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

[Jim Fehlig <jfehlig@suse.com>]

On 08/25/2015 04:40 AM, Andrew Cooper wrote:
> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
> ---
> v2:
>   * %Revision and #History, following template review
>   * Grammar fixes
> ---
>   docs/features/migration.pandoc |  100 ++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 100 insertions(+)
>   create mode 100644 docs/features/migration.pandoc
>
> diff --git a/docs/features/migration.pandoc b/docs/features/migration.pandoc
> new file mode 100644
> index 0000000..0fd227f
> --- /dev/null
> +++ b/docs/features/migration.pandoc
> @@ -0,0 +1,100 @@
> +% 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.

s/migrate/migration/ ?

> +
> +# 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

Xen 4.6 sees the introduction of Migration v2.
Or, Xen 4.6 introduces Migration v2.

> +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.

A question better asked when you posted the patches, but is there a way to 
detect if the receiver is V2 capable? I suspect sending a V2 stream to a 
receiver capable only of V1 is not advised :-).

Also, commit id introducing the change would be helpful info. Looks like 
3a9ace01 in this case.

> +
> +# 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

* cpuid levelling

Regards,
Jim

> +
> +# 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
> +---------- -------- -------- -------------------------------------------