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Re: [Qemu-devel] [PATCH v2] docs: Convert migration.txt to rst
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From: |
Daniel P. Berrange |
|
Subject: |
Re: [Qemu-devel] [PATCH v2] docs: Convert migration.txt to rst |
|
Date: |
Fri, 15 Dec 2017 15:52:31 +0000 |
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User-agent: |
Mutt/1.9.1 (2017-09-22) |
On Fri, Dec 15, 2017 at 03:45:07PM +0000, Dr. David Alan Gilbert (git) wrote:
> From: "Dr. David Alan Gilbert" <address@hidden>
>
> Mostly just manual conversion with very minor fixes.
>
> Signed-off-by: Dr. David Alan Gilbert <address@hidden>
> ---
Reviewed-by: Daniel P. Berrange <address@hidden>
>
> v2
> Fixed issues found by Peter and Kashyap.
> Remove the detailed QEMUFile explanation and point to the header
> and QIOChannel code [after discussion with Daniel on Jay Zhou's patch]
>
> docs/devel/{migration.txt => migration.rst} | 476
> +++++++++++++++-------------
> 1 file changed, 250 insertions(+), 226 deletions(-)
> rename docs/devel/{migration.txt => migration.rst} (58%)
>
> diff --git a/docs/devel/migration.txt b/docs/devel/migration.rst
> similarity index 58%
> rename from docs/devel/migration.txt
> rename to docs/devel/migration.rst
> index 4030703726..bf97080dac 100644
> --- a/docs/devel/migration.txt
> +++ b/docs/devel/migration.rst
> @@ -1,4 +1,6 @@
> -= Migration =
> +=========
> +Migration
> +=========
>
> QEMU has code to load/save the state of the guest that it is running.
> These are two complementary operations. Saving the state just does
> @@ -26,7 +28,8 @@ 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 ===
> +Types of migration
> +==================
>
> Now that we have talked about live migration, there are several ways
> to do migration:
> @@ -41,49 +44,21 @@ All these four migration protocols use the same
> infrastructure to
> save/restore state devices. This infrastructure is shared with the
> savevm/loadvm functionality.
>
> -=== State Live Migration ===
> +State Live Migration
> +====================
>
> This is used for RAM and block devices. It is not yet ported to vmstate.
> <Fill more information here>
>
> -=== What is the common infrastructure ===
> +Common infrastructure
> +=====================
>
> -QEMU uses a QEMUFile abstraction to be able to do migration. Any type
> -of migration that wants to use QEMU infrastructure has to create a
> -QEMUFile with:
> +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/`).
>
> -QEMUFile *qemu_fopen_ops(void *opaque,
> - QEMUFilePutBufferFunc *put_buffer,
> - QEMUFileGetBufferFunc *get_buffer,
> - QEMUFileCloseFunc *close);
> -
> -The functions have the following functionality:
> -
> -This function writes a chunk of data to a file at the given position.
> -The pos argument can be ignored if the file is only used for
> -streaming. The handler should try to write all of the data it can.
> -
> -typedef int (QEMUFilePutBufferFunc)(void *opaque, const uint8_t *buf,
> - int64_t pos, int size);
> -
> -Read a chunk of data from a file at the given position. The pos argument
> -can be ignored if the file is only be used for streaming. The number of
> -bytes actually read should be returned.
> -
> -typedef int (QEMUFileGetBufferFunc)(void *opaque, uint8_t *buf,
> - int64_t pos, int size);
> -
> -Close a file and return an error code.
> -
> -typedef int (QEMUFileCloseFunc)(void *opaque);
> -
> -You can use any internal state that you need using the opaque void *
> -pointer that is passed to all functions.
> -
> -The important functions for us are put_buffer()/get_buffer() that
> -allow to write/read a buffer into the QEMUFile.
> -
> -=== How to save the state of one device ===
> +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.
> @@ -93,34 +68,38 @@ 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
> +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 ===
> +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.
>
> -int register_savevm(DeviceState *dev,
> - const char *idstr,
> - int instance_id,
> - int version_id,
> - SaveStateHandler *save_state,
> - LoadStateHandler *load_state,
> - void *opaque);
> +.. 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);
> + 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
> +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.
>
> -=== VMState ===
> +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
> @@ -135,31 +114,36 @@ save/load functions.
>
> An example (from hw/input/pckbd.c)
>
> -static const VMStateDescription vmstate_kbd = {
> - .name = "pckbd",
> - .version_id = 3,
> - .minimum_version_id = 3,
> - .fields = (VMStateField[]) {
> - VMSTATE_UINT8(write_cmd, KBDState),
> - VMSTATE_UINT8(status, KBDState),
> - VMSTATE_UINT8(mode, KBDState),
> - VMSTATE_UINT8(pending, KBDState),
> - VMSTATE_END_OF_LIST()
> - }
> -};
> +.. code:: c
> +
> + static const VMStateDescription vmstate_kbd = {
> + .name = "pckbd",
> + .version_id = 3,
> + .minimum_version_id = 3,
> + .fields = (VMStateField[]) {
> + VMSTATE_UINT8(write_cmd, KBDState),
> + VMSTATE_UINT8(status, KBDState),
> + VMSTATE_UINT8(mode, KBDState),
> + VMSTATE_UINT8(pending, KBDState),
> + VMSTATE_END_OF_LIST()
> + }
> + };
>
> We are declaring the state with name "pckbd".
> -The version_id is 3, and the fields are 4 uint8_t in a KBDState structure.
> +The `version_id` is 3, and the fields are 4 uint8_t in a KBDState structure.
> We registered this with:
>
> +.. code:: c
> +
> 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
> +You can search for ``VMSTATE_*`` macros for lots of types used in QEMU in
> include/hw/hw.h.
>
> -=== More about versions ===
> +More about versions
> +-------------------
>
> Version numbers are intended for major incompatible changes to the
> migration of a device, and using them breaks backwards-migration
> @@ -168,22 +152,23 @@ compatibility; in general most changes can be made by
> adding Subsections
>
> 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
> +- `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
> +- `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.
> + ignored if there is no `load_state_old` handler.
>
> 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
> +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.
>
> Saving state will always create a section with the 'version_id' value
> and thus can't be loaded by any older QEMU.
>
> -=== Massaging functions ===
> +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
> @@ -194,24 +179,24 @@ 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);
> +- ``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);
> +- ``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);
> +- ``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.
> +massage the state that is transferred.
>
> If you use memory API functions that update memory layout outside
> initialization (i.e., in response to a guest action), this is a strong
> -indication that you need to call these functions in a post_load callback.
> +indication that you need to call these functions in a `post_load` callback.
> Examples of such memory API functions are:
>
> - memory_region_add_subregion()
> @@ -221,7 +206,8 @@ Examples of such memory API functions are:
> - memory_region_set_address()
> - memory_region_set_alias_offset()
>
> -=== Subsections ===
> +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
> @@ -251,52 +237,54 @@ value that it uses.
>
> Example:
>
> -static bool ide_drive_pio_state_needed(void *opaque)
> -{
> - IDEState *s = opaque;
> -
> - return ((s->status & DRQ_STAT) != 0)
> - || (s->bus->error_status & BM_STATUS_PIO_RETRY);
> -}
> -
> -const VMStateDescription vmstate_ide_drive_pio_state = {
> - .name = "ide_drive/pio_state",
> - .version_id = 1,
> - .minimum_version_id = 1,
> - .pre_save = ide_drive_pio_pre_save,
> - .post_load = ide_drive_pio_post_load,
> - .needed = ide_drive_pio_state_needed,
> - .fields = (VMStateField[]) {
> - VMSTATE_INT32(req_nb_sectors, IDEState),
> - VMSTATE_VARRAY_INT32(io_buffer, IDEState, io_buffer_total_len, 1,
> - vmstate_info_uint8, uint8_t),
> - VMSTATE_INT32(cur_io_buffer_offset, IDEState),
> - VMSTATE_INT32(cur_io_buffer_len, IDEState),
> - VMSTATE_UINT8(end_transfer_fn_idx, IDEState),
> - VMSTATE_INT32(elementary_transfer_size, IDEState),
> - VMSTATE_INT32(packet_transfer_size, IDEState),
> - VMSTATE_END_OF_LIST()
> - }
> -};
> -
> -const VMStateDescription vmstate_ide_drive = {
> - .name = "ide_drive",
> - .version_id = 3,
> - .minimum_version_id = 0,
> - .post_load = ide_drive_post_load,
> - .fields = (VMStateField[]) {
> - .... several fields ....
> - VMSTATE_END_OF_LIST()
> - },
> - .subsections = (const VMStateDescription*[]) {
> - &vmstate_ide_drive_pio_state,
> - NULL
> - }
> -};
> +.. code:: c
> +
> + static bool ide_drive_pio_state_needed(void *opaque)
> + {
> + IDEState *s = opaque;
> +
> + return ((s->status & DRQ_STAT) != 0)
> + || (s->bus->error_status & BM_STATUS_PIO_RETRY);
> + }
> +
> + const VMStateDescription vmstate_ide_drive_pio_state = {
> + .name = "ide_drive/pio_state",
> + .version_id = 1,
> + .minimum_version_id = 1,
> + .pre_save = ide_drive_pio_pre_save,
> + .post_load = ide_drive_pio_post_load,
> + .needed = ide_drive_pio_state_needed,
> + .fields = (VMStateField[]) {
> + VMSTATE_INT32(req_nb_sectors, IDEState),
> + VMSTATE_VARRAY_INT32(io_buffer, IDEState, io_buffer_total_len, 1,
> + vmstate_info_uint8, uint8_t),
> + VMSTATE_INT32(cur_io_buffer_offset, IDEState),
> + VMSTATE_INT32(cur_io_buffer_len, IDEState),
> + VMSTATE_UINT8(end_transfer_fn_idx, IDEState),
> + VMSTATE_INT32(elementary_transfer_size, IDEState),
> + VMSTATE_INT32(packet_transfer_size, IDEState),
> + VMSTATE_END_OF_LIST()
> + }
> + };
> +
> + const VMStateDescription vmstate_ide_drive = {
> + .name = "ide_drive",
> + .version_id = 3,
> + .minimum_version_id = 0,
> + .post_load = ide_drive_post_load,
> + .fields = (VMStateField[]) {
> + .... several fields ....
> + VMSTATE_END_OF_LIST()
> + },
> + .subsections = (const VMStateDescription*[]) {
> + &vmstate_ide_drive_pio_state,
> + NULL
> + }
> + };
>
> Here we have a subsection for the pio state. We only need to
> save/send this state when we are in the middle of a pio operation
> -(that is what ide_drive_pio_state_needed() checks). If DRQ_STAT is
> +(that is what ``ide_drive_pio_state_needed()`` checks). If DRQ_STAT is
> not enabled, the values on that fields are garbage and don't need to
> be sent.
>
> @@ -304,11 +292,12 @@ Using a condition function that checks a 'property' to
> determine whether
> to send a subsection allows backwards migration compatibility when
> new subsections are added.
>
> -For example;
> - a) Add a new property using DEFINE_PROP_BOOL - e.g. support-foo and
> +For example:
> +
> + a) Add a new property using ``DEFINE_PROP_BOOL`` - e.g. support-foo and
> default it to true.
> - b) Add an entry to the HW_COMPAT_ for the previous version
> - that sets the property to false.
> + b) Add an entry to the ``HW_COMPAT_`` for the previous version that sets
> + the property to false.
> c) Add a static bool support_foo function that tests the property.
> d) Add a subsection with a .needed set to the support_foo function
> e) (potentially) Add a pre_load that sets up a default value for 'foo'
> @@ -332,25 +321,30 @@ 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.
>
> -= Not sending existing elements =
> +Not sending existing elements
> +-----------------------------
> +
> +Sometimes members of the VMState are no longer needed:
>
> -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.
> + - removing them will break migration compatibility
> +
> + - making them version dependent and bumping the version will break
> backwards migration compatibility.
>
> The best way is to:
> - a) Add a new property/compatibility/function in the same way for
> subsections
> - above.
> +
> + 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.:
> - VMSTATE_UINT32(foo, barstruct)
> +
> + ``VMSTATE_UINT32(foo, barstruct)``
> +
> becomes
> - 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.
> + ``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.
>
> -= Return path =
> +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
> @@ -360,19 +354,23 @@ However, some uses need two way communication; in
> particular the Postcopy
> destination needs to be able to request pages on demand from the source.
>
> 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
> +``qemu_file_get_return_path(QEMUFile* fwdpath)`` gives the QEMUFile* for the
> return
> path.
>
> Source side
> +
> Forward path - written by migration thread
> Return path - opened by main thread, read by return-path thread
>
> Destination side
> +
> Forward path - read by main thread
> Return path - opened by main thread, written by main thread AND
> postcopy
> - thread (protected by rp_mutex)
> + thread (protected by rp_mutex)
> +
> +Postcopy
> +========
>
> -= Postcopy =
> 'Postcopy' migration is a way to deal with migrations that refuse to converge
> (or take too long to converge) its plus side is that there is an upper bound
> on
> the amount of migration traffic and time it takes, the down side is that
> during
> @@ -386,27 +384,30 @@ a fault that's translated by QEMU into a request to the
> source QEMU.
> Postcopy can be combined with precopy (i.e. normal migration) so that if
> precopy
> doesn't finish in a given time the switch is made to postcopy.
>
> -=== Enabling postcopy ===
> +Enabling postcopy
> +-----------------
>
> To enable postcopy, issue this command on the monitor prior to the
> start of migration:
>
> -migrate_set_capability postcopy-ram on
> +``migrate_set_capability postcopy-ram on``
>
> The normal commands are then used to start a migration, which is still
> started in precopy mode. Issuing:
>
> -migrate_start_postcopy
> +``migrate_start_postcopy``
>
> will now cause the transition from precopy to postcopy.
> It can be issued immediately after migration is started or any
> time later on. Issuing it after the end of a migration is harmless.
>
> -Note: During the postcopy phase, the bandwidth limits set using
> -migrate_set_speed is ignored (to avoid delaying requested pages that
> -the destination is waiting for).
> +.. note::
> + During the postcopy phase, the bandwidth limits set using
> + ``migrate_set_speed`` is ignored (to avoid delaying requested pages that
> + the destination is waiting for).
>
> -=== Postcopy device transfer ===
> +Postcopy device transfer
> +------------------------
>
> Loading of device data may cause the device emulation to access guest RAM
> that may trigger faults that have to be resolved by the source, as such
> @@ -416,6 +417,7 @@ before the device load begins to free the stream up.
> This is achieved by
> 'packaging' the device data into a blob that's read in one go.
>
> Source behaviour
> +----------------
>
> Until postcopy is entered the migration stream is identical to normal
> precopy, except for the addition of a 'postcopy advise' command at
> @@ -423,13 +425,14 @@ the beginning, to tell the destination that postcopy
> might happen.
> When postcopy starts the source sends the page discard data and then
> forms the 'package' containing:
>
> - Command: 'postcopy listen'
> - The device state
> - A series of sections, identical to the precopy streams device state
> stream
> - containing everything except postcopiable devices (i.e. RAM)
> - Command: 'postcopy run'
> + - Command: 'postcopy listen'
> + - The device state
>
> -The 'package' is sent as the data part of a Command: 'CMD_PACKAGED', and the
> + A series of sections, identical to the precopy streams device state
> stream
> + containing everything except postcopiable devices (i.e. RAM)
> + - Command: 'postcopy run'
> +
> +The 'package' is sent as the data part of a Command: ``CMD_PACKAGED``, and
> the
> contents are formatted in the same way as the main migration stream.
>
> During postcopy the source scans the list of dirty pages and sends them
> @@ -441,82 +444,100 @@ to be sent quickly in the hope that those pages are
> likely to be used
> by the destination soon.
>
> Destination behaviour
> +---------------------
>
> Initially the destination looks the same as precopy, with a single thread
> reading the migration stream; the 'postcopy advise' and 'discard' commands
> are processed to change the way RAM is managed, but don't affect the stream
> processing.
>
> -------------------------------------------------------------------------------
> - 1 2 3 4 5 6 7
> -main -----DISCARD-CMD_PACKAGED ( LISTEN DEVICE DEVICE DEVICE RUN )
> -thread | |
> - | (page request)
> - | \___
> - v \
> -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 '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.
> -
> -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 ===
> +::
> +
> +
> ------------------------------------------------------------------------------
> + 1 2 3 4 5 6 7
> + main -----DISCARD-CMD_PACKAGED ( LISTEN DEVICE DEVICE DEVICE RUN )
> + thread | |
> + | (page request)
> + | \___
> + v \
> + 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 '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.
> +
> +- 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
> - 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.
> - The destination will fail early in migration at this point if the
> - required OS support is not present.
> - (Triggered by reception of POSTCOPY_ADVISE command)
> -
> - 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
> - 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
> - on processing the blob. With this thread able to process page
> - reception, the destination now 'sensitises' the RAM to detect
> - any access to missing pages (on Linux using the 'userfault'
> - system).
> -
> - 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
> - state, the migration is now complete.
> -
> -=== Source side page maps ===
> + - 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.
> + The destination will fail early in migration at this point if the
> + required OS support is not present.
> + (Triggered by reception of POSTCOPY_ADVISE command)
> +
> + - 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
> + 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
> + on processing the blob. With this thread able to process page
> + reception, the destination now 'sensitises' the RAM to detect
> + any access to missing pages (on Linux using the 'userfault'
> + system).
> +
> + - 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
> + state, the migration is now complete.
> +
> +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,
> --
> 2.14.3
>
Regards,
Daniel
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