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authorDr. David Alan Gilbert <dgilbert@redhat.com>2017-02-10 11:03:59 +0000
committerDr. David Alan Gilbert <dgilbert@redhat.com>2017-02-28 11:30:22 +0000
commit5f9412bbac3a6906b2277d6b8aea02bc12a8464d (patch)
tree459d4cb357ee265d2c1149f7a4a019adb75f43d0
parent128e4e108949b35dbe351fe122a3e34b834e185a (diff)
migration: Update docs to discourage version bumps
Version bumps break backwards migration; update the docs to explain to people that's bad and how to avoid it. Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com> Reviewed-by: Markus Armbruster <armbru@redhat.com> Message-Id: <20170210110359.8210-1-dgilbert@redhat.com> Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
-rw-r--r--docs/migration.txt58
1 files changed, 58 insertions, 0 deletions
diff --git a/docs/migration.txt b/docs/migration.txt
index 6503c17685..c8b0304d5e 100644
--- a/docs/migration.txt
+++ b/docs/migration.txt
@@ -161,6 +161,11 @@ include/hw/hw.h.
=== More about versions ===
+Version numbers are intended for major incompatible changes to the
+migration of a device, and using them breaks backwards-migration
+compatibility; in general most changes can be made by adding Subsections
+(see below) or _TEST macros (see below) which won't break compatibility.
+
You can see that there are several version fields:
- version_id: the maximum version_id supported by VMState for that device.
@@ -175,6 +180,9 @@ 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 ===
Sometimes, it is not enough to be able to save the state directly
@@ -292,6 +300,56 @@ save/send this state when we are in the middle of a pio operation
not enabled, the values on that fields are garbage and don't need to
be sent.
+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
+ 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.
+
+Now that subsection will not be generated when using an older
+machine type and the migration stream will be accepted by older
+QEMU versions. pre-load functions can be used to initialise state
+on the newer version so that they default to suitable values
+when loading streams created by older QEMU versions that do not
+generate the subsection.
+
+In some cases subsections are added for data that had been accidentally
+omitted by earlier versions; if the missing data causes the migration
+process to succeed but the guest to behave badly then it may be better
+to send the subsection and cause the migration to explicitly fail
+with the unknown subsection error. If the bad behaviour only happens
+with certain data values, making the subsection conditional on
+the data value (rather than the machine type) allows migrations to succeed
+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 =
+
+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.
+
+The best way is to:
+ 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)
+ 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.
+
= Return path =
In most migration scenarios there is only a single data path that runs