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Diffstat (limited to 'docs/migration.txt')
| -rw-r--r-- | docs/migration.txt | 71 |
1 files changed, 71 insertions, 0 deletions
diff --git a/docs/migration.txt b/docs/migration.txt index 6503c17685..1b940a829b 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 @@ -482,3 +540,16 @@ 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 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 + RAM if it doesn't have enough hugepages, triggering (b) to fail. + 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, + and until the full page is transferred the destination thread is blocked. |