migration: update docs
Update the migration docs:
Among other changes:
* Added a general list of advice for device authors
* Reordered the section on conditional state (subsections etc)
into the order we prefer.
* Add a note about firmware
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Balamuruhan S <bala24@linux.vnet.ibm.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Message-Id: <20180503191059.19576-1-dgilbert@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
This commit is contained in:
Dr. David Alan Gilbert
Juan Quintela
parent
d37297dc66
commit
edd7080692
@ -28,11 +28,11 @@ the guest to be stopped. Typically the time that the guest is
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unresponsive during live migration is the low hundred of milliseconds
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(notice that this depends on a lot of things).
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Types of migration
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==================
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Transports
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==========
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Now that we have talked about live migration, there are several ways
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to do migration:
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The migration stream is normally just a byte stream that can be passed
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over any transport.
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- tcp migration: do the migration using tcp sockets
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- unix migration: do the migration using unix sockets
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@ -40,16 +40,16 @@ to do migration:
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- fd migration: do the migration using an file descriptor that is
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passed to QEMU. QEMU doesn't care how this file descriptor is opened.
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All these four migration protocols use the same infrastructure to
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In addition, support is included for migration using RDMA, which
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transports the page data using ``RDMA``, where the hardware takes care of
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transporting the pages, and the load on the CPU is much lower. While the
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internals of RDMA migration are a bit different, this isn't really visible
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outside the RAM migration code.
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All these migration protocols use the same infrastructure to
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save/restore state devices. This infrastructure is shared with the
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savevm/loadvm functionality.
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State Live Migration
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====================
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This is used for RAM and block devices. It is not yet ported to vmstate.
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<Fill more information here>
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Common infrastructure
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=====================
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@ -57,60 +57,75 @@ The files, sockets or fd's that carry the migration stream are abstracted by
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the ``QEMUFile`` type (see `migration/qemu-file.h`). In most cases this
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is connected to a subtype of ``QIOChannel`` (see `io/`).
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Saving the state of one device
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==============================
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The state of a device is saved using intermediate buffers. There are
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some helper functions to assist this saving.
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For most devices, the state is saved in a single call to the migration
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infrastructure; these are *non-iterative* devices. The data for these
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devices is sent at the end of precopy migration, when the CPUs are paused.
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There are also *iterative* devices, which contain a very large amount of
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data (e.g. RAM or large tables). See the iterative device section below.
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There is a new concept that we have to explain here: device state
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version. When we migrate a device, we save/load the state as a series
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of fields. Some times, due to bugs or new functionality, we need to
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change the state to store more/different information. We use the
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version to identify each time that we do a change. Each version is
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associated with a series of fields saved. The `save_state` always saves
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the state as the newer version. But `load_state` sometimes is able to
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load state from an older version.
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General advice for device developers
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------------------------------------
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Legacy way
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----------
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- The migration state saved should reflect the device being modelled rather
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than the way your implementation works. That way if you change the implementation
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later the migration stream will stay compatible. That model may include
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internal state that's not directly visible in a register.
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This way is going to disappear as soon as all current users are ported to VMSTATE.
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- When saving a migration stream the device code may walk and check
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the state of the device. These checks might fail in various ways (e.g.
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discovering internal state is corrupt or that the guest has done something bad).
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Consider carefully before asserting/aborting at this point, since the
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normal response from users is that *migration broke their VM* since it had
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apparently been running fine until then. In these error cases, the device
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should log a message indicating the cause of error, and should consider
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putting the device into an error state, allowing the rest of the VM to
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continue execution.
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Each device has to register two functions, one to save the state and
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another to load the state back.
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- The migration might happen at an inconvenient point,
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e.g. right in the middle of the guest reprogramming the device, during
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guest reboot or shutdown or while the device is waiting for external IO.
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It's strongly preferred that migrations do not fail in this situation,
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since in the cloud environment migrations might happen automatically to
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VMs that the administrator doesn't directly control.
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.. code:: c
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- If you do need to fail a migration, ensure that sufficient information
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is logged to identify what went wrong.
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int register_savevm(DeviceState *dev,
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const char *idstr,
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int instance_id,
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int version_id,
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SaveStateHandler *save_state,
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LoadStateHandler *load_state,
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void *opaque);
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- The destination should treat an incoming migration stream as hostile
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(which we do to varying degrees in the existing code). Check that offsets
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into buffers and the like can't cause overruns. Fail the incoming migration
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in the case of a corrupted stream like this.
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typedef void SaveStateHandler(QEMUFile *f, void *opaque);
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typedef int LoadStateHandler(QEMUFile *f, void *opaque, int version_id);
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- Take care with internal device state or behaviour that might become
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migration version dependent. For example, the order of PCI capabilities
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is required to stay constant across migration. Another example would
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be that a special case handled by subsections (see below) might become
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much more common if a default behaviour is changed.
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The important functions for the device state format are the `save_state`
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and `load_state`. Notice that `load_state` receives a version_id
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parameter to know what state format is receiving. `save_state` doesn't
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have a version_id parameter because it always uses the latest version.
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- The state of the source should not be changed or destroyed by the
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outgoing migration. Migrations timing out or being failed by
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higher levels of management, or failures of the destination host are
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not unusual, and in that case the VM is restarted on the source.
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Note that the management layer can validly revert the migration
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even though the QEMU level of migration has succeeded as long as it
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does it before starting execution on the destination.
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- Buses and devices should be able to explicitly specify addresses when
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instantiated, and management tools should use those. For example,
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when hot adding USB devices it's important to specify the ports
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and addresses, since implicit ordering based on the command line order
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may be different on the destination. This can result in the
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device state being loaded into the wrong device.
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VMState
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-------
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The legacy way of saving/loading state of the device had the problem
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that we have to maintain two functions in sync. If we did one change
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in one of them and not in the other, we would get a failed migration.
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VMState changed the way that state is saved/loaded. Instead of using
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a function to save the state and another to load it, it was changed to
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a declarative way of what the state consisted of. Now VMState is able
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to interpret that definition to be able to load/save the state. As
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the state is declared only once, it can't go out of sync in the
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save/load functions.
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Most device data can be described using the ``VMSTATE`` macros (mostly defined
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in ``include/migration/vmstate.h``).
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An example (from hw/input/pckbd.c)
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@ -137,103 +152,99 @@ We registered this with:
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vmstate_register(NULL, 0, &vmstate_kbd, s);
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Note: talk about how vmstate <-> qdev interact, and what the instance ids mean.
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For devices that are `qdev` based, we can register the device in the class
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init function:
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You can search for ``VMSTATE_*`` macros for lots of types used in QEMU in
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include/hw/hw.h.
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.. code:: c
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More about versions
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-------------------
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dc->vmsd = &vmstate_kbd_isa;
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Version numbers are intended for major incompatible changes to the
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migration of a device, and using them breaks backwards-migration
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compatibility; in general most changes can be made by adding Subsections
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(see below) or _TEST macros (see below) which won't break compatibility.
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The VMState macros take care of ensuring that the device data section
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is formatted portably (normally big endian) and make some compile time checks
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against the types of the fields in the structures.
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You can see that there are several version fields:
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VMState macros can include other VMStateDescriptions to store substructures
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(see ``VMSTATE_STRUCT_``), arrays (``VMSTATE_ARRAY_``) and variable length
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arrays (``VMSTATE_VARRAY_``). Various other macros exist for special
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cases.
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- `version_id`: the maximum version_id supported by VMState for that device.
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- `minimum_version_id`: the minimum version_id that VMState is able to understand
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for that device.
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- `minimum_version_id_old`: For devices that were not able to port to vmstate, we can
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assign a function that knows how to read this old state. This field is
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ignored if there is no `load_state_old` handler.
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Note that the format on the wire is still very raw; i.e. a VMSTATE_UINT32
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ends up with a 4 byte bigendian representation on the wire; in the future
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it might be possible to use a more structured format.
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So, VMState is able to read versions from minimum_version_id to
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version_id. And the function ``load_state_old()`` (if present) is able to
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load state from minimum_version_id_old to minimum_version_id. This
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function is deprecated and will be removed when no more users are left.
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Legacy way
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----------
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Saving state will always create a section with the 'version_id' value
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and thus can't be loaded by any older QEMU.
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This way is going to disappear as soon as all current users are ported to VMSTATE;
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although converting existing code can be tricky, and thus 'soon' is relative.
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Massaging functions
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-------------------
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Each device has to register two functions, one to save the state and
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another to load the state back.
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Sometimes, it is not enough to be able to save the state directly
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from one structure, we need to fill the correct values there. One
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example is when we are using kvm. Before saving the cpu state, we
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need to ask kvm to copy to QEMU the state that it is using. And the
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opposite when we are loading the state, we need a way to tell kvm to
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load the state for the cpu that we have just loaded from the QEMUFile.
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.. code:: c
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The functions to do that are inside a vmstate definition, and are called:
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int register_savevm_live(DeviceState *dev,
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const char *idstr,
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int instance_id,
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int version_id,
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SaveVMHandlers *ops,
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void *opaque);
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- ``int (*pre_load)(void *opaque);``
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Two functions in the ``ops`` structure are the `save_state`
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and `load_state` functions. Notice that `load_state` receives a version_id
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parameter to know what state format is receiving. `save_state` doesn't
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have a version_id parameter because it always uses the latest version.
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This function is called before we load the state of one device.
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Note that because the VMState macros still save the data in a raw
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format, in many cases it's possible to replace legacy code
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with a carefully constructed VMState description that matches the
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byte layout of the existing code.
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- ``int (*post_load)(void *opaque, int version_id);``
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Changing migration data structures
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----------------------------------
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This function is called after we load the state of one device.
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- ``int (*pre_save)(void *opaque);``
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This function is called before we save the state of one device.
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Example: You can look at hpet.c, that uses the three function to
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massage the state that is transferred.
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If you use memory API functions that update memory layout outside
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initialization (i.e., in response to a guest action), this is a strong
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indication that you need to call these functions in a `post_load` callback.
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Examples of such memory API functions are:
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- memory_region_add_subregion()
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- memory_region_del_subregion()
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- memory_region_set_readonly()
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- memory_region_set_enabled()
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- memory_region_set_address()
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- memory_region_set_alias_offset()
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When we migrate a device, we save/load the state as a series
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of fields. Sometimes, due to bugs or new functionality, we need to
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change the state to store more/different information. Changing the migration
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state saved for a device can break migration compatibility unless
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care is taken to use the appropriate techniques. In general QEMU tries
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to maintain forward migration compatibility (i.e. migrating from
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QEMU n->n+1) and there are users who benefit from backward compatibility
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as well.
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Subsections
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-----------
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The use of version_id allows to be able to migrate from older versions
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to newer versions of a device. But not the other way around. This
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makes very complicated to fix bugs in stable branches. If we need to
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add anything to the state to fix a bug, we have to disable migration
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to older versions that don't have that bug-fix (i.e. a new field).
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The most common structure change is adding new data, e.g. when adding
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a newer form of device, or adding that state that you previously
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forgot to migrate. This is best solved using a subsection.
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But sometimes, that bug-fix is only needed sometimes, not always. For
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instance, if the device is in the middle of a DMA operation, it is
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using a specific functionality, ....
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It is impossible to create a way to make migration from any version to
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any other version to work. But we can do better than only allowing
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migration from older versions to newer ones. For that fields that are
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only needed sometimes, we add the idea of subsections. A subsection
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is "like" a device vmstate, but with a particularity, it has a Boolean
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function that tells if that values are needed to be sent or not. If
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this functions returns false, the subsection is not sent.
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A subsection is "like" a device vmstate, but with a particularity, it
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has a Boolean function that tells if that values are needed to be sent
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or not. If this functions returns false, the subsection is not sent.
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Subsections have a unique name, that is looked for on the receiving
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side.
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On the receiving side, if we found a subsection for a device that we
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don't understand, we just fail the migration. If we understand all
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the subsections, then we load the state with success.
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the subsections, then we load the state with success. There's no check
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that a subsection is loaded, so a newer QEMU that knows about a subsection
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can (with care) load a stream from an older QEMU that didn't send
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the subsection.
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If the new data is only needed in a rare case, then the subsection
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can be made conditional on that case and the migration will still
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succeed to older QEMUs in most cases. This is OK for data that's
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critical, but in some use cases it's preferred that the migration
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should succeed even with the data missing. To support this the
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subsection can be connected to a device property and from there
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to a versioned machine type.
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One important note is that the post_load() function is called "after"
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loading all subsections, because a newer subsection could change same
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value that it uses.
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value that it uses. A flag, and the combination of pre_load and post_load
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can be used to detect whether a subsection was loaded, and to
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fall back on default behaviour when the subsection isn't present.
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Example:
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@ -288,9 +299,13 @@ save/send this state when we are in the middle of a pio operation
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not enabled, the values on that fields are garbage and don't need to
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be sent.
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Connecting subsections to properties
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------------------------------------
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Using a condition function that checks a 'property' to determine whether
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to send a subsection allows backwards migration compatibility when
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new subsections are added.
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to send a subsection allows backward migration compatibility when
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new subsections are added, especially when combined with versioned
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machine types.
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For example:
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@ -305,21 +320,7 @@ For example:
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Now that subsection will not be generated when using an older
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machine type and the migration stream will be accepted by older
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QEMU versions. pre-load functions can be used to initialise state
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on the newer version so that they default to suitable values
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when loading streams created by older QEMU versions that do not
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generate the subsection.
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In some cases subsections are added for data that had been accidentally
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omitted by earlier versions; if the missing data causes the migration
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process to succeed but the guest to behave badly then it may be better
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to send the subsection and cause the migration to explicitly fail
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with the unknown subsection error. If the bad behaviour only happens
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with certain data values, making the subsection conditional on
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the data value (rather than the machine type) allows migrations to succeed
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in most cases. In general the preference is to tie the subsection to
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the machine type, and allow reliable migrations, unless the behaviour
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from omission of the subsection is really bad.
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QEMU versions.
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Not sending existing elements
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-----------------------------
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@ -328,9 +329,13 @@ Sometimes members of the VMState are no longer needed:
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- removing them will break migration compatibility
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- making them version dependent and bumping the version will break backwards migration compatibility.
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- making them version dependent and bumping the version will break backward migration
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compatibility.
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The best way is to:
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Adding a dummy field into the migration stream is normally the best way to preserve
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compatibility.
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If the field really does need to be removed then:
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a) Add a new property/compatibility/function in the same way for subsections above.
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b) replace the VMSTATE macro with the _TEST version of the macro, e.g.:
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@ -342,18 +347,208 @@ The best way is to:
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``VMSTATE_UINT32_TEST(foo, barstruct, pre_version_baz)``
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Sometime in the future when we no longer care about the ancient versions these can be killed off.
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Note that for backward compatibility it's important to fill in the structure with
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data that the destination will understand.
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Any difference in the predicates on the source and destination will end up
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with different fields being enabled and data being loaded into the wrong
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fields; for this reason conditional fields like this are very fragile.
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Versions
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--------
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Version numbers are intended for major incompatible changes to the
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||||
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
|
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load state from an older version.
|
||||
|
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You can see that there are several 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.
|
||||
- `minimum_version_id_old`: For devices that were not able to port to vmstate, we can
|
||||
assign a function that knows how to read this old state. This field is
|
||||
ignored if there is no `load_state_old` handler.
|
||||
|
||||
VMState is able to read versions from minimum_version_id to
|
||||
version_id. And the function ``load_state_old()`` (if present) is able to
|
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load state from minimum_version_id_old to minimum_version_id. This
|
||||
function is deprecated and will be removed when no more users are left.
|
||||
|
||||
There are *_V* forms of many ``VMSTATE_`` macros to load fields for version dependent fields,
|
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e.g.
|
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|
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.. code:: c
|
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|
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VMSTATE_UINT16_V(ip_id, Slirp, 2),
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|
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only loads that field for versions 2 and newer.
|
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|
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Saving state will always create a section with the 'version_id' value
|
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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.
|
||||
|
||||
Example: You can look at hpet.c, that uses the three function 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 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 memory API functions are:
|
||||
|
||||
- memory_region_add_subregion()
|
||||
- memory_region_del_subregion()
|
||||
- memory_region_set_readonly()
|
||||
- memory_region_set_enabled()
|
||||
- memory_region_set_address()
|
||||
- memory_region_set_alias_offset()
|
||||
|
||||
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 ``save_live_pending`` function that is called repeatedly and must
|
||||
indicate how much more data the iterative data must save. The core
|
||||
migration code will use this to determine when to pause the CPUs
|
||||
and complete the migration.
|
||||
|
||||
- A ``save_live_iterate`` function (called after ``save_live_pending``
|
||||
when there is significant data still to be sent). It 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
|
||||
-----------
|
||||
|
||||
In most migration scenarios there is only a single data path that runs
|
||||
from the source VM to the destination, typically along a single fd (although
|
||||
possibly with another fd or similar for some fast way of throwing pages across).
|
||||
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.
|
||||
|
||||
However, some uses need two way communication; in particular the Postcopy
|
||||
destination needs to be able to request pages on demand from the source.
|
||||
|
||||
For these scenarios there is a 'return path' from the destination to the source;
|
||||
``qemu_file_get_return_path(QEMUFile* fwdpath)`` gives the QEMUFile* for the return
|
||||
path.
|
||||
|
||||
@ -632,3 +827,28 @@ Retro-fitting postcopy to existing clients is possible:
|
||||
identified and the implication understood; for example if the
|
||||
guest memory access is made while holding a lock then all other
|
||||
threads waiting for that lock will also be blocked.
|
||||
|
||||
Firmware
|
||||
========
|
||||
|
||||
Migration migrates the copies of RAM and ROM, and thus when running
|
||||
on the destination it includes the firmware from the source. Even after
|
||||
resetting a VM, the old firmware is used. Only once QEMU has been restarted
|
||||
is the new firmware in use.
|
||||
|
||||
- Changes in firmware size can cause changes in the required RAMBlock size
|
||||
to hold the firmware and thus migration can fail. In practice it's best
|
||||
to pad firmware images to convenient powers of 2 with plenty of space
|
||||
for growth.
|
||||
|
||||
- Care should be taken with device emulation code so that newer
|
||||
emulation code can work with older firmware to allow forward migration.
|
||||
|
||||
- Care should be taken with newer firmware so that backward migration
|
||||
to older systems with older device emulation code will work.
|
||||
|
||||
In some cases it may be best to tie specific firmware versions to specific
|
||||
versioned machine types to cut down on the combinations that will need
|
||||
support. This is also useful when newer versions of firmware outgrow
|
||||
the padding.
|
||||
|
||||
|
||||
Loading…
Reference in New Issue
Block a user