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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. + |