docs: Convert migration.txt to rst
Mostly just manual conversion with very minor fixes. Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com> Reviewed-by: Daniel P. Berrange <berrange@redhat.com> Reviewed-by: Kashyap Chamarthy <kchamart@redhat.com> Reviewed-by: Peter Xu <peterx@redhat.com> Signed-off-by: Juan Quintela <quintela@redhat.com>
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@ -1,4 +1,6 @@
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= Migration =
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=========
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Migration
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=========
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QEMU has code to load/save the state of the guest that it is running.
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These are two complementary operations. Saving the state just does
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@ -26,7 +28,8 @@ the guest to be stopped. Typically the time that the guest is
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unresponsive during live migration is the low hundred of milliseconds
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(notice that this depends on a lot of things).
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=== Types of migration ===
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Types of migration
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==================
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Now that we have talked about live migration, there are several ways
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to do migration:
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@ -41,49 +44,21 @@ All these four migration protocols use the same infrastructure to
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save/restore state devices. This infrastructure is shared with the
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savevm/loadvm functionality.
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=== State Live Migration ===
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State Live Migration
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====================
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This is used for RAM and block devices. It is not yet ported to vmstate.
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<Fill more information here>
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=== What is the common infrastructure ===
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Common infrastructure
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=====================
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QEMU uses a QEMUFile abstraction to be able to do migration. Any type
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of migration that wants to use QEMU infrastructure has to create a
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QEMUFile with:
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The files, sockets or fd's that carry the migration stream are abstracted by
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the ``QEMUFile`` type (see `migration/qemu-file.h`). In most cases this
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is connected to a subtype of ``QIOChannel`` (see `io/`).
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QEMUFile *qemu_fopen_ops(void *opaque,
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QEMUFilePutBufferFunc *put_buffer,
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QEMUFileGetBufferFunc *get_buffer,
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QEMUFileCloseFunc *close);
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The functions have the following functionality:
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This function writes a chunk of data to a file at the given position.
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The pos argument can be ignored if the file is only used for
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streaming. The handler should try to write all of the data it can.
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typedef int (QEMUFilePutBufferFunc)(void *opaque, const uint8_t *buf,
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int64_t pos, int size);
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Read a chunk of data from a file at the given position. The pos argument
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can be ignored if the file is only be used for streaming. The number of
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bytes actually read should be returned.
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typedef int (QEMUFileGetBufferFunc)(void *opaque, uint8_t *buf,
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int64_t pos, int size);
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Close a file and return an error code.
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typedef int (QEMUFileCloseFunc)(void *opaque);
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You can use any internal state that you need using the opaque void *
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pointer that is passed to all functions.
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The important functions for us are put_buffer()/get_buffer() that
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allow to write/read a buffer into the QEMUFile.
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=== How to save the state of one device ===
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Saving the state of one device
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==============================
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The state of a device is saved using intermediate buffers. There are
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some helper functions to assist this saving.
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@ -93,18 +68,21 @@ version. When we migrate a device, we save/load the state as a series
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of fields. Some times, due to bugs or new functionality, we need to
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change the state to store more/different information. We use the
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version to identify each time that we do a change. Each version is
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associated with a series of fields saved. The save_state always saves
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the state as the newer version. But load_state sometimes is able to
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associated with a series of fields saved. The `save_state` always saves
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the state as the newer version. But `load_state` sometimes is able to
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load state from an older version.
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=== Legacy way ===
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Legacy way
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----------
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This way is going to disappear as soon as all current users are ported to VMSTATE.
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Each device has to register two functions, one to save the state and
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another to load the state back.
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int register_savevm(DeviceState *dev,
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.. code:: c
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int register_savevm(DeviceState *dev,
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const char *idstr,
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int instance_id,
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int version_id,
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@ -112,15 +90,16 @@ int register_savevm(DeviceState *dev,
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LoadStateHandler *load_state,
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void *opaque);
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typedef void SaveStateHandler(QEMUFile *f, void *opaque);
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typedef int LoadStateHandler(QEMUFile *f, void *opaque, int version_id);
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typedef void SaveStateHandler(QEMUFile *f, void *opaque);
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typedef int LoadStateHandler(QEMUFile *f, void *opaque, int version_id);
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The important functions for the device state format are the save_state
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and load_state. Notice that load_state receives a version_id
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parameter to know what state format is receiving. save_state doesn't
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The important functions for the device state format are the `save_state`
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and `load_state`. Notice that `load_state` receives a version_id
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parameter to know what state format is receiving. `save_state` doesn't
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have a version_id parameter because it always uses the latest version.
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=== VMState ===
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VMState
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-------
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The legacy way of saving/loading state of the device had the problem
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that we have to maintain two functions in sync. If we did one change
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@ -135,7 +114,9 @@ save/load functions.
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An example (from hw/input/pckbd.c)
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static const VMStateDescription vmstate_kbd = {
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.. code:: c
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static const VMStateDescription vmstate_kbd = {
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.name = "pckbd",
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.version_id = 3,
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.minimum_version_id = 3,
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@ -146,20 +127,23 @@ static const VMStateDescription vmstate_kbd = {
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VMSTATE_UINT8(pending, KBDState),
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VMSTATE_END_OF_LIST()
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}
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};
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};
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We are declaring the state with name "pckbd".
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The version_id is 3, and the fields are 4 uint8_t in a KBDState structure.
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The `version_id` is 3, and the fields are 4 uint8_t in a KBDState structure.
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We registered this with:
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.. code:: c
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vmstate_register(NULL, 0, &vmstate_kbd, s);
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Note: talk about how vmstate <-> qdev interact, and what the instance ids mean.
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You can search for VMSTATE_* macros for lots of types used in QEMU in
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You can search for ``VMSTATE_*`` macros for lots of types used in QEMU in
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include/hw/hw.h.
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=== More about versions ===
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More about versions
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-------------------
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Version numbers are intended for major incompatible changes to the
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migration of a device, and using them breaks backwards-migration
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@ -168,22 +152,23 @@ compatibility; in general most changes can be made by adding Subsections
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You can see that there are several version fields:
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- version_id: the maximum version_id supported by VMState for that device.
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- minimum_version_id: the minimum version_id that VMState is able to understand
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- `version_id`: the maximum version_id supported by VMState for that device.
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- `minimum_version_id`: the minimum version_id that VMState is able to understand
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for that device.
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- minimum_version_id_old: For devices that were not able to port to vmstate, we can
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- `minimum_version_id_old`: For devices that were not able to port to vmstate, we can
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assign a function that knows how to read this old state. This field is
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ignored if there is no load_state_old handler.
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ignored if there is no `load_state_old` handler.
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So, VMState is able to read versions from minimum_version_id to
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version_id. And the function load_state_old() (if present) is able to
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version_id. And the function ``load_state_old()`` (if present) is able to
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load state from minimum_version_id_old to minimum_version_id. This
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function is deprecated and will be removed when no more users are left.
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Saving state will always create a section with the 'version_id' value
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and thus can't be loaded by any older QEMU.
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=== Massaging functions ===
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Massaging functions
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-------------------
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Sometimes, it is not enough to be able to save the state directly
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from one structure, we need to fill the correct values there. One
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@ -194,24 +179,24 @@ load the state for the cpu that we have just loaded from the QEMUFile.
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The functions to do that are inside a vmstate definition, and are called:
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- int (*pre_load)(void *opaque);
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- ``int (*pre_load)(void *opaque);``
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This function is called before we load the state of one device.
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- int (*post_load)(void *opaque, int version_id);
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- ``int (*post_load)(void *opaque, int version_id);``
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This function is called after we load the state of one device.
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- int (*pre_save)(void *opaque);
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- ``int (*pre_save)(void *opaque);``
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This function is called before we save the state of one device.
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Example: You can look at hpet.c, that uses the three function to
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massage the state that is transferred.
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massage the state that is transferred.
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If you use memory API functions that update memory layout outside
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initialization (i.e., in response to a guest action), this is a strong
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indication that you need to call these functions in a post_load callback.
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indication that you need to call these functions in a `post_load` callback.
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Examples of such memory API functions are:
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- memory_region_add_subregion()
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@ -221,7 +206,8 @@ Examples of such memory API functions are:
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- memory_region_set_address()
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- memory_region_set_alias_offset()
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=== Subsections ===
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Subsections
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-----------
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The use of version_id allows to be able to migrate from older versions
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to newer versions of a device. But not the other way around. This
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@ -251,15 +237,17 @@ value that it uses.
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Example:
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static bool ide_drive_pio_state_needed(void *opaque)
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{
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.. code:: c
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static bool ide_drive_pio_state_needed(void *opaque)
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{
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IDEState *s = opaque;
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return ((s->status & DRQ_STAT) != 0)
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|| (s->bus->error_status & BM_STATUS_PIO_RETRY);
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}
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}
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const VMStateDescription vmstate_ide_drive_pio_state = {
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const VMStateDescription vmstate_ide_drive_pio_state = {
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.name = "ide_drive/pio_state",
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.version_id = 1,
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.minimum_version_id = 1,
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VMSTATE_INT32(packet_transfer_size, IDEState),
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VMSTATE_END_OF_LIST()
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}
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};
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};
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const VMStateDescription vmstate_ide_drive = {
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const VMStateDescription vmstate_ide_drive = {
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.name = "ide_drive",
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.version_id = 3,
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.minimum_version_id = 0,
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&vmstate_ide_drive_pio_state,
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NULL
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}
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};
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};
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Here we have a subsection for the pio state. We only need to
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save/send this state when we are in the middle of a pio operation
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(that is what ide_drive_pio_state_needed() checks). If DRQ_STAT is
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(that is what ``ide_drive_pio_state_needed()`` checks). If DRQ_STAT is
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not enabled, the values on that fields are garbage and don't need to
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be sent.
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@ -304,11 +292,12 @@ Using a condition function that checks a 'property' to determine whether
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to send a subsection allows backwards migration compatibility when
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new subsections are added.
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For example;
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a) Add a new property using DEFINE_PROP_BOOL - e.g. support-foo and
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For example:
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a) Add a new property using ``DEFINE_PROP_BOOL`` - e.g. support-foo and
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default it to true.
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b) Add an entry to the HW_COMPAT_ for the previous version
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that sets the property to false.
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b) Add an entry to the ``HW_COMPAT_`` for the previous version that sets
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the property to false.
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c) Add a static bool support_foo function that tests the property.
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d) Add a subsection with a .needed set to the support_foo function
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e) (potentially) Add a pre_load that sets up a default value for 'foo'
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@ -332,25 +321,30 @@ in most cases. In general the preference is to tie the subsection to
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the machine type, and allow reliable migrations, unless the behaviour
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from omission of the subsection is really bad.
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= Not sending existing elements =
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Not sending existing elements
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-----------------------------
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Sometimes members of the VMState are no longer needed;
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removing them will break migration compatibility
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making them version dependent and bumping the version will break backwards
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migration compatibility.
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Sometimes members of the VMState are no longer needed:
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- removing them will break migration compatibility
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- making them version dependent and bumping the version will break backwards migration compatibility.
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The best way is to:
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a) Add a new property/compatibility/function in the same way for subsections
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above.
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a) Add a new property/compatibility/function in the same way for subsections above.
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b) replace the VMSTATE macro with the _TEST version of the macro, e.g.:
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VMSTATE_UINT32(foo, barstruct)
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``VMSTATE_UINT32(foo, barstruct)``
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becomes
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VMSTATE_UINT32_TEST(foo, barstruct, pre_version_baz)
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Sometime in the future when we no longer care about the ancient
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versions these can be killed off.
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``VMSTATE_UINT32_TEST(foo, barstruct, pre_version_baz)``
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= Return path =
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Sometime in the future when we no longer care about the ancient versions these can be killed off.
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Return path
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-----------
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In most migration scenarios there is only a single data path that runs
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from the source VM to the destination, typically along a single fd (although
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@ -360,19 +354,23 @@ However, some uses need two way communication; in particular the Postcopy
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destination needs to be able to request pages on demand from the source.
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For these scenarios there is a 'return path' from the destination to the source;
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qemu_file_get_return_path(QEMUFile* fwdpath) gives the QEMUFile* for the return
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``qemu_file_get_return_path(QEMUFile* fwdpath)`` gives the QEMUFile* for the return
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path.
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Source side
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Forward path - written by migration thread
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Return path - opened by main thread, read by return-path thread
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Destination side
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Forward path - read by main thread
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Return path - opened by main thread, written by main thread AND postcopy
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thread (protected by rp_mutex)
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= Postcopy =
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Postcopy
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========
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'Postcopy' migration is a way to deal with migrations that refuse to converge
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(or take too long to converge) its plus side is that there is an upper bound on
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the amount of migration traffic and time it takes, the down side is that during
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@ -386,27 +384,30 @@ a fault that's translated by QEMU into a request to the source QEMU.
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Postcopy can be combined with precopy (i.e. normal migration) so that if precopy
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doesn't finish in a given time the switch is made to postcopy.
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=== Enabling postcopy ===
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Enabling postcopy
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-----------------
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To enable postcopy, issue this command on the monitor prior to the
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start of migration:
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migrate_set_capability postcopy-ram on
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``migrate_set_capability postcopy-ram on``
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The normal commands are then used to start a migration, which is still
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started in precopy mode. Issuing:
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migrate_start_postcopy
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``migrate_start_postcopy``
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will now cause the transition from precopy to postcopy.
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It can be issued immediately after migration is started or any
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time later on. Issuing it after the end of a migration is harmless.
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Note: During the postcopy phase, the bandwidth limits set using
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migrate_set_speed is ignored (to avoid delaying requested pages that
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the destination is waiting for).
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.. note::
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During the postcopy phase, the bandwidth limits set using
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``migrate_set_speed`` is ignored (to avoid delaying requested pages that
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the destination is waiting for).
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=== Postcopy device transfer ===
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Postcopy device transfer
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------------------------
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Loading of device data may cause the device emulation to access guest RAM
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that may trigger faults that have to be resolved by the source, as such
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|
@ -416,6 +417,7 @@ before the device load begins to free the stream up. This is achieved by
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'packaging' the device data into a blob that's read in one go.
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Source behaviour
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----------------
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Until postcopy is entered the migration stream is identical to normal
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precopy, except for the addition of a 'postcopy advise' command at
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|
@ -423,13 +425,14 @@ the beginning, to tell the destination that postcopy might happen.
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When postcopy starts the source sends the page discard data and then
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forms the 'package' containing:
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Command: 'postcopy listen'
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The device state
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- Command: 'postcopy listen'
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- The device state
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A series of sections, identical to the precopy streams device state stream
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containing everything except postcopiable devices (i.e. RAM)
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Command: 'postcopy run'
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- Command: 'postcopy run'
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The 'package' is sent as the data part of a Command: 'CMD_PACKAGED', and the
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The 'package' is sent as the data part of a Command: ``CMD_PACKAGED``, and the
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contents are formatted in the same way as the main migration stream.
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During postcopy the source scans the list of dirty pages and sends them
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|
@ -441,49 +444,58 @@ to be sent quickly in the hope that those pages are likely to be used
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by the destination soon.
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Destination behaviour
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---------------------
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Initially the destination looks the same as precopy, with a single thread
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reading the migration stream; the 'postcopy advise' and 'discard' commands
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are processed to change the way RAM is managed, but don't affect the stream
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processing.
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------------------------------------------------------------------------------
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::
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------------------------------------------------------------------------------
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1 2 3 4 5 6 7
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main -----DISCARD-CMD_PACKAGED ( LISTEN DEVICE DEVICE DEVICE RUN )
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thread | |
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main -----DISCARD-CMD_PACKAGED ( LISTEN DEVICE DEVICE DEVICE RUN )
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thread | |
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| (page request)
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| \___
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v \
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listen thread: --- page -- page -- page -- page -- page --
|
||||
listen thread: --- page -- page -- page -- page -- page --
|
||||
|
||||
a b c
|
||||
------------------------------------------------------------------------------
|
||||
------------------------------------------------------------------------------
|
||||
|
||||
On receipt of CMD_PACKAGED (1)
|
||||
All the data associated with the package - the ( ... ) section in the
|
||||
diagram - is read into memory, and the main thread recurses into
|
||||
qemu_loadvm_state_main to process the contents of the package (2)
|
||||
which contains commands (3,6) and devices (4...)
|
||||
- On receipt of ``CMD_PACKAGED`` (1)
|
||||
|
||||
On receipt of 'postcopy listen' - 3 -(i.e. the 1st command in the package)
|
||||
a new thread (a) is started that takes over servicing the migration stream,
|
||||
while the main thread carries on loading the package. It loads normal
|
||||
background page data (b) but if during a device load a fault happens (5) the
|
||||
returned page (c) is loaded by the listen thread allowing the main threads
|
||||
device load to carry on.
|
||||
All the data associated with the package - the ( ... ) section in the diagram -
|
||||
is read into memory, and the main thread recurses into qemu_loadvm_state_main
|
||||
to process the contents of the package (2) which contains commands (3,6) and
|
||||
devices (4...)
|
||||
|
||||
The last thing in the CMD_PACKAGED is a 'RUN' command (6) letting the destination
|
||||
CPUs start running.
|
||||
At the end of the CMD_PACKAGED (7) the main thread returns to normal running behaviour
|
||||
and is no longer used by migration, while the listen thread carries
|
||||
on servicing page data until the end of migration.
|
||||
- On receipt of 'postcopy listen' - 3 -(i.e. the 1st command in the package)
|
||||
|
||||
=== Postcopy states ===
|
||||
a new thread (a) is started that takes over servicing the migration stream,
|
||||
while the main thread carries on loading the package. It loads normal
|
||||
background page data (b) but if during a device load a fault happens (5)
|
||||
the returned page (c) is loaded by the listen thread allowing the main
|
||||
threads device load to carry on.
|
||||
|
||||
- The last thing in the ``CMD_PACKAGED`` is a 'RUN' command (6)
|
||||
|
||||
letting the destination CPUs start running. At the end of the
|
||||
``CMD_PACKAGED`` (7) the main thread returns to normal running behaviour and
|
||||
is no longer used by migration, while the listen thread carries on servicing
|
||||
page data until the end of migration.
|
||||
|
||||
Postcopy states
|
||||
---------------
|
||||
|
||||
Postcopy moves through a series of states (see postcopy_state) from
|
||||
ADVISE->DISCARD->LISTEN->RUNNING->END
|
||||
|
||||
Advise: Set at the start of migration if postcopy is enabled, even
|
||||
- Advise
|
||||
|
||||
Set at the start of migration if postcopy is enabled, even
|
||||
if it hasn't had the start command; here the destination
|
||||
checks that its OS has the support needed for postcopy, and performs
|
||||
setup to ensure the RAM mappings are suitable for later postcopy.
|
||||
|
@ -491,13 +503,17 @@ ADVISE->DISCARD->LISTEN->RUNNING->END
|
|||
required OS support is not present.
|
||||
(Triggered by reception of POSTCOPY_ADVISE command)
|
||||
|
||||
Discard: Entered on receipt of the first 'discard' command; prior to
|
||||
- Discard
|
||||
|
||||
Entered on receipt of the first 'discard' command; prior to
|
||||
the first Discard being performed, hugepages are switched off
|
||||
(using madvise) to ensure that no new huge pages are created
|
||||
during the postcopy phase, and to cause any huge pages that
|
||||
have discards on them to be broken.
|
||||
|
||||
Listen: The first command in the package, POSTCOPY_LISTEN, switches
|
||||
- Listen
|
||||
|
||||
The first command in the package, POSTCOPY_LISTEN, switches
|
||||
the destination state to Listen, and starts a new thread
|
||||
(the 'listen thread') which takes over the job of receiving
|
||||
pages off the migration stream, while the main thread carries
|
||||
|
@ -506,17 +522,22 @@ ADVISE->DISCARD->LISTEN->RUNNING->END
|
|||
any access to missing pages (on Linux using the 'userfault'
|
||||
system).
|
||||
|
||||
Running: POSTCOPY_RUN causes the destination to synchronise all
|
||||
- Running
|
||||
|
||||
POSTCOPY_RUN causes the destination to synchronise all
|
||||
state and start the CPUs and IO devices running. The main
|
||||
thread now finishes processing the migration package and
|
||||
now carries on as it would for normal precopy migration
|
||||
(although it can't do the cleanup it would do as it
|
||||
finishes a normal migration).
|
||||
|
||||
End: The listen thread can now quit, and perform the cleanup of migration
|
||||
- End
|
||||
|
||||
The listen thread can now quit, and perform the cleanup of migration
|
||||
state, the migration is now complete.
|
||||
|
||||
=== Source side page maps ===
|
||||
Source side page maps
|
||||
---------------------
|
||||
|
||||
The source side keeps two bitmaps during postcopy; 'the migration bitmap'
|
||||
and 'unsent map'. The 'migration bitmap' is basically the same as in
|
||||
|
@ -529,6 +550,7 @@ The 'unsent map' is used for the transition to postcopy. It is a bitmap that
|
|||
has a bit cleared whenever a page is sent to the destination, however during
|
||||
the transition to postcopy mode it is combined with the migration bitmap
|
||||
to form a set of pages that:
|
||||
|
||||
a) Have been sent but then redirtied (which must be discarded)
|
||||
b) Have not yet been sent - which also must be discarded to cause any
|
||||
transparent huge pages built during precopy to be broken.
|
||||
|
@ -540,15 +562,17 @@ request for a page that has already been sent is ignored. Duplicate requests
|
|||
such as this can happen as a page is sent at about the same time the
|
||||
destination accesses it.
|
||||
|
||||
=== Postcopy with hugepages ===
|
||||
Postcopy with hugepages
|
||||
-----------------------
|
||||
|
||||
Postcopy now works with hugetlbfs backed memory:
|
||||
|
||||
a) The linux kernel on the destination must support userfault on hugepages.
|
||||
b) The huge-page configuration on the source and destination VMs must be
|
||||
identical; i.e. RAMBlocks on both sides must use the same page size.
|
||||
c) Note that -mem-path /dev/hugepages will fall back to allocating normal
|
||||
c) Note that ``-mem-path /dev/hugepages`` will fall back to allocating normal
|
||||
RAM if it doesn't have enough hugepages, triggering (b) to fail.
|
||||
Using -mem-prealloc enforces the allocation using hugepages.
|
||||
Using ``-mem-prealloc`` enforces the allocation using hugepages.
|
||||
d) Care should be taken with the size of hugepage used; postcopy with 2MB
|
||||
hugepages works well, however 1GB hugepages are likely to be problematic
|
||||
since it takes ~1 second to transfer a 1GB hugepage across a 10Gbps link,
|
Loading…
Reference in a new issue