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[Xen-devel] [PATCH for-4.6 2/2] docs: Migration feature document

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
 docs/features/migration.pandoc |   90 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 90 insertions(+)
 create mode 100644 docs/features/migration.pandoc

diff --git a/docs/features/migration.pandoc b/docs/features/migration.pandoc
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+% Migration
+# Basics
+--------------- -------------
+        Status: **Supported**
+  Architecture: x86
+     Component: Toolstack
+--------------- -------------
+# Overview
+Migration is a mechanism to move a virtual machine while the VM is running.
+Live migration moves a running virtual machine between two physical servers,
+but the same mechanism can be used for non-live migrate (pause and copy) and
+suspend/resume from disk.
+# User details
+No hardware requirements, although hypervisor logdirty support is required for
+live migration.
+From the command line, `xl migrate/save/restore` are the top level
+interactions.  e.g.
+    xl create my-vm.cfg
+    xl migrate my-vm localhost
+    xl create my-vm.cfg
+    xl save my-vm /path/to/save/file
+    xl restore /path/to/save/file
+Xen 4.6 sees the instruction of Migration v2.  There is no change for people
+using `xl`, although the `libxl` API has had an extension.
+# Technical details
+Migration is of formed of several layers.  `libxc` is responsible for the
+contents of the VM (ram, vcpus, etc) and the live migration loop, while
+`libxl` is responsible for items such as emulator state.
+The format of the migration v2 stream is specified in two documents, and is
+architecture neutral.  Compatibility with legacy streams is maintained via the
+`convert-legacy-stream` script which transforms a legacy stream into a
+migration v2 stream.
+* Documents
+    * `docs/specs/libxc-migration-stream.pandoc`
+    * `docs/specs/libxl-migration-stream.pandoc`
+* `libxc`
+    * `tools/libxc/xc_sr_*.[hc]`
+* `libxl`
+    * `tools/libxl/libxl_stream_{read,write}.c`
+* Scripts
+    * `tools/python/xen/migration/*.py`
+    * `tools/python/scripts/convert-legacy-stream`
+    * `tools/python/scripts/verify-stream-v2`
+Users of the `libxl` API have a new parameter `stream_version` in
+`domain_restore_params` which is used to distinguish between legacy and v2
+migration streams, and hence whether legacy conversion is required.
+# Limitations
+Hypervisor logdirty support is incompatible with hardware passthrough, as
+IOMMU faults cannot be used to track writes.
+While not a bug in migration specifically, VMs are very sensitive to changes
+in cpuid information, and cpuid levelling support currently has its issues.
+Extreme care should be taken when migrating VMs between non-identical CPUs
+until the cpuid levelling improvements are complete.
+# Areas for improvement
+* Arm support
+* Linear P2M support for x86 PV
+* Live looping parameters
+# Known issues
+* x86 HVM guest physmap operations (not reflected in logdirty bitmap)
+* x86 HVM with PoD pages (attempts to map cause PoD allocations)
+* x86 HVM with nested-virt (no relevant information included in the stream)
+* x86 PV ballooning (P2M marked dirty, target frame not marked)
+* x86 PV P2M structure changes (not noticed, stale mappings used)

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