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Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com> Reviewed-by: Amit Shah <amit.shah@redhat.com> Reviewed-by: Juan Quintela <quintela@redhat.com> Signed-off-by: Juan Quintela <quintela@redhat.com>
485 lines
20 KiB
Plaintext
485 lines
20 KiB
Plaintext
= Migration =
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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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that, saves the state for each device that the guest is running.
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Restoring a guest is just the opposite operation: we need to load the
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state of each device.
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For this to work, QEMU has to be launched with the same arguments the
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two times. I.e. it can only restore the state in one guest that has
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the same devices that the one it was saved (this last requirement can
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be relaxed a bit, but for now we can consider that configuration has
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to be exactly the same).
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Once that we are able to save/restore a guest, a new functionality is
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requested: migration. This means that QEMU is able to start in one
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machine and being "migrated" to another machine. I.e. being moved to
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another machine.
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Next was the "live migration" functionality. This is important
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because some guests run with a lot of state (specially RAM), and it
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can take a while to move all state from one machine to another. Live
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migration allows the guest to continue running while the state is
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transferred. Only while the last part of the state is transferred has
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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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Now that we have talked about live migration, there are several ways
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to do migration:
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- tcp migration: do the migration using tcp sockets
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- unix migration: do the migration using unix sockets
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- exec migration: do the migration using the stdin/stdout through a process.
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- fd migration: do the migration using an file descriptor that is
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passed to QEMU. QEMU doesn't care how this file descriptor is opened.
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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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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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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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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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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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There is a new concept that we have to explain here: device state
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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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load state from an older version.
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=== Legacy way ===
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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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const char *idstr,
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int instance_id,
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int version_id,
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SaveStateHandler *save_state,
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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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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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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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in one of them and not in the other, we would get a failed migration.
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VMState changed the way that state is saved/loaded. Instead of using
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a function to save the state and another to load it, it was changed to
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a declarative way of what the state consisted of. Now VMState is able
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to interpret that definition to be able to load/save the state. As
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the state is declared only once, it can't go out of sync in the
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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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.name = "pckbd",
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.version_id = 3,
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.minimum_version_id = 3,
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.fields = (VMStateField[]) {
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VMSTATE_UINT8(write_cmd, KBDState),
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VMSTATE_UINT8(status, KBDState),
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VMSTATE_UINT8(mode, KBDState),
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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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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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We registered this with:
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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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include/hw/hw.h.
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=== More about versions ===
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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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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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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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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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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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=== Massaging functions ===
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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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example is when we are using kvm. Before saving the cpu state, we
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need to ask kvm to copy to QEMU the state that it is using. And the
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opposite when we are loading the state, we need a way to tell kvm to
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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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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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This function is called after we load the state of one device.
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- void (*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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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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Examples of such memory API functions are:
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- memory_region_add_subregion()
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- memory_region_del_subregion()
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- memory_region_set_readonly()
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- memory_region_set_enabled()
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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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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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makes very complicated to fix bugs in stable branches. If we need to
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add anything to the state to fix a bug, we have to disable migration
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to older versions that don't have that bug-fix (i.e. a new field).
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But sometimes, that bug-fix is only needed sometimes, not always. For
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instance, if the device is in the middle of a DMA operation, it is
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using a specific functionality, ....
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It is impossible to create a way to make migration from any version to
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any other version to work. But we can do better than only allowing
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migration from older versions to newer ones. For that fields that are
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only needed sometimes, we add the idea of subsections. A subsection
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is "like" a device vmstate, but with a particularity, it has a Boolean
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function that tells if that values are needed to be sent or not. If
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this functions returns false, the subsection is not sent.
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On the receiving side, if we found a subsection for a device that we
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don't understand, we just fail the migration. If we understand all
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the subsections, then we load the state with success.
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One important note is that the post_load() function is called "after"
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loading all subsections, because a newer subsection could change same
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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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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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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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.pre_save = ide_drive_pio_pre_save,
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.post_load = ide_drive_pio_post_load,
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.needed = ide_drive_pio_state_needed,
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.fields = (VMStateField[]) {
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VMSTATE_INT32(req_nb_sectors, IDEState),
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VMSTATE_VARRAY_INT32(io_buffer, IDEState, io_buffer_total_len, 1,
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vmstate_info_uint8, uint8_t),
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VMSTATE_INT32(cur_io_buffer_offset, IDEState),
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VMSTATE_INT32(cur_io_buffer_len, IDEState),
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VMSTATE_UINT8(end_transfer_fn_idx, IDEState),
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VMSTATE_INT32(elementary_transfer_size, IDEState),
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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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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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.post_load = ide_drive_post_load,
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.fields = (VMStateField[]) {
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.... several fields ....
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VMSTATE_END_OF_LIST()
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},
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.subsections = (const VMStateDescription*[]) {
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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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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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not enabled, the values on that fields are garbage and don't need to
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be sent.
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= Return path =
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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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possibly with another fd or similar for some fast way of throwing pages across).
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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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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' 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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the postcopy phase, a failure of *either* side or the network connection causes
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the guest to be lost.
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In postcopy the destination CPUs are started before all the memory has been
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transferred, and accesses to pages that are yet to be transferred cause
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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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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 x-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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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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=== Postcopy device transfer ===
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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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the migration stream has to be able to respond with page data *during* the
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device load, and hence the device data has to be read from the stream completely
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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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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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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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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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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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to the destination without being requested (in much the same way as precopy),
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however when a page request is received from the destination, the dirty page
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scanning restarts from the requested location. This causes requested pages
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to be sent quickly, and also causes pages directly after the requested page
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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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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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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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| (page request)
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| \___
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v \
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listen thread: --- page -- page -- page -- page -- page --
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a b c
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------------------------------------------------------------------------------
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On receipt of CMD_PACKAGED (1)
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All the data associated with the package - the ( ... ) section in the
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diagram - is read into memory (into a QEMUSizedBuffer), and the main thread
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recurses into qemu_loadvm_state_main to process the contents of the package (2)
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which contains commands (3,6) and devices (4...)
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On receipt of 'postcopy listen' - 3 -(i.e. the 1st command in the package)
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a new thread (a) is started that takes over servicing the migration stream,
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while the main thread carries on loading the package. It loads normal
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background page data (b) but if during a device load a fault happens (5) the
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returned page (c) is loaded by the listen thread allowing the main threads
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device load to carry on.
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The last thing in the CMD_PACKAGED is a 'RUN' command (6) letting the destination
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CPUs start running.
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At the end of the CMD_PACKAGED (7) the main thread returns to normal running behaviour
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and is no longer used by migration, while the listen thread carries
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on servicing page data until the end of migration.
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=== Postcopy states ===
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Postcopy moves through a series of states (see postcopy_state) from
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ADVISE->DISCARD->LISTEN->RUNNING->END
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Advise: Set at the start of migration if postcopy is enabled, even
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if it hasn't had the start command; here the destination
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checks that its OS has the support needed for postcopy, and performs
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setup to ensure the RAM mappings are suitable for later postcopy.
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The destination will fail early in migration at this point if the
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required OS support is not present.
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(Triggered by reception of POSTCOPY_ADVISE command)
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Discard: Entered on receipt of the first 'discard' command; prior to
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the first Discard being performed, hugepages are switched off
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(using madvise) to ensure that no new huge pages are created
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during the postcopy phase, and to cause any huge pages that
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have discards on them to be broken.
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Listen: The first command in the package, POSTCOPY_LISTEN, switches
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the destination state to Listen, and starts a new thread
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(the 'listen thread') which takes over the job of receiving
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pages off the migration stream, while the main thread carries
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on processing the blob. With this thread able to process page
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reception, the destination now 'sensitises' the RAM to detect
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any access to missing pages (on Linux using the 'userfault'
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system).
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Running: POSTCOPY_RUN causes the destination to synchronise all
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state and start the CPUs and IO devices running. The main
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thread now finishes processing the migration package and
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now carries on as it would for normal precopy migration
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(although it can't do the cleanup it would do as it
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finishes a normal migration).
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End: The listen thread can now quit, and perform the cleanup of migration
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state, the migration is now complete.
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=== Source side page maps ===
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The source side keeps two bitmaps during postcopy; 'the migration bitmap'
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and 'unsent map'. The 'migration bitmap' is basically the same as in
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the precopy case, and holds a bit to indicate that page is 'dirty' -
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i.e. needs sending. During the precopy phase this is updated as the CPU
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dirties pages, however during postcopy the CPUs are stopped and nothing
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should dirty anything any more.
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The 'unsent map' is used for the transition to postcopy. It is a bitmap that
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has a bit cleared whenever a page is sent to the destination, however during
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the transition to postcopy mode it is combined with the migration bitmap
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to form a set of pages that:
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a) Have been sent but then redirtied (which must be discarded)
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b) Have not yet been sent - which also must be discarded to cause any
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transparent huge pages built during precopy to be broken.
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Note that the contents of the unsentmap are sacrificed during the calculation
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of the discard set and thus aren't valid once in postcopy. The dirtymap
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is still valid and is used to ensure that no page is sent more than once. Any
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request for a page that has already been sent is ignored. Duplicate requests
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such as this can happen as a page is sent at about the same time the
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destination accesses it.
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