[Qemu-devel] [PATCH v2] migration: update docs

[Dr. David Alan Gilbert (git)]

From: "Dr. David Alan Gilbert" <address@hidden>

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 <address@hidden>
---
 docs/devel/migration.rst | 531 +++++++++++++++++++++++++++------------
 1 file changed, 375 insertions(+), 156 deletions(-)

diff --git a/docs/devel/migration.rst b/docs/devel/migration.rst
index 9342a8af06..d21db81daf 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.
-
-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:
-
-- `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.
+For devices that are `qdev` based, we can register the device in the class
+init function:
 
-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.
+.. code:: c
 
-Saving state will always create a section with the 'version_id' value
-and thus can't be loaded by any older QEMU.
+    dc->vmsd = &vmstate_kbd_isa;
 
-Massaging functions
--------------------
+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.
 
-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.
+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.
 
-The functions to do that are inside a vmstate definition, and are called:
+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.
 
-- ``int (*pre_load)(void *opaque);``
+Legacy way
+----------
 
-  This function is called before we load the state of one device.
+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 (*post_load)(void *opaque, int version_id);``
+Each device has to register two functions, one to save the state and
+another to load the state back.
 
-  This function is called after we load the state of one device.
+.. code:: c
 
-- ``int (*pre_save)(void *opaque);``
+  int register_savevm_live(DeviceState *dev,
+                           const char *idstr,
+                           int instance_id,
+                           int version_id,
+                           SaveVMHandlers *ops,
+                           void *opaque);
 
-  This function is called before we save 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.
 
-Example: You can look at hpet.c, that uses the three function to
-massage the state that is transferred.
+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.
 
-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:
+Changing migration data structures
+----------------------------------
 
-  - 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.  Some times, 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).
-
-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, ....
+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.
 
-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 +329,12 @@ 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 +346,208 @@ 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 +826,28 @@ 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