Explain how to use analyze-migration.py, this may help. Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com> Message-Id: <20200330174852.456148-1-marcandre.lureau@redhat.com> Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com> Reviewed-by: Daniel P. Berrangé <berrange@redhat.com> Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
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883 lines
35 KiB
ReStructuredText
=========
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Migration
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=========
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QEMU has code to load/save the state of the guest that it is running.
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These are two complementary operations. Saving the state just does
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that, saves the state for each device that the guest is running.
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Restoring a guest is just the opposite operation: we need to load the
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state of each device.
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For this to work, QEMU has to be launched with the same arguments the
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two times. I.e. it can only restore the state in one guest that has
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the same devices that the one it was saved (this last requirement can
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be relaxed a bit, but for now we can consider that configuration has
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to be exactly the same).
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Once that we are able to save/restore a guest, a new functionality is
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requested: migration. This means that QEMU is able to start in one
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machine and being "migrated" to another machine. I.e. being moved to
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another machine.
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Next was the "live migration" functionality. This is important
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because some guests run with a lot of state (specially RAM), and it
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can take a while to move all state from one machine to another. Live
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migration allows the guest to continue running while the state is
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transferred. Only while the last part of the state is transferred has
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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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Transports
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==========
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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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- exec migration: do the migration using the stdin/stdout through a process.
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- fd migration: do the migration using a 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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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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Debugging
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=========
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The migration stream can be analyzed thanks to `scripts/analyze_migration.py`.
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Example usage:
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.. code-block:: shell
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$ qemu-system-x86_64
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(qemu) migrate "exec:cat > mig"
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$ ./scripts/analyze_migration.py -f mig
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{
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"ram (3)": {
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"section sizes": {
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"pc.ram": "0x0000000008000000",
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...
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See also ``analyze_migration.py -h`` help for more options.
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Common infrastructure
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=====================
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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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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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General advice for device developers
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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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- 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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- 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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- 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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- 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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- 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 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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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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.. code:: c
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static const VMStateDescription vmstate_kbd = {
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.name = "pckbd",
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.version_id = 3,
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.minimum_version_id = 3,
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.fields = (VMStateField[]) {
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VMSTATE_UINT8(write_cmd, KBDState),
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VMSTATE_UINT8(status, KBDState),
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VMSTATE_UINT8(mode, KBDState),
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VMSTATE_UINT8(pending, KBDState),
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VMSTATE_END_OF_LIST()
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}
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};
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We are declaring the state with name "pckbd".
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The `version_id` is 3, and the fields are 4 uint8_t in a KBDState structure.
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We registered this with:
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.. code:: c
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vmstate_register(NULL, 0, &vmstate_kbd, s);
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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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.. code:: c
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dc->vmsd = &vmstate_kbd_isa;
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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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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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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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Legacy way
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----------
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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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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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.. code:: c
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int register_savevm_live(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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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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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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Changing migration data structures
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----------------------------------
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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 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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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. 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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The 'pre_load' and 'post_load' functions on subsections are only
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called if the subsection is loaded.
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One important note is that the outer post_load() function is called "after"
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loading all subsections, because a newer subsection could change the same
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value that it uses. A flag, and the combination of outer pre_load and
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post_load 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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.. code:: c
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static bool ide_drive_pio_state_needed(void *opaque)
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{
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IDEState *s = opaque;
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return ((s->status & DRQ_STAT) != 0)
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|| (s->bus->error_status & BM_STATUS_PIO_RETRY);
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}
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const VMStateDescription vmstate_ide_drive_pio_state = {
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.name = "ide_drive/pio_state",
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.version_id = 1,
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.minimum_version_id = 1,
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.pre_save = ide_drive_pio_pre_save,
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.post_load = ide_drive_pio_post_load,
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.needed = ide_drive_pio_state_needed,
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.fields = (VMStateField[]) {
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VMSTATE_INT32(req_nb_sectors, IDEState),
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VMSTATE_VARRAY_INT32(io_buffer, IDEState, io_buffer_total_len, 1,
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vmstate_info_uint8, uint8_t),
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VMSTATE_INT32(cur_io_buffer_offset, IDEState),
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VMSTATE_INT32(cur_io_buffer_len, IDEState),
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VMSTATE_UINT8(end_transfer_fn_idx, IDEState),
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VMSTATE_INT32(elementary_transfer_size, IDEState),
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VMSTATE_INT32(packet_transfer_size, IDEState),
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VMSTATE_END_OF_LIST()
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}
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};
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const VMStateDescription vmstate_ide_drive = {
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.name = "ide_drive",
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.version_id = 3,
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.minimum_version_id = 0,
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.post_load = ide_drive_post_load,
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.fields = (VMStateField[]) {
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.... several fields ....
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VMSTATE_END_OF_LIST()
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},
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.subsections = (const VMStateDescription*[]) {
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&vmstate_ide_drive_pio_state,
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NULL
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}
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};
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Here we have a subsection for the pio state. We only need to
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save/send this state when we are in the middle of a pio operation
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(that is what ``ide_drive_pio_state_needed()`` checks). If DRQ_STAT is
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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 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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a) Add a new property using ``DEFINE_PROP_BOOL`` - e.g. support-foo and
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default it to true.
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b) Add an entry to the ``hw_compat_`` for the previous version that sets
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the property to false.
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c) Add a static bool support_foo function that tests the property.
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d) Add a subsection with a .needed set to the support_foo function
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e) (potentially) Add an outer pre_load that sets up a default value
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for 'foo' to be used if the subsection isn't loaded.
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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.
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Not sending existing elements
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-----------------------------
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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 backward migration
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compatibility.
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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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``VMSTATE_UINT32(foo, barstruct)``
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becomes
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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
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compatibility; in general most changes can be made by adding Subsections
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(see above) or _TEST macros (see above) which won't break compatibility.
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Each version is 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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You can see that there are several version fields:
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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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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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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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.. code:: c
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VMSTATE_UINT16_V(ip_id, Slirp, 2),
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only loads that field for versions 2 and newer.
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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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Massaging functions
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-------------------
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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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The functions to do that are inside a vmstate definition, and are called:
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- ``int (*pre_load)(void *opaque);``
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This function is called before we load the state of one device.
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- ``int (*post_load)(void *opaque, int version_id);``
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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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- ``int (*post_save)(void *opaque);``
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This function is called after we save the state of one device
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(even upon failure, unless the call to pre_save returned an error).
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Example: You can look at hpet.c, that uses the first three functions
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to massage the state that is transferred.
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The ``VMSTATE_WITH_TMP`` macro may be useful when the migration
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data doesn't match the stored device data well; it allows an
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intermediate temporary structure to be populated with migration
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data and then transferred to the main structure.
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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_nonvolatile()
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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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Iterative device migration
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--------------------------
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Some devices, such as RAM, Block storage or certain platform devices,
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have large amounts of data that would mean that the CPUs would be
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paused for too long if they were sent in one section. For these
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devices an *iterative* approach is taken.
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The iterative devices generally don't use VMState macros
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(although it may be possible in some cases) and instead use
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qemu_put_*/qemu_get_* macros to read/write data to the stream. Specialist
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versions exist for high bandwidth IO.
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An iterative device must provide:
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- A ``save_setup`` function that initialises the data structures and
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transmits a first section containing information on the device. In the
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case of RAM this transmits a list of RAMBlocks and sizes.
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- A ``load_setup`` function that initialises the data structures on the
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destination.
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- A ``save_live_pending`` function that is called repeatedly and must
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indicate how much more data the iterative data must save. The core
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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
|
|
-----------
|
|
|
|
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)
|
|
|
|
Postcopy
|
|
========
|
|
|
|
'Postcopy' migration is a way to deal with migrations that refuse to converge
|
|
(or take too long to converge) its plus side is that there is an upper bound on
|
|
the amount of migration traffic and time it takes, the down side is that during
|
|
the postcopy phase, a failure of *either* side or the network connection causes
|
|
the guest to be lost.
|
|
|
|
In postcopy the destination CPUs are started before all the memory has been
|
|
transferred, and accesses to pages that are yet to be transferred cause
|
|
a fault that's translated by QEMU into a request to the source QEMU.
|
|
|
|
Postcopy can be combined with precopy (i.e. normal migration) so that if precopy
|
|
doesn't finish in a given time the switch is made to postcopy.
|
|
|
|
Enabling postcopy
|
|
-----------------
|
|
|
|
To enable postcopy, issue this command on the monitor (both source and
|
|
destination) prior to the start of migration:
|
|
|
|
``migrate_set_capability postcopy-ram on``
|
|
|
|
The normal commands are then used to start a migration, which is still
|
|
started in precopy mode. Issuing:
|
|
|
|
``migrate_start_postcopy``
|
|
|
|
will now cause the transition from precopy to postcopy.
|
|
It can be issued immediately after migration is started or any
|
|
time later on. Issuing it after the end of a migration is harmless.
|
|
|
|
Blocktime is a postcopy live migration metric, intended to show how
|
|
long the vCPU was in state of interruptable sleep due to pagefault.
|
|
That metric is calculated both for all vCPUs as overlapped value, and
|
|
separately for each vCPU. These values are calculated on destination
|
|
side. To enable postcopy blocktime calculation, enter following
|
|
command on destination monitor:
|
|
|
|
``migrate_set_capability postcopy-blocktime on``
|
|
|
|
Postcopy blocktime can be retrieved by query-migrate qmp command.
|
|
postcopy-blocktime value of qmp command will show overlapped blocking
|
|
time for all vCPU, postcopy-vcpu-blocktime will show list of blocking
|
|
time per vCPU.
|
|
|
|
.. note::
|
|
During the postcopy phase, the bandwidth limits set using
|
|
``migrate_set_speed`` is ignored (to avoid delaying requested pages that
|
|
the destination is waiting for).
|
|
|
|
Postcopy device transfer
|
|
------------------------
|
|
|
|
Loading of device data may cause the device emulation to access guest RAM
|
|
that may trigger faults that have to be resolved by the source, as such
|
|
the migration stream has to be able to respond with page data *during* the
|
|
device load, and hence the device data has to be read from the stream completely
|
|
before the device load begins to free the stream up. This is achieved by
|
|
'packaging' the device data into a blob that's read in one go.
|
|
|
|
Source behaviour
|
|
----------------
|
|
|
|
Until postcopy is entered the migration stream is identical to normal
|
|
precopy, except for the addition of a 'postcopy advise' command at
|
|
the beginning, to tell the destination that postcopy might happen.
|
|
When postcopy starts the source sends the page discard data and then
|
|
forms the 'package' containing:
|
|
|
|
- Command: 'postcopy listen'
|
|
- The device state
|
|
|
|
A series of sections, identical to the precopy streams device state stream
|
|
containing everything except postcopiable devices (i.e. RAM)
|
|
- Command: 'postcopy run'
|
|
|
|
The 'package' is sent as the data part of a Command: ``CMD_PACKAGED``, and the
|
|
contents are formatted in the same way as the main migration stream.
|
|
|
|
During postcopy the source scans the list of dirty pages and sends them
|
|
to the destination without being requested (in much the same way as precopy),
|
|
however when a page request is received from the destination, the dirty page
|
|
scanning restarts from the requested location. This causes requested pages
|
|
to be sent quickly, and also causes pages directly after the requested page
|
|
to be sent quickly in the hope that those pages are likely to be used
|
|
by the destination soon.
|
|
|
|
Destination behaviour
|
|
---------------------
|
|
|
|
Initially the destination looks the same as precopy, with a single thread
|
|
reading the migration stream; the 'postcopy advise' and 'discard' commands
|
|
are processed to change the way RAM is managed, but don't affect the stream
|
|
processing.
|
|
|
|
::
|
|
|
|
------------------------------------------------------------------------------
|
|
1 2 3 4 5 6 7
|
|
main -----DISCARD-CMD_PACKAGED ( LISTEN DEVICE DEVICE DEVICE RUN )
|
|
thread | |
|
|
| (page request)
|
|
| \___
|
|
v \
|
|
listen thread: --- page -- page -- page -- page -- page --
|
|
|
|
a b c
|
|
------------------------------------------------------------------------------
|
|
|
|
- On receipt of ``CMD_PACKAGED`` (1)
|
|
|
|
All the data associated with the package - the ( ... ) section in the diagram -
|
|
is read into memory, and the main thread recurses into qemu_loadvm_state_main
|
|
to process the contents of the package (2) which contains commands (3,6) and
|
|
devices (4...)
|
|
|
|
- On receipt of 'postcopy listen' - 3 -(i.e. the 1st command in the package)
|
|
|
|
a new thread (a) is started that takes over servicing the migration stream,
|
|
while the main thread carries on loading the package. It loads normal
|
|
background page data (b) but if during a device load a fault happens (5)
|
|
the returned page (c) is loaded by the listen thread allowing the main
|
|
threads device load to carry on.
|
|
|
|
- The last thing in the ``CMD_PACKAGED`` is a 'RUN' command (6)
|
|
|
|
letting the destination CPUs start running. At the end of the
|
|
``CMD_PACKAGED`` (7) the main thread returns to normal running behaviour and
|
|
is no longer used by migration, while the listen thread carries on servicing
|
|
page data until the end of migration.
|
|
|
|
Postcopy states
|
|
---------------
|
|
|
|
Postcopy moves through a series of states (see postcopy_state) from
|
|
ADVISE->DISCARD->LISTEN->RUNNING->END
|
|
|
|
- Advise
|
|
|
|
Set at the start of migration if postcopy is enabled, even
|
|
if it hasn't had the start command; here the destination
|
|
checks that its OS has the support needed for postcopy, and performs
|
|
setup to ensure the RAM mappings are suitable for later postcopy.
|
|
The destination will fail early in migration at this point if the
|
|
required OS support is not present.
|
|
(Triggered by reception of POSTCOPY_ADVISE command)
|
|
|
|
- Discard
|
|
|
|
Entered on receipt of the first 'discard' command; prior to
|
|
the first Discard being performed, hugepages are switched off
|
|
(using madvise) to ensure that no new huge pages are created
|
|
during the postcopy phase, and to cause any huge pages that
|
|
have discards on them to be broken.
|
|
|
|
- Listen
|
|
|
|
The first command in the package, POSTCOPY_LISTEN, switches
|
|
the destination state to Listen, and starts a new thread
|
|
(the 'listen thread') which takes over the job of receiving
|
|
pages off the migration stream, while the main thread carries
|
|
on processing the blob. With this thread able to process page
|
|
reception, the destination now 'sensitises' the RAM to detect
|
|
any access to missing pages (on Linux using the 'userfault'
|
|
system).
|
|
|
|
- Running
|
|
|
|
POSTCOPY_RUN causes the destination to synchronise all
|
|
state and start the CPUs and IO devices running. The main
|
|
thread now finishes processing the migration package and
|
|
now carries on as it would for normal precopy migration
|
|
(although it can't do the cleanup it would do as it
|
|
finishes a normal migration).
|
|
|
|
- End
|
|
|
|
The listen thread can now quit, and perform the cleanup of migration
|
|
state, the migration is now complete.
|
|
|
|
Source side page maps
|
|
---------------------
|
|
|
|
The source side keeps two bitmaps during postcopy; 'the migration bitmap'
|
|
and 'unsent map'. The 'migration bitmap' is basically the same as in
|
|
the precopy case, and holds a bit to indicate that page is 'dirty' -
|
|
i.e. needs sending. During the precopy phase this is updated as the CPU
|
|
dirties pages, however during postcopy the CPUs are stopped and nothing
|
|
should dirty anything any more.
|
|
|
|
The 'unsent map' is used for the transition to postcopy. It is a bitmap that
|
|
has a bit cleared whenever a page is sent to the destination, however during
|
|
the transition to postcopy mode it is combined with the migration bitmap
|
|
to form a set of pages that:
|
|
|
|
a) Have been sent but then redirtied (which must be discarded)
|
|
b) Have not yet been sent - which also must be discarded to cause any
|
|
transparent huge pages built during precopy to be broken.
|
|
|
|
Note that the contents of the unsentmap are sacrificed during the calculation
|
|
of the discard set and thus aren't valid once in postcopy. The dirtymap
|
|
is still valid and is used to ensure that no page is sent more than once. Any
|
|
request for a page that has already been sent is ignored. Duplicate requests
|
|
such as this can happen as a page is sent at about the same time the
|
|
destination accesses it.
|
|
|
|
Postcopy with hugepages
|
|
-----------------------
|
|
|
|
Postcopy now works with hugetlbfs backed memory:
|
|
|
|
a) The linux kernel on the destination must support userfault on hugepages.
|
|
b) The huge-page configuration on the source and destination VMs must be
|
|
identical; i.e. RAMBlocks on both sides must use the same page size.
|
|
c) Note that ``-mem-path /dev/hugepages`` will fall back to allocating normal
|
|
RAM if it doesn't have enough hugepages, triggering (b) to fail.
|
|
Using ``-mem-prealloc`` enforces the allocation using hugepages.
|
|
d) Care should be taken with the size of hugepage used; postcopy with 2MB
|
|
hugepages works well, however 1GB hugepages are likely to be problematic
|
|
since it takes ~1 second to transfer a 1GB hugepage across a 10Gbps link,
|
|
and until the full page is transferred the destination thread is blocked.
|
|
|
|
Postcopy with shared memory
|
|
---------------------------
|
|
|
|
Postcopy migration with shared memory needs explicit support from the other
|
|
processes that share memory and from QEMU. There are restrictions on the type of
|
|
memory that userfault can support shared.
|
|
|
|
The Linux kernel userfault support works on `/dev/shm` memory and on `hugetlbfs`
|
|
(although the kernel doesn't provide an equivalent to `madvise(MADV_DONTNEED)`
|
|
for hugetlbfs which may be a problem in some configurations).
|
|
|
|
The vhost-user code in QEMU supports clients that have Postcopy support,
|
|
and the `vhost-user-bridge` (in `tests/`) and the DPDK package have changes
|
|
to support postcopy.
|
|
|
|
The client needs to open a userfaultfd and register the areas
|
|
of memory that it maps with userfault. The client must then pass the
|
|
userfaultfd back to QEMU together with a mapping table that allows
|
|
fault addresses in the clients address space to be converted back to
|
|
RAMBlock/offsets. The client's userfaultfd is added to the postcopy
|
|
fault-thread and page requests are made on behalf of the client by QEMU.
|
|
QEMU performs 'wake' operations on the client's userfaultfd to allow it
|
|
to continue after a page has arrived.
|
|
|
|
.. note::
|
|
There are two future improvements that would be nice:
|
|
a) Some way to make QEMU ignorant of the addresses in the clients
|
|
address space
|
|
b) Avoiding the need for QEMU to perform ufd-wake calls after the
|
|
pages have arrived
|
|
|
|
Retro-fitting postcopy to existing clients is possible:
|
|
a) A mechanism is needed for the registration with userfault as above,
|
|
and the registration needs to be coordinated with the phases of
|
|
postcopy. In vhost-user extra messages are added to the existing
|
|
control channel.
|
|
b) Any thread that can block due to guest memory accesses must be
|
|
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.
|
|
|