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authorDr. David Alan Gilbert <dgilbert@redhat.com>2018-05-03 20:10:59 +0100
committerJuan Quintela <quintela@redhat.com>2018-05-15 22:13:08 +0200
commitedd7080692cb43147352d736cce0fb728787f2b4 (patch)
tree674db7de2e038c4f512c2be1e030f6fed4ded33a /docs
parentd37297dc66202c33f9cafbc48ccae629e7d6dc31 (diff)
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migration: update docs
Update the migration docs: Among other changes: * Added a general list of advice for device authors * Reordered the section on conditional state (subsections etc) into the order we prefer. * Add a note about firmware Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com> Reviewed-by: Peter Xu <peterx@redhat.com> Reviewed-by: Balamuruhan S <bala24@linux.vnet.ibm.com> Reviewed-by: Juan Quintela <quintela@redhat.com> Message-Id: <20180503191059.19576-1-dgilbert@redhat.com> Signed-off-by: Juan Quintela <quintela@redhat.com>
Diffstat (limited to 'docs')
-rw-r--r--docs/devel/migration.rst532
1 files changed, 376 insertions, 156 deletions
diff --git a/docs/devel/migration.rst b/docs/devel/migration.rst
index 9342a8a..40f136f 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.
+