[PATCH v2 for-4.6 2/2] docs: Migration feature document

[Andrew Cooper <andrew.cooper3@citrix.com>]

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
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+% Migration
+% Revision 1
+
+\clearpage
+
+# 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
+
+or
+
+    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 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)
+
+# History
+
+------------------------------------------------------------------------
+Date       Revision Version  Notes
+---------- -------- -------- -------------------------------------------
+2015-10-24 1        Xen 4.6  Document written
+---------- -------- -------- -------------------------------------------
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1.7.10.4