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Re: [Qemu-devel] [PATCH] docs: Convert migration.txt to rst
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From: |
Peter Xu |
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Subject: |
Re: [Qemu-devel] [PATCH] docs: Convert migration.txt to rst |
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Date: |
Tue, 12 Dec 2017 23:10:48 +0800 |
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User-agent: |
Mutt/1.9.1 (2017-09-22) |
On Tue, Dec 12, 2017 at 01:56:00PM +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: Peter Xu <address@hidden>
Some nits below, but r-b is fine with/without changing them. Mostly
it's about wrapping lines. I think .rst supports wrapping lines?
Thanks,
> ---
> docs/devel/{migration.txt => migration.rst} | 326
> +++++++++++++++-------------
> 1 file changed, 176 insertions(+), 150 deletions(-)
> rename docs/devel/{migration.txt => migration.rst} (74%)
>
> diff --git a/docs/devel/migration.txt b/docs/devel/migration.rst
> similarity index 74%
> rename from docs/devel/migration.txt
> rename to docs/devel/migration.rst
> index 4030703726..7d64298cd7 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:
> @@ -41,21 +44,25 @@ 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:
>
> -QEMUFile *qemu_fopen_ops(void *opaque,
> - QEMUFilePutBufferFunc *put_buffer,
> - QEMUFileGetBufferFunc *get_buffer,
> - QEMUFileCloseFunc *close);
> +.. code:: c
> +
> + QEMUFile *qemu_fopen_ops(void *opaque,
> + QEMUFilePutBufferFunc *put_buffer,
> + QEMUFileGetBufferFunc *get_buffer,
> + QEMUFileCloseFunc *close);
>
> The functions have the following functionality:
>
> @@ -63,19 +70,25 @@ 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);
> +.. code:: c
> +
> + 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);
> +.. code:: c
> +
> + 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);
> +.. code:: c
> +
> + 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.
> @@ -83,7 +96,8 @@ 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.
> @@ -97,30 +111,34 @@ 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
> 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
> @@ -135,23 +153,27 @@ 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.
> 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.
> @@ -159,7 +181,8 @@ Note: talk about how vmstate <-> qdev interact, and what
> the instance ids mean.
> You can search for VMSTATE_* macros for lots of types used in QEMU in
> include/hw/hw.h.
>
> -=== More about versions ===
> +More about versions
> +-------------------
>
> Version numbers are intended for major incompatible changes to the
> migration of a device, and using them breaks backwards-migration
> @@ -183,7 +206,8 @@ 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
> @@ -194,20 +218,19 @@ 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.
> +Example: You can look at hpet.c, that uses the three function to massage the
> state that is transferred.
This change seems meaningless, I would prefer keep it wrapped, but I'm
fine with it.
>
> If you use memory API functions that update memory layout outside
> initialization (i.e., in response to a guest action), this is a strong
> @@ -221,7 +244,8 @@ 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
> @@ -251,48 +275,50 @@ 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
> @@ -305,14 +331,11 @@ 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
> - default it to true.
> - b) Add an entry to the HW_COMPAT_ for the previous version
> - that sets the property to false.
> + 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.
> 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'
> - to be used if the subsection isn't loaded.
> + e) (potentially) Add a pre_load that sets up a default value for 'foo' to
> be used if the subsection isn't loaded.
(same to these lines)
>
> Now that subsection will not be generated when using an older
> machine type and the migration stream will be accepted by older
> @@ -332,25 +355,28 @@ 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;
> - 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)``
>
> -= Return path =
> + Sometime in the future when we no longer care about the ancient versions
> these can be killed off.
> +
> +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
> @@ -369,10 +395,11 @@ path.
>
> 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)
> + Return path - opened by main thread, written by main thread AND
> postcopy thread (protected by rp_mutex)
(same here)
> +
> +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
> @@ -386,17 +413,18 @@ 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
> @@ -406,7 +434,8 @@ 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 +445,7 @@ 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
> @@ -423,11 +453,10 @@ 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
> + A series of sections, identical to the precopy streams device state
> stream containing everything except postcopiable devices (i.e. RAM)
(a super long line, fine too)
> + - 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.
> @@ -441,44 +470,38 @@ 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.
(here the lines can be wrapped too?)
> +
> +Postcopy states
> +---------------
>
> Postcopy moves through a series of states (see postcopy_state) from
> ADVISE->DISCARD->LISTEN->RUNNING->END
> @@ -516,7 +539,8 @@ ADVISE->DISCARD->LISTEN->RUNNING->END
> 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 +553,7 @@ 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.
> @@ -540,7 +565,8 @@ 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.
> --
> 2.14.3
>
--
Peter Xu