On Thu, May 03, 2018 at 08:10:59PM +0100, Dr. David Alan Gilbert (git) wrote: > From: "Dr. David Alan Gilbert" <dgilb...@redhat.com> > > Update the migration docs: > > Among other changes: > * Added a general list of advice for device authors > * Reordered the section on conditional state (subsections etc) > into the order we prefer. > * Add a note about firmware > > Signed-off-by: Dr. David Alan Gilbert <dgilb...@redhat.com> > Reviewed-by: Peter Xu <pet...@redhat.com> > --- > docs/devel/migration.rst | 532 +++++++++++++++++++++++++++------------ > 1 file changed, 376 insertions(+), 156 deletions(-) > Reviewed-by: Balamuruhan S <bal...@linux.vnet.ibm.com>
> diff --git a/docs/devel/migration.rst b/docs/devel/migration.rst > index 9342a8af06..40f136f6be 100644 > --- a/docs/devel/migration.rst > +++ b/docs/devel/migration.rst > @@ -28,11 +28,11 @@ the guest to be stopped. Typically the time that the > guest is > unresponsive during live migration is the low hundred of milliseconds > (notice that this depends on a lot of things). > > -Types of migration > -================== > +Transports > +========== > > -Now that we have talked about live migration, there are several ways > -to do migration: > +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 > @@ -40,16 +40,16 @@ to do migration: > - fd migration: do the migration using an file descriptor that is > passed to QEMU. QEMU doesn't care how this file descriptor is opened. > > -All these four migration protocols use the same infrastructure to > +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. > > -State Live Migration > -==================== > - > -This is used for RAM and block devices. It is not yet ported to vmstate. > -<Fill more information here> > - > Common infrastructure > ===================== > > @@ -57,60 +57,75 @@ 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 > ============================== > > -The state of a device is saved using intermediate buffers. There are > -some helper functions to assist this saving. > - > -There is a new concept that we have to explain here: device state > -version. When we migrate a device, we save/load the state as a series > -of fields. Some times, due to bugs or new functionality, we need to > -change the state to store more/different information. We use the > -version to identify each time that we do a change. Each version is > -associated with a series of fields saved. The `save_state` always saves > -the state as the newer version. But `load_state` sometimes is able to > -load state from an older version. > - > -Legacy way > ----------- > - > -This way is going to disappear as soon as all current users are ported to > VMSTATE. > - > -Each device has to register two functions, one to save the state and > -another to load the state back. > - > -.. code:: c > - > - int register_savevm(DeviceState *dev, > - const char *idstr, > - int instance_id, > - int version_id, > - SaveStateHandler *save_state, > - LoadStateHandler *load_state, > - void *opaque); > - > - typedef void SaveStateHandler(QEMUFile *f, void *opaque); > - typedef int LoadStateHandler(QEMUFile *f, void *opaque, int version_id); > - > -The important functions for the device state format are the `save_state` > -and `load_state`. 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. > +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 > ------- > > -The legacy way of saving/loading state of the device had the problem > -that we have to maintain two functions in sync. If we did one change > -in one of them and not in the other, we would get a failed migration. > - > -VMState changed the way that state is saved/loaded. Instead of using > -a function to save the state and another to load it, it was changed to > -a declarative way of what the state consisted of. Now VMState is able > -to interpret that definition to be able to load/save the state. As > -the state is declared only once, it can't go out of sync in the > -save/load functions. > +Most device data can be described using the ``VMSTATE`` macros (mostly > defined > +in ``include/migration/vmstate.h``). > > An example (from hw/input/pckbd.c) > > @@ -137,103 +152,99 @@ We registered this with: > > vmstate_register(NULL, 0, &vmstate_kbd, s); > > -Note: talk about how vmstate <-> qdev interact, and what the instance ids > mean. > +For devices that are `qdev` based, we can register the device in the class > +init function: > > -You can search for ``VMSTATE_*`` macros for lots of types used in QEMU in > -include/hw/hw.h. > - > -More about versions > -------------------- > - > -Version numbers are intended for major incompatible changes to the > -migration of a device, and using them breaks backwards-migration > -compatibility; in general most changes can be made by adding Subsections > -(see below) or _TEST macros (see below) which won't break compatibility. > - > -You can see that there are several version fields: > +.. code:: c > > -- `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. > + dc->vmsd = &vmstate_kbd_isa; > > -So, 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. > +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. > > -Saving state will always create a section with the 'version_id' value > -and thus can't be loaded by any older QEMU. > +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. > > -Massaging functions > -------------------- > +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. > > -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. > +Legacy way > +---------- > > -The functions to do that are inside a vmstate definition, and are called: > +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. > > -- ``int (*pre_load)(void *opaque);`` > +Each device has to register two functions, one to save the state and > +another to load the state back. > > - This function is called before we load the state of one device. > +.. code:: c > > -- ``int (*post_load)(void *opaque, int version_id);`` > + int register_savevm_live(DeviceState *dev, > + const char *idstr, > + int instance_id, > + int version_id, > + SaveVMHandlers *ops, > + void *opaque); > > - This function is called after we load the state of one device. > +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. > > -- ``int (*pre_save)(void *opaque);`` > +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. > > - This function is called before we save the state of one device. > +Changing migration data structures > +---------------------------------- > > -Example: You can look at hpet.c, that uses the three function to > -massage the state that is transferred. > - > -If you use memory API functions that update memory layout outside > -initialization (i.e., in response to a guest action), this is a strong > -indication that you need to call these functions in a `post_load` callback. > -Examples of such memory API functions are: > - > - - memory_region_add_subregion() > - - memory_region_del_subregion() > - - memory_region_set_readonly() > - - memory_region_set_enabled() > - - memory_region_set_address() > - - memory_region_set_alias_offset() > +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 use of version_id allows to be able to migrate from older versions > -to newer versions of a device. But not the other way around. This > -makes very complicated to fix bugs in stable branches. If we need to > -add anything to the state to fix a bug, we have to disable migration > -to older versions that don't have that bug-fix (i.e. a new field). > +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. > > -But sometimes, that bug-fix is only needed sometimes, not always. For > -instance, if the device is in the middle of a DMA operation, it is > -using a specific functionality, .... > - > -It is impossible to create a way to make migration from any version to > -any other version to work. But we can do better than only allowing > -migration from older versions to newer ones. For that fields that are > -only needed sometimes, we add the idea of subsections. 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. > +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. > +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. > > One important note is that the post_load() function is called "after" > loading all subsections, because a newer subsection could change same > -value that it uses. > +value that it uses. A flag, and the combination of 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: > > @@ -288,9 +299,13 @@ save/send this state when we are in the middle of a pio > operation > 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 backwards migration compatibility when > -new subsections are added. > +to send a subsection allows backward migration compatibility when > +new subsections are added, especially when combined with versioned > +machine types. > > For example: > > @@ -305,21 +320,7 @@ For example: > > Now that subsection will not be generated when using an older > machine type and the migration stream will be accepted by older > -QEMU versions. pre-load functions can be used to initialise state > -on the newer version so that they default to suitable values > -when loading streams created by older QEMU versions that do not > -generate the subsection. > - > -In some cases subsections are added for data that had been accidentally > -omitted by earlier versions; if the missing data causes the migration > -process to succeed but the guest to behave badly then it may be better > -to send the subsection and cause the migration to explicitly fail > -with the unknown subsection error. If the bad behaviour only happens > -with certain data values, making the subsection conditional on > -the data value (rather than the machine type) allows migrations to succeed > -in most cases. In general the preference is to tie the subsection to > -the machine type, and allow reliable migrations, unless the behaviour > -from omission of the subsection is really bad. > +QEMU versions. > > Not sending existing elements > ----------------------------- > @@ -328,9 +329,13 @@ 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 > backwards 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. > > -The best way is to: > +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.: > @@ -342,18 +347,208 @@ The best way is to: > ``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. > + > +Example: You can look at hpet.c, that uses the three function to > +massage the state that is transferred. > + > +The ``VMSTATE_WITH_TMP`` macro may be useful when the migration > +data doesn't match the stored device data well; it allows an > +intermediate temporary structure to be populated with migration > +data and then transferred to the main structure. > + > +If you use memory API functions that update memory layout outside > +initialization (i.e., in response to a guest action), this is a strong > +indication that you need to call these functions in a `post_load` callback. > +Examples of such memory API functions are: > + > + - memory_region_add_subregion() > + - memory_region_del_subregion() > + - memory_region_set_readonly() > + - memory_region_set_enabled() > + - memory_region_set_address() > + - memory_region_set_alias_offset() > + > +Iterative device migration > +-------------------------- > + > +Some devices, such as RAM, Block storage or certain platform devices, > +have large amounts of data that would mean that the CPUs would be > +paused for too long if they were sent in one section. For these > +devices an *iterative* approach is taken. > + > +The iterative devices generally don't use VMState macros > +(although it may be possible in some cases) and instead use > +qemu_put_*/qemu_get_* macros to read/write data to the stream. Specialist > +versions exist for high bandwidth IO. > + > + > +An iterative device must provide: > + > + - A ``save_setup`` function that initialises the data structures and > + transmits a first section containing information on the device. In the > + case of RAM this transmits a list of RAMBlocks and sizes. > + > + - A ``load_setup`` function that initialises the data structures on the > + destination. > + > + - A ``save_live_pending`` function that is called repeatedly and must > + indicate how much more data the iterative data must save. The core > + migration code will use this to determine when to pause the CPUs > + and complete the migration. > + > + - A ``save_live_iterate`` function (called after ``save_live_pending`` > + when there is significant data still to be sent). It should send > + a chunk of data until the point that stream bandwidth limits tell it > + to stop. Each call generates one section. > + > + - A ``save_live_complete_precopy`` function that must transmit the > + last section for the device containing any remaining data. > + > + - A ``load_state`` function used to load sections generated by > + any of the save functions that generate sections. > + > + - ``cleanup`` functions for both save and load that are called > + at the end of migration. > + > +Note that the contents of the sections for iterative migration tend > +to be open-coded by the devices; care should be taken in parsing > +the results and structuring the stream to make them easy to validate. > + > +Device ordering > +--------------- > + > +There are cases in which the ordering of device loading matters; for > +example in some systems where a device may assert an interrupt during > loading, > +if the interrupt controller is loaded later then it might lose the state. > + > +Some ordering is implicitly provided by the order in which the machine > +definition creates devices, however this is somewhat fragile. > + > +The ``MigrationPriority`` enum provides a means of explicitly enforcing > +ordering. Numerically higher priorities are loaded earlier. > +The priority is set by setting the ``priority`` field of the top level > +``VMStateDescription`` for the device. > + > +Stream structure > +================ > + > +The stream tries to be word and endian agnostic, allowing migration between > hosts > +of different characteristics running the same VM. > + > + - Header > + > + - Magic > + - Version > + - VM configuration section > + > + - Machine type > + - Target page bits > + - List of sections > + Each section contains a device, or one iteration of a device save. > + > + - section type > + - section id > + - ID string (First section of each device) > + - instance id (First section of each device) > + - version id (First section of each device) > + - <device data> > + - Footer mark > + - EOF mark > + - VM Description structure > + Consisting of a JSON description of the contents for analysis only > + > +The ``device data`` in each section consists of the data produced > +by the code described above. For non-iterative devices they have a single > +section; iterative devices have an initial and last section and a set > +of parts in between. > +Note that there is very little checking by the common code of the integrity > +of the ``device data`` contents, that's up to the devices themselves. > +The ``footer mark`` provides a little bit of protection for the case where > +the receiving side reads more or less data than expected. > + > +The ``ID string`` is normally unique, having been formed from a bus name > +and device address, PCI devices and storage devices hung off PCI controllers > +fit this pattern well. Some devices are fixed single instances (e.g. > "pc-ram"). > +Others (especially either older devices or system devices which for > +some reason don't have a bus concept) make use of the ``instance id`` > +for otherwise identically named devices. > > Return path > ----------- > > -In most migration scenarios there is only a single data path that runs > -from the source VM to the destination, typically along a single fd (although > -possibly with another fd or similar for some fast way of throwing pages > across). > - > -However, some uses need two way communication; in particular the Postcopy > -destination needs to be able to request pages on demand from the source. > +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. > > -For these scenarios there is a 'return path' from the destination to the > source; > ``qemu_file_get_return_path(QEMUFile* fwdpath)`` gives the QEMUFile* for the > return > path. > > @@ -632,3 +827,28 @@ Retro-fitting postcopy to existing clients is possible: > identified and the implication understood; for example if the > guest memory access is made while holding a lock then all other > threads waiting for that lock will also be blocked. > + > +Firmware > +======== > + > +Migration migrates the copies of RAM and ROM, and thus when running > +on the destination it includes the firmware from the source. Even after > +resetting a VM, the old firmware is used. Only once QEMU has been restarted > +is the new firmware in use. > + > +- Changes in firmware size can cause changes in the required RAMBlock size > + to hold the firmware and thus migration can fail. In practice it's best > + to pad firmware images to convenient powers of 2 with plenty of space > + for growth. > + > +- Care should be taken with device emulation code so that newer > + emulation code can work with older firmware to allow forward migration. > + > +- Care should be taken with newer firmware so that backward migration > + to older systems with older device emulation code will work. > + > +In some cases it may be best to tie specific firmware versions to specific > +versioned machine types to cut down on the combinations that will need > +support. This is also useful when newer versions of firmware outgrow > +the padding. > + > -- > 2.17.0 >