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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Mon, Aug 24, 2015 at 06:37:02PM +0100, Andrew Cooper wrote:
> 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
> new file mode 100644
> index 0000000..e0422f9
> --- /dev/null
> +++ b/docs/features/migration.pandoc
> @@ -0,0 +1,90 @@
> +% 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 of formed of several layers.  `libxc` is responsible for the

Extraneous "of".

> +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)

TBH I think "Areas for improvement" and "Known issues" sections are very
cryptic to normal users, but I don't have any suggestion to make them
better.

In any case, any document is better than no documents. I'm all for this
doc going in as soon as possible.

Wei.

> -- 
> 1.7.10.4
> 
> 
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