1 Introduction

For the purposes of this document, xl is used as a representation of any implementer of the libxl API. xl should be considered completely interchangeable with alternates, such as libvirt or xenopsd-xl.

1.1 Purpose

The domain image format 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 domain image format used in Xen 4.5 and earlier (the legacy format)

This design addresses the above points, allowing for a completely self-contained, extensible stream with each layer responsible for its own appropriate information.

1.2 Not Yet Included

The following features are not yet fully specified and will be included in a future draft.

2 Overview

The image format consists of a Header, followed by 1 or more Records. Each record consists of a type and length field, followed by any type-specific data.

3 Header

The header identifies the stream as a libxl stream, including the version of this specification that it complies with.

All fields in this header shall be in big-endian byte order, regardless of the setting of the endianness bit.

 0     1     2     3     4     5     6     7 octet
+-------------------------------------------------+
| ident                                           |
+-----------------------+-------------------------+
| version               | options                 |
+-----------------------+-------------------------+
Field Description
ident 0x4c6962786c466d74 ("LibxlFmt" in ASCII).
version 0x00000002. The version of this specification.
options bit 0: Endianness. 0 = little-endian, 1 = big-endian.
bit 1: Legacy Format. If set, this stream was created by the legacy conversion tool.
bits 2-31: Reserved.

The endianness shall be 0 (little-endian) for images generated on an i386, x86_64, or arm host.

4 Record Overview

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

 0     1     2     3     4     5     6     7 octet
+-----------------------+-------------------------+
| type                  | body_length             |
+-----------+-----------+-------------------------+
| body...                                         |
...
|           | padding (0 to 7 octets)             |
+-----------+-------------------------------------+
Field Description
type 0x00000000: END
0x00000001: LIBXC_CONTEXT
0x00000002: EMULATOR_XENSTORE_DATA
0x00000003: EMULATOR_CONTEXT
0x00000004: CHECKPOINT_END
0x00000005: CHECKPOINT_STATE
0x00000006 - 0x7FFFFFFF: Reserved for future mandatory records.
0x80000000 - 0xFFFFFFFF: Reserved for future optional records.
body_length Length in octets of the record body.
body Content of the record.
padding 0 to 7 octets of zeros to pad the whole record to a multiple of 8 octets.

4.1 Emulator Records

Several records are specifically for emulators, and have a common sub header.

 0     1     2     3     4     5     6     7 octet
+------------------------+------------------------+
| emulator_id            | index                  |
+------------------------+------------------------+
| record specific data                            |
...
+-------------------------------------------------+
Field Description
emulator_id 0x00000000: Unknown (In the case of a legacy stream)
0x00000001: Qemu Traditional
0x00000002: Qemu Upstream
0x00000003 - 0xFFFFFFFF: Reserved for future emulators.
index Index of this emulator for the domain.

5 Records

5.1 END

A end record marks the end of the image, and shall be the final record in the stream.

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

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

5.2 LIBXC_CONTEXT

A libxc context record is a marker, indicating that the stream should be handed to xc_domain_restore(). libxc shall be responsible for reading its own image format from the stream.

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

The libxc context record contains no fields; its body_length is 01.

5.3 EMULATOR_XENSTORE_DATA

A set of xenstore key/value pairs for a specific emulator associated with the domain.

 0     1     2     3     4     5     6     7 octet
+------------------------+------------------------+
| emulator_id            | index                  |
+------------------------+------------------------+
| xenstore key/value data                         |
...
+-------------------------------------------------+

Xenstore key/value data are encoded as a packed sequence of (key, value) tuples. Each (key, value) tuple is a packed pair of NUL terminated octets, conforming to xenstore protocol character encoding (keys strictly as alphanumeric ASCII and -/_@, values expected to be human-readable ASCII).

Keys shall be relative to to the device models xenstore tree for the new domain. At the time of writing, keys are relative to the path

/local/domain/$dm_domid/device-model/$domid/

although this path is free to change moving forward, thus should not be assumed.

5.4 EMULATOR_CONTEXT

A context blob for a specific emulator associated with the domain.

 0     1     2     3     4     5     6     7 octet
+------------------------+------------------------+
| emulator_id            | index                  |
+------------------------+------------------------+
| emulator_ctx                                    |
...
+-------------------------------------------------+

The emulator_ctx is a binary blob interpreted by the emulator identified by emulator_id. Its format is unspecified.

5.5 CHECKPOINT_END

A checkpoint end record marks the end of a checkpoint in the image.

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

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

5.6 CHECKPOINT_STATE

A checkpoint state record contains the control information for checkpoint. It is only used by COLO, more detail please reference README.colo.

 0     1     2     3     4     5     6     7 octet
+------------------------+------------------------+
| control_id             | padding                |
+------------------------+------------------------+
Field Description
control_id 0x00000000: Secondary VM is out of sync, start a new checkpoint (Primary -> Secondary)
0x00000001: Secondary VM is suspended (Secondary -> Primary)
0x00000002: Secondary VM is ready (Secondary -> Primary)
0x00000003: Secondary VM is resumed (Secondary -> Primary)

In COLO, Primary is running in below loop:

  1. Suspend primary vm
    1. Suspend primary vm
    2. Read CHECKPOINT_SVM_SUSPENDED sent by secondary
  2. Checkpoint
  3. Resume primary vm
    1. Read CHECKPOINT_SVM_READY from secondary
    2. Resume primary vm
    3. Read CHECKPOINT_SVM_RESUMED from secondary
  4. Wait a new checkpoint
    1. Send CHECKPOINT_NEW to secondary

While Secondary is running in below loop:

  1. Resume secondary vm
    1. Send CHECKPOINT_SVM_READY to primary
    2. Resume secondary vm
    3. Send CHECKPOINT_SVM_RESUMED to primary
  2. Wait a new checkpoint
    1. Read CHECKPOINT_NEW from primary
  3. Suspend secondary vm
    1. Suspend secondary vm
    2. Send CHECKPOINT_SVM_SUSPENDED to primary
  4. Checkpoint

6 Future Extensions

All changes to this specification should bump the revision number in the title block.

All changes to the header require the header 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.


  1. The sending side cannot calculate ahead of time how much data libxc might write into the stream, especially for live migration where the quantity of data is partially proportional to the elapsed time.