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[Qemu-devel] [PATCH] docs: Convert migration.txt to rst
From: |
Dr. David Alan Gilbert (git) |
Subject: |
[Qemu-devel] [PATCH] docs: Convert migration.txt to rst |
Date: |
Tue, 12 Dec 2017 13:56:00 +0000 |
From: "Dr. David Alan Gilbert" <address@hidden>
Mostly just manual conversion with very minor fixes.
Signed-off-by: Dr. David Alan Gilbert <address@hidden>
---
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.
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.
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)
+
+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)
+ - 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.
+
+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
- [Qemu-devel] [PATCH] docs: Convert migration.txt to rst,
Dr. David Alan Gilbert (git) <=