Xen project Mailing List

On Mon, Feb 10, 2014 at 9:20 AM, David Vrabel <david.vrabel@xxxxxxxxxx> wrote:

Here is a draft of a proposal for a new domain save image format. It
does not currently cover all use cases (e.g., images for HVM guest are
not considered).

http://xenbits.xen.org/people/dvrabel/domain-save-format-B.pdf

Introduction
============

Revision History
----------------

--------------------------------------------------------------------
Version Date Changes
------- ----------- ----------------------------------------------
Draft A 6 Feb 2014 Initial draft.

Draft B 10 Feb 2014 Corrected image header field widths.

Minor updates and clarifications.
--------------------------------------------------------------------

Purpose
-------

The _domain save image_ is the context of a running domain used for
snapshots of a domain or for transferring domains between hosts during
migration.

There are a number of problems with the format of the domain save
image used in Xen 4.4 and earlier (the _legacy format_).

* Dependant on toolstack word size. A number of fields within the
image are native types such as `unsigned long` which have different
sizes between 32-bit and 64-bit hosts. This prevents domains from
being migrated between 32-bit and 64-bit hosts.

* There is no header identifying the image.

* The image has no version information.

A new format that addresses the above is required.

ARM does not yet have have a domain save image format specified and
the format described in this specification should be suitable.

I suggest keeping the processing overhead in mind when designing the new

image format. Some key things have been addressed, such as making sure data

is always padded to maintain alignment. But there are also some aspects of this

proposal that seem awfully unnecessary.. More details below.

Overview
========

The image format consists of two main sections:

* _Headers_
* _Records_

Headers
-------

There are two headers: the _image header_, and the _domain header_.
The image header describes the format of the image (version etc.).
The _domain header_ contains general information about the domain
(architecture, type etc.).

Records
-------

The main part of the format is a sequence of different _records_.
Each record type contains information about the domain context. At a
minimum there is a END record marking the end of the records section.

Fields
------

All the fields within the headers and records have a fixed width.

Fields are always aligned to their size.

Padding and reserved fields are set to zero on save and must be
ignored during restore.

So far so good.

Integer (numeric) fields in the image header are always in big-endian
byte order.

Integer fields in the domain header and in the records are in the
endianess described in the image header (which will typically be the
native ordering).

Its tempting to adopt all the TCP-style madness for transferring a set of

structured data. Why this endian-ness mess? Am I missing something here?

I am assuming that a lion's share of Xen's deployment is on x86

(not including Amazon). So that leaves ARM. Why not let these

processors take the hit of endian-ness conversion?

Headers
=======

Image Header
------------

The image header identifies an image as a Xen domain save image. It
includes the version of this specification that the image complies
with.

Tools supporting version _V_ of the specification shall always save
images using version _V_. Tools shall support restoring from version
_V_ and version _V_ - 1. Tools may additionally support restoring
from earlier versions.

The marker field can be used to distinguish between legacy images and
those corresponding to this specification. Legacy images will have at
one or more zero bits within the first 8 octets of the image.

Fields within the image header are always in _big-endian_ byte order,
regardless of the setting of the endianness bit.

and more endian-ness mess.

0 1 2 3 4 5 6 7 octet
+-------------------------------------------------+
| marker |
+-----------------------+-------------------------+
| id | version |
+-----------+-----------+-------------------------+
| options | |
+-----------+-------------------------------------+

--------------------------------------------------------------------
Field Description
----------- --------------------------------------------------------
marker 0xFFFFFFFFFFFFFFFF.

id 0x58454E46 ("XENF" in ASCII).

version 0x00000001. The version of this specification.

options bit 0: Endianness. 0 = little-endian, 1 = big-endian.

bit 1-15: Reserved.
--------------------------------------------------------------------

Domain Header
-------------

The domain header includes general properties of the domain.

0 1 2 3 4 5 6 7 octet
+-----------+-----------+-----------+-------------+
| arch | type | page_shift| (reserved) |
+-----------+-----------+-----------+-------------+

--------------------------------------------------------------------
Field Description
----------- --------------------------------------------------------
arch 0x0000: Reserved.

0x0001: x86.

0x0002: ARM.

type 0x0000: Reserved.

0x0001: x86 PV.

0x0002 - 0xFFFF: Reserved.

page_shift Size of a guest page as a power of two.

i.e., page size = 2^page_shift^.
--------------------------------------------------------------------

Records
=======

A record has a record header, type specific data and a trailing
footer. If body_length is not a multiple of 8, the body is padded
with zeroes to align the checksum field on an 8 octet boundary.

0 1 2 3 4 5 6 7 octet
+-----------------------+-------------------------+
| type | body_length |
+-----------+-----------+-------------------------+
| options | (reserved) |
+-----------+-------------------------------------+
...
Record body of length body_length octets followed by
0 to 7 octets of padding.
...
+-----------------------+-------------------------+
| checksum | (reserved) |
+-----------------------+-------------------------+

I am assuming that you the checksum field is present only

for debugging purposes? Otherwise, I see no reason for the

computational overhead, given that we are already sending data

over a reliable channel + IIRC we already have an image-wide checksum

when saving the image to disk.

If debugging is the only use case, then I guess, the type field

can be prefixed with a 1/0 bit, eliminating the need for the

1-bit checkum options field + 7-byte padding. Similarly, if debugging

mode is not set, why waste another 8 bytes in the end for the checksum field.

Unless you think there may be more types with need of special options,

Feel free to correct me if I am missing something elementary here..

--------------------------------------------------------------------
Field Description
----------- -------------------------------------------------------
type 0x00000000: END

0x00000001: PAGE_DATA

0x00000002: VCPU_INFO

0x00000003: VCPU_CONTEXT

0x00000004: X86_PV_INFO

0x00000005: P2M

0x00000006 - 0xFFFFFFFF: Reserved

body_length Length in octets of the record body.

options Bit 0: 0 - checksum invalid, 1 = checksum valid.

Bit 1-15: Reserved.

checksum CRC-32 checksum of the record body (including any trailing
padding), or 0x00000000 if the checksum field is invalid.
--------------------------------------------------------------------

The following sub-sections specify the record body format for each of
the record types.

END
----

A end record marks the end of the image.

0 1 2 3 4 5 6 7 octet
+-------------------------------------------------+

The end record contains no fields; its body_length is 0.

PAGE_DATA
---------

The bulk of an image consists of many PAGE_DATA records containing the
memory contents.

0 1 2 3 4 5 6 7 octet
+-----------------------+-------------------------+
| count (C) | (reserved) |
+-----------------------+-------------------------+
| pfn[0] |
+-------------------------------------------------+
...
+-------------------------------------------------+
| pfn[C-1] |
+-------------------------------------------------+
| page_data[0]... |
...
+-------------------------------------------------+
| page_data[N-1]... |
...
+-------------------------------------------------+

--------------------------------------------------------------------
Field Description
----------- --------------------------------------------------------
count Number of pages described in this record.

pfn An array of count PFNs. Bits 63-60 contain
the XEN\_DOMCTL\_PFINFO_* value for that PFN.

page_data page_size octets of uncompressed page contents for each page
set as present in the pfn array.
--------------------------------------------------------------------

s/uncompressed/(compressed/uncompressed)/

(Remus sends compressed data)

VCPU_INFO
---------

> [ This is a combination of parts of the extended-info and
> XC_SAVE_ID_VCPU_INFO chunks. ]

The VCPU_INFO record includes the maximum possible VCPU ID. This will
be followed a VCPU_CONTEXT record for each online VCPU.

0 1 2 3 4 5 6 7 octet
+-----------------------+------------------------+
| max_vcpu_id | (reserved) |
+-----------------------+------------------------+

--------------------------------------------------------------------
Field Description
----------- ---------------------------------------------------
max_vcpu_id Maximum possible VCPU ID.
--------------------------------------------------------------------

VCPU_CONTEXT
------------

The context for a single VCPU.

0 1 2 3 4 5 6 7 octet
+-----------------------+-------------------------+
| vcpu_id | (reserved) |
+-----------------------+-------------------------+
| vcpu_ctx... |
...
+-------------------------------------------------+

--------------------------------------------------------------------
Field Description
----------- ---------------------------------------------------
vcpu_id The VCPU ID.

vcpu_ctx Context for this VCPU.
--------------------------------------------------------------------

[ vcpu_ctx format TBD. ]

X86_PV_INFO
-----------

> [ This record replaces part of the extended-info chunk. ]

0 1 2 3 4 5 6 7 octet
+-----+-----+-----+-------------------------------+
| w | ptl | o | (reserved) |
+-----+-----+-----+-------------------------------+

--------------------------------------------------------------------
Field Description
----------- ---------------------------------------------------
guest_width (w) Guest width in octets (either 4 or 8).

pt_levels (ptl) Number of page table levels (either 3 or 4).

options (o) Bit 0: 0 - no VMASST_pae_extended_cr3,
1 - VMASST_pae_extended_cr3.

Bit 1-7: Reserved.
--------------------------------------------------------------------

P2M
---

[ This is a more flexible replacement for the old p2m_size field and
p2m array. ]

The P2M record contains a portion of the source domain's P2M.
Multiple P2M records may be sent if the source P2M changes during the
stream.

0 1 2 3 4 5 6 7 octet
+-------------------------------------------------+
| pfn_begin |
+-------------------------------------------------+
| pfn_end |
+-------------------------------------------------+
| mfn[0] |
+-------------------------------------------------+
...
+-------------------------------------------------+
| mfn[N-1] |
+-------------------------------------------------+

--------------------------------------------------------------------
Field Description
----------- --------------------------------------------------------
pfn_begin The first PFN in this portion of the P2M

pfn_end One past the last PFN in this portion of the P2M.

mfn Array of (pfn_end - pfn-begin) MFNs corresponding to
the set of PFNs in the range [pfn_begin, pfn_end).
--------------------------------------------------------------------

Layout
======

The set of valid records depends on the guest architecture and type.

x86 PV Guest
------------

An x86 PV guest image will have in this order:

1. Image header
2. Domain header
3. X86_PV_INFO record
4. At least one P2M record
5. At least one PAGE_DATA record

6. VCPU_INFO record
6. At least one VCPU_CONTEXT record

7. END record

There seems to be a bunch of info missing. Here are some

missing elements that I can recall at the moment:

a) there is no support for sending over one time markers that switch the

receiver's operating mode in the middle of a data stream.

E.g., XC_SAVE_ENABLE_COMPRESSION, XC_SAVE_ID_LAST_CHECKPOINT, etc.

XC_SAVE_ENABLE_VERIFY_MODE,

b) in pv case, the tail also has a list of unmapped PFNs at the end of every iteration.

c) XC_SAVE_ID_TOOLSTACK -- used by xl to pass device context information (generally

for HVMs).

Legacy Images (x86 only)
========================

Restoring legacy images from older tools shall be handled by
translating the legacy format image into this new format.

It shall not be possible to save in the legacy format.

There are two different legacy images depending on whether they were
generated by a 32-bit or a 64-bit toolstack. These shall be
distinguished by inspecting octets 4-7 in the image. If these are
zero then it is a 64-bit image.

Toolstack Field Value
--------- ----- -----
64-bit Bit 31-63 of the p2m_size field 0 (since p2m_size < 2^32^)
32-bit extended-info chunk ID (PV) 0xFFFFFFFF
32-bit Chunk type (HVM) < 0
32-bit Page count (HVM) > 0

Table: Possible values for octet 4-7 in legacy images

This assumes the presence of the extended-info chunk which was
introduced in Xen 3.0.

Future Extensions
=================

All changes to this format require the image version to be increased.

The format may be extended by adding additional record types.

Extending an existing record type must be done by adding a new record
type. This allows old images with the old record to still be
restored.

The image header may be extended by _appending_ additional fields. In
particular, the `marker`, `id` and `version` fields must never change
size or location.

Re: [Xen-devel] Domain Save Image Format proposal (draft B)