[Qemu-devel] [PATCH] migration: Update docs to discourage version bumps

Dr. David Alan Gilbert (git) posted 1 patch 158 weeks ago
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git fetch https://github.com/patchew-project/qemu tags/patchew/20170209181746.29896-1-dgilbert@redhat.com
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docs/migration.txt | 42 ++++++++++++++++++++++++++++++++++++++++++
1 file changed, 42 insertions(+)

[Qemu-devel] [PATCH] migration: Update docs to discourage version bumps

Posted by Dr. David Alan Gilbert (git) 158 weeks ago
From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>

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>
 docs/migration.txt | 42 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 42 insertions(+)

diff --git a/docs/migration.txt b/docs/migration.txt
index b462ead..f375f4b 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.
+The VMState with the 'version_id' value will always be generated 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 +300,40 @@ 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.
+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
+   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.
+= 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