible changes to the migration of a device, and using them breaks backward-migration compatibility; in general most changes can be made by adding Subsections (see above) or _TEST macros (see above) which won’t break compatibility.

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.

You can see that there are two version fields:

- `version_id`: the maximum version_id supported by VMState for that device.

- `minimum_version_id`: the minimum version_id that VMState is able to understand for that device.

VMState is able to read versions from minimum_version_id to version_id.

There are _V forms of many `VMSTATE_` macros to load fields for version dependent fields, e.g.

    VMSTATE_UINT16_V(ip_id, Slirp, 2),

only loads that field for versions 2 and newer.

Saving state will always create a section with the ‘version_id’ value and thus can’t be loaded by any older QEMU.

### Massaging functions

Sometimes, it is not enough to be able to save the state directly from one structure, we need to fill the correct values there. One example is when we are using kvm. Before saving the cpu state, we need to ask kvm to copy to QEMU the state that it is using. And the opposite when we are loading the state, we need a way to tell kvm to load the state for the cpu that we have just loaded from the QEMUFile.

The functions to do that are inside a vmstate definition, and are called:

- `int (*pre_load)(void *opaque);`

  This function is called before we load the state of one device.

- `int (*post_load)(void *opaque, int version_id);`

  This function is called after we load the state of one device.

- `int (*pre_save)(void *opaque);`

  This function is called before we save the state of one device.

- `int (*post_save)(void *opaque);`

  This function is called after we save the state of one device (even upon failure, unless the call to pre_save returned an error).

Example: You can look at hpet.c, that uses the first three functions to massage the state that is transferred.

The `VMSTATE_WITH_TMP` macro may be useful when the migration data doesn’t match the stored device data well; it allows an intermediate temporary structure to be populated with migration data and then transferred to the main structure.

If you use memory or portio_list API functions that update memory layout outside initialization (i.e., in response to a guest action), this is a strong indication that you need to call these functions in a `post_load` callback. Examples of such API functions are:

> - memory_region_add_subregion()

> - memory_region_del_subregion()

> - memory_region_set_readonly()

> - memory_region_set_nonvolatile()

> - memory_region_set_enabled()

> - memory_region_set_address()

> - memory_region_set_alias_offset()

> - portio_list_set_address()

> - portio_list_set_enabled()

### Iterative device migration

Some devices, such as RAM, Block storage or certain platform devices, have large amounts of data that would mean that the CPUs would be paused for too long if they were sent in one section. For these devices an iterative approach is taken.

The iterative devices generally don’t use VMState macros (although it may be possible in some cases) and instead use qemu_put_*/qemu_get_* macros to read/write data to the stream. Specialist versions exist for high bandwidth IO.

An iterative device must provide:

> - A `save_setup` function that initialises the data structures and transmits a first section containing information on the device. In the case of RAM this transmits a list of RAMBlocks and sizes.

> - A `load_setup` function that initialises the data structures on the destination.

> - A `state_pending_exact` function that indicates how much more data we must save. The core migration code will use this to determine when to pause the CPUs and complete the migration.

> - A `state_pending_estimate` function that indicates how much more data we must save. When the estimated amount is smaller than the threshold, we call `state_pending_exact`.

> - A `save_live_iterate` function should send a chunk of data until the point that stream bandwidth limits tell it to stop. Each call generates one section.

> - A `save_live_complete_precopy` function that must transmit the last section for the device containing any remaining data.

> - A `load_state` function used to load sections generated by any of the save functions that generate sections.

> - `cleanup` functions for both save and load that are called at the end of migration.

Note that the contents of the sections for iterative migration tend to be open-coded by the devices; care should be taken in parsing the results and structuring the stream to make them easy to validate.

### Device ordering

There are cases in which the ordering of device loading matters; for example in some systems where a device may assert an interrupt during loading, if the interrupt controller is loaded later then it might lose the state.

Some ordering is implicitly provided by the order in which the machine definition creates devices, however this is somewhat fragile.

The `MigrationPriority` enum provides a means of explicitly enforcing ordering. Numerically higher priorities are loaded earlier. The priority is set by setting the `priority` field of the top level `VMStateDescription` for the device.

## Stream structure

The stream tries to be word and endian agnostic, allowing migration between hosts of different characteristics running the same VM.

> - Header

>   - Magic

>   - Version

>   - VM configuration section

>     > - Machine type

>     > - Target page bits

> - List of sections Each section contains a device, or one iteration of a device save.

>   - section type

>   - section id

>   - ID string (First section of each device)

>   - instance id (First section of each device)

>   - version id (First section of each device)

>   - <device data>

>   - Footer mark

> - EOF mark

> - VM Description structure Consisting of a JSON description of the contents for analysis only

The `device data` in each section consists of the data produced by the code described above. For non-iterative devices they have a single section; iterative devices have an initial and last section and a set of parts in between. Note that there is very little checking by the common code of the integrity of the `device data` contents, that’s up to the devices themselves. The `footer mark` provides a little bit of protection for the case where the receiving side reads more or less data than expected.

The `ID string` is normally unique, having been formed from a bus name and device address, PCI devices and storage devices hung off PCI controllers fit this pattern well. Some devices are fixed single instances (e.g. “pc-ram”). Others (especially either older devices or system devices which for some reason don’t have a bus concept) make use of the `instance id` for otherwise identically named devices.

### Return path

Only a unidirectional stream is required for normal migration, however a `return path` can be created when bidirectional communication is desired. This is primarily used by postcopy, but is also used to return a success flag to the source at the end of migration.

`qemu_file_get_return_path(QEMUFile* fwdpath)` gives the QEMUFile* for the return path.

> Source side

> > Forward path - written by migration thread Return path - opened by main thread, read by return-path thread

> Destination side

> > Forward path - read by main thread Return path - opened by main thread, written by main thread AND postcopy thread (protected by rp_mutex)