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diff --git a/docs/devel/migration.rst b/docs/devel/migration.rst new file mode 100644 index 000000000..240125348 --- /dev/null +++ b/docs/devel/migration.rst @@ -0,0 +1,883 @@ +========= +Migration +========= + +QEMU has code to load/save the state of the guest that it is running. +These are two complementary operations. Saving the state just does +that, saves the state for each device that the guest is running. +Restoring a guest is just the opposite operation: we need to load the +state of each device. + +For this to work, QEMU has to be launched with the same arguments the +two times. I.e. it can only restore the state in one guest that has +the same devices that the one it was saved (this last requirement can +be relaxed a bit, but for now we can consider that configuration has +to be exactly the same). + +Once that we are able to save/restore a guest, a new functionality is +requested: migration. This means that QEMU is able to start in one +machine and being "migrated" to another machine. I.e. being moved to +another machine. + +Next was the "live migration" functionality. This is important +because some guests run with a lot of state (specially RAM), and it +can take a while to move all state from one machine to another. Live +migration allows the guest to continue running while the state is +transferred. Only while the last part of the state is transferred has +the guest to be stopped. Typically the time that the guest is +unresponsive during live migration is the low hundred of milliseconds +(notice that this depends on a lot of things). + +Transports +========== + +The migration stream is normally just a byte stream that can be passed +over any transport. + +- tcp migration: do the migration using tcp sockets +- unix migration: do the migration using unix sockets +- exec migration: do the migration using the stdin/stdout through a process. +- fd migration: do the migration using a file descriptor that is + passed to QEMU. QEMU doesn't care how this file descriptor is opened. + +In addition, support is included for migration using RDMA, which +transports the page data using ``RDMA``, where the hardware takes care of +transporting the pages, and the load on the CPU is much lower. While the +internals of RDMA migration are a bit different, this isn't really visible +outside the RAM migration code. + +All these migration protocols use the same infrastructure to +save/restore state devices. This infrastructure is shared with the +savevm/loadvm functionality. + +Debugging +========= + +The migration stream can be analyzed thanks to ``scripts/analyze-migration.py``. + +Example usage: + +.. code-block:: shell + + $ qemu-system-x86_64 -display none -monitor stdio + (qemu) migrate "exec:cat > mig" + (qemu) q + $ ./scripts/analyze-migration.py -f mig + { + "ram (3)": { + "section sizes": { + "pc.ram": "0x0000000008000000", + ... + +See also ``analyze-migration.py -h`` help for more options. + +Common infrastructure +===================== + +The files, sockets or fd's that carry the migration stream are abstracted by +the ``QEMUFile`` type (see ``migration/qemu-file.h``). In most cases this +is connected to a subtype of ``QIOChannel`` (see ``io/``). + + +Saving the state of one device +============================== + +For most devices, the state is saved in a single call to the migration +infrastructure; these are *non-iterative* devices. The data for these +devices is sent at the end of precopy migration, when the CPUs are paused. +There are also *iterative* devices, which contain a very large amount of +data (e.g. RAM or large tables). See the iterative device section below. + +General advice for device developers +------------------------------------ + +- The migration state saved should reflect the device being modelled rather + than the way your implementation works. That way if you change the implementation + later the migration stream will stay compatible. That model may include + internal state that's not directly visible in a register. + +- When saving a migration stream the device code may walk and check + the state of the device. These checks might fail in various ways (e.g. + discovering internal state is corrupt or that the guest has done something bad). + Consider carefully before asserting/aborting at this point, since the + normal response from users is that *migration broke their VM* since it had + apparently been running fine until then. In these error cases, the device + should log a message indicating the cause of error, and should consider + putting the device into an error state, allowing the rest of the VM to + continue execution. + +- The migration might happen at an inconvenient point, + e.g. right in the middle of the guest reprogramming the device, during + guest reboot or shutdown or while the device is waiting for external IO. + It's strongly preferred that migrations do not fail in this situation, + since in the cloud environment migrations might happen automatically to + VMs that the administrator doesn't directly control. + +- If you do need to fail a migration, ensure that sufficient information + is logged to identify what went wrong. + +- The destination should treat an incoming migration stream as hostile + (which we do to varying degrees in the existing code). Check that offsets + into buffers and the like can't cause overruns. Fail the incoming migration + in the case of a corrupted stream like this. + +- Take care with internal device state or behaviour that might become + migration version dependent. For example, the order of PCI capabilities + is required to stay constant across migration. Another example would + be that a special case handled by subsections (see below) might become + much more common if a default behaviour is changed. + +- The state of the source should not be changed or destroyed by the + outgoing migration. Migrations timing out or being failed by + higher levels of management, or failures of the destination host are + not unusual, and in that case the VM is restarted on the source. + Note that the management layer can validly revert the migration + even though the QEMU level of migration has succeeded as long as it + does it before starting execution on the destination. + +- Buses and devices should be able to explicitly specify addresses when + instantiated, and management tools should use those. For example, + when hot adding USB devices it's important to specify the ports + and addresses, since implicit ordering based on the command line order + may be different on the destination. This can result in the + device state being loaded into the wrong device. + +VMState +------- + +Most device data can be described using the ``VMSTATE`` macros (mostly defined +in ``include/migration/vmstate.h``). + +An example (from hw/input/pckbd.c) + +.. code:: c + + static const VMStateDescription vmstate_kbd = { + .name = "pckbd", + .version_id = 3, + .minimum_version_id = 3, + .fields = (VMStateField[]) { + VMSTATE_UINT8(write_cmd, KBDState), + VMSTATE_UINT8(status, KBDState), + VMSTATE_UINT8(mode, KBDState), + VMSTATE_UINT8(pending, KBDState), + VMSTATE_END_OF_LIST() + } + }; + +We are declaring the state with name "pckbd". +The ``version_id`` is 3, and the fields are 4 uint8_t in a KBDState structure. +We registered this with: + +.. code:: c + + vmstate_register(NULL, 0, &vmstate_kbd, s); + +For devices that are ``qdev`` based, we can register the device in the class +init function: + +.. code:: c + + dc->vmsd = &vmstate_kbd_isa; + +The VMState macros take care of ensuring that the device data section +is formatted portably (normally big endian) and make some compile time checks +against the types of the fields in the structures. + +VMState macros can include other VMStateDescriptions to store substructures +(see ``VMSTATE_STRUCT_``), arrays (``VMSTATE_ARRAY_``) and variable length +arrays (``VMSTATE_VARRAY_``). Various other macros exist for special +cases. + +Note that the format on the wire is still very raw; i.e. a VMSTATE_UINT32 +ends up with a 4 byte bigendian representation on the wire; in the future +it might be possible to use a more structured format. + +Legacy way +---------- + +This way is going to disappear as soon as all current users are ported to VMSTATE; +although converting existing code can be tricky, and thus 'soon' is relative. + +Each device has to register two functions, one to save the state and +another to load the state back. + +.. code:: c + + int register_savevm_live(const char *idstr, + int instance_id, + int version_id, + SaveVMHandlers *ops, + void *opaque); + +Two functions in the ``ops`` structure are the ``save_state`` +and ``load_state`` functions. Notice that ``load_state`` receives a version_id +parameter to know what state format is receiving. ``save_state`` doesn't +have a version_id parameter because it always uses the latest version. + +Note that because the VMState macros still save the data in a raw +format, in many cases it's possible to replace legacy code +with a carefully constructed VMState description that matches the +byte layout of the existing code. + +Changing migration data structures +---------------------------------- + +When we migrate a device, we save/load the state as a series +of fields. Sometimes, due to bugs or new functionality, we need to +change the state to store more/different information. Changing the migration +state saved for a device can break migration compatibility unless +care is taken to use the appropriate techniques. In general QEMU tries +to maintain forward migration compatibility (i.e. migrating from +QEMU n->n+1) and there are users who benefit from backward compatibility +as well. + +Subsections +----------- + +The most common structure change is adding new data, e.g. when adding +a newer form of device, or adding that state that you previously +forgot to migrate. This is best solved using a subsection. + +A subsection is "like" a device vmstate, but with a particularity, it +has a Boolean function that tells if that values are needed to be sent +or not. If this functions returns false, the subsection is not sent. +Subsections have a unique name, that is looked for on the receiving +side. + +On the receiving side, if we found a subsection for a device that we +don't understand, we just fail the migration. If we understand all +the subsections, then we load the state with success. There's no check +that a subsection is loaded, so a newer QEMU that knows about a subsection +can (with care) load a stream from an older QEMU that didn't send +the subsection. + +If the new data is only needed in a rare case, then the subsection +can be made conditional on that case and the migration will still +succeed to older QEMUs in most cases. This is OK for data that's +critical, but in some use cases it's preferred that the migration +should succeed even with the data missing. To support this the +subsection can be connected to a device property and from there +to a versioned machine type. + +The 'pre_load' and 'post_load' functions on subsections are only +called if the subsection is loaded. + +One important note is that the outer post_load() function is called "after" +loading all subsections, because a newer subsection could change the same +value that it uses. A flag, and the combination of outer pre_load and +post_load can be used to detect whether a subsection was loaded, and to +fall back on default behaviour when the subsection isn't present. + +Example: + +.. code:: c + + static bool ide_drive_pio_state_needed(void *opaque) + { + IDEState *s = opaque; + + return ((s->status & DRQ_STAT) != 0) + || (s->bus->error_status & BM_STATUS_PIO_RETRY); + } + + const VMStateDescription vmstate_ide_drive_pio_state = { + .name = "ide_drive/pio_state", + .version_id = 1, + .minimum_version_id = 1, + .pre_save = ide_drive_pio_pre_save, + .post_load = ide_drive_pio_post_load, + .needed = ide_drive_pio_state_needed, + .fields = (VMStateField[]) { + VMSTATE_INT32(req_nb_sectors, IDEState), + VMSTATE_VARRAY_INT32(io_buffer, IDEState, io_buffer_total_len, 1, + vmstate_info_uint8, uint8_t), + VMSTATE_INT32(cur_io_buffer_offset, IDEState), + VMSTATE_INT32(cur_io_buffer_len, IDEState), + VMSTATE_UINT8(end_transfer_fn_idx, IDEState), + VMSTATE_INT32(elementary_transfer_size, IDEState), + VMSTATE_INT32(packet_transfer_size, IDEState), + VMSTATE_END_OF_LIST() + } + }; + + const VMStateDescription vmstate_ide_drive = { + .name = "ide_drive", + .version_id = 3, + .minimum_version_id = 0, + .post_load = ide_drive_post_load, + .fields = (VMStateField[]) { + .... several fields .... + VMSTATE_END_OF_LIST() + }, + .subsections = (const VMStateDescription*[]) { + &vmstate_ide_drive_pio_state, + NULL + } + }; + +Here we have a subsection for the pio state. We only need to +save/send this state when we are in the middle of a pio operation +(that is what ``ide_drive_pio_state_needed()`` checks). If DRQ_STAT is +not enabled, the values on that fields are garbage and don't need to +be sent. + +Connecting subsections to properties +------------------------------------ + +Using a condition function that checks a 'property' to determine whether +to send a subsection allows backward migration compatibility when +new subsections are added, especially when combined with versioned +machine types. + +For example: + + a) Add a new property using ``DEFINE_PROP_BOOL`` - e.g. support-foo and + default it to true. + b) Add an entry to the ``hw_compat_`` for the previous version that sets + the property to false. + c) Add a static bool support_foo function that tests the property. + d) Add a subsection with a .needed set to the support_foo function + e) (potentially) Add an outer pre_load that sets up a default value + for 'foo' to be used if the subsection isn't loaded. + +Now that subsection will not be generated when using an older +machine type and the migration stream will be accepted by older +QEMU versions. + +Not sending existing elements +----------------------------- + +Sometimes members of the VMState are no longer needed: + + - removing them will break migration compatibility + + - making them version dependent and bumping the version will break backward migration + compatibility. + +Adding a dummy field into the migration stream is normally the best way to preserve +compatibility. + +If the field really does need to be removed then: + + a) Add a new property/compatibility/function in the same way for subsections above. + b) replace the VMSTATE macro with the _TEST version of the macro, e.g.: + + ``VMSTATE_UINT32(foo, barstruct)`` + + becomes + + ``VMSTATE_UINT32_TEST(foo, barstruct, pre_version_baz)`` + + Sometime in the future when we no longer care about the ancient versions these can be killed off. + Note that for backward compatibility it's important to fill in the structure with + data that the destination will understand. + +Any difference in the predicates on the source and destination will end up +with different fields being enabled and data being loaded into the wrong +fields; for this reason conditional fields like this are very fragile. + +Versions +-------- + +Version numbers are intended for major incompatible changes to the +migration of a device, and using them breaks backward-migration +compatibility; in general most changes can be made by adding Subsections +(see above) or _TEST macros (see above) which won't break compatibility. + +Each version is associated with a series of fields saved. The ``save_state`` always saves +the state as the newer version. But ``load_state`` sometimes is able to +load state from an older version. + +You can see that there are 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 +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, +e.g. + +.. code:: c + + VMSTATE_UINT16_V(ip_id, Slirp, 2), + +only loads that field for versions 2 and newer. + +Saving state will always create a section with the 'version_id' value +and thus can't be loaded by any older QEMU. + +Massaging functions +------------------- + +Sometimes, it is not enough to be able to save the state directly +from one structure, we need to fill the correct values there. One +example is when we are using kvm. Before saving the cpu state, we +need to ask kvm to copy to QEMU the state that it is using. And the +opposite when we are loading the state, we need a way to tell kvm to +load the state for the cpu that we have just loaded from the QEMUFile. + +The functions to do that are inside a vmstate definition, and are called: + +- ``int (*pre_load)(void *opaque);`` + + This function is called before we load the state of one device. + +- ``int (*post_load)(void *opaque, int version_id);`` + + This function is called after we load the state of one device. + +- ``int (*pre_save)(void *opaque);`` + + This function is called before we save the state of one device. + +- ``int (*post_save)(void *opaque);`` + + This function is called after we save the state of one device + (even upon failure, unless the call to pre_save returned an error). + +Example: You can look at hpet.c, that uses the first three functions +to massage the state that is transferred. + +The ``VMSTATE_WITH_TMP`` macro may be useful when the migration +data doesn't match the stored device data well; it allows an +intermediate temporary structure to be populated with migration +data and then transferred to the main structure. + +If you use memory 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_nonvolatile() + - 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 +----------- + +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 interruptible 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_parameter`` 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. + |