The design for Non-Cooperative Migration of Guests[1] explains that extra save records are required in the migrations stream to allow a guest running PV drivers to be migrated without its co-operation. Moreover the save records must include details of registered xenstore watches as well as content; information that cannot currently be recovered from xenstored, and hence some extension to the xenstored implementations will also be required.
As a similar set of data is needed for transferring xenstore data from one instance to another when live updating xenstored this document proposes an image format for a ‘migration stream’ suitable for both purposes.
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 data mandated by the record type. At minimum there will be a single record of type END (defined below).
The header identifies the stream as a xenstore stream, including the version of the specification that it complies with.
All fields in this header must 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 | flags |
+-------------------------------+-------------------------------+| Field | Description |
|---|---|
ident |
0x78656e73746f7265 (‘xenstore’ in ASCII) |
version |
0x00000001 (the version of the specification) |
flags |
0 (LSB): Endianness: 0 = little, 1 = big |
| 1-31: Reserved (must be zero) |
Records immediately follow the header and have the following format:
0 1 2 3 4 5 6 7 octet
+-------+-------+-------+-------+-------+-------+-------+-------+
| type | len |
+-------------------------------+-------------------------------+
| body
...
| | padding (0 to 7 octets) |
+-------+-------------------------------------------------------+NOTE: padding octets here and in all subsequent format specifications must be written as zero and should be ignored when the stream is read.
| Field | Description |
|---|---|
type |
0x00000000: END |
| 0x00000001: GLOBAL_DATA | |
| 0x00000002: CONNECTION_DATA | |
| 0x00000003: WATCH_DATA | |
| 0x00000004: TRANSACTION_DATA | |
| 0x00000005: NODE_DATA | |
| 0x00000006 - 0xFFFFFFFF: reserved for future use | |
len |
The length (in octets) of body |
body |
The type-specific record data |
Some records will depend on other records in the migration stream. Records upon which other records depend must always appear earlier in the stream.
The various formats of the type-specific data are described in the following sections:
The end record marks the end of the image, and is 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.
This record is only relevant for live update. It contains details of global xenstored state that needs to be restored.
0 1 2 3 octet
+-------+-------+-------+-------+
| rw-socket-fd |
+-------------------------------+
| evtchn-fd |
+-------------------------------+| Field | Description |
|---|---|
rw-socket-fd |
The file descriptor of the socket accepting |
| read-write connections | |
evtchn-fd |
The file descriptor used to communicate with |
| the event channel driver |
xenstored will resume in the original process context. Hence rw-socket-fd simply specifies the file descriptor of the socket. Sockets are not always used, however, and so -1 will be used to denote an unused socket.
For live update the image format will contain a CONNECTION_DATA record for each connection to xenstore. For migration it will only contain a record for the domain being migrated.
0 1 2 3 4 5 6 7 octet
+-------+-------+-------+-------+-------+-------+-------+-------+
| conn-id | conn-type | |
+-------------------------------+---------------+---------------+
| conn-spec
...
+---------------+---------------+-------------------------------+
| in-data-len | out-resp-len | out-data-len |
+---------------+---------------+-------------------------------+
| data
...| Field | Description |
|---|---|
conn-id |
A non-zero number used to identify this |
| connection in subsequent connection-specific | |
| records | |
conn-type |
0x0000: shared ring |
| 0x0001: socket | |
| 0x0002 - 0xFFFF: reserved for future use | |
conn-spec |
See below |
in-data-len |
The length (in octets) of any data read |
| from the connection not yet processed | |
out-resp-len |
The length (in octets) of a partial response |
| not yet written to the connection | |
out-data-len |
The length (in octets) of any pending data |
| not yet written to the connection, including | |
a partial response (see out-resp-len) |
|
data |
Pending data: first in-data-len octets of |
| read data, then out-data-len octets of | |
| written data (any of both may be empty) |
In case of live update the connection record for the connection via which the live update command was issued will contain the response for the live update command in the pending not yet written data.
The format of conn-spec is dependent upon conn-type.
For shared ring connections it is as follows:
0 1 2 3 4 5 6 7 octet
+---------------+---------------+---------------+---------------+
| domid | tdomid | evtchn |
+-------------------------------+-------------------------------+| Field | Description |
|---|---|
domid |
The domain-id that owns the shared page |
tdomid |
The domain-id that domid acts on behalf of if |
| it has been subject to an SET_TARGET | |
| operation [2] or DOMID_INVALID [3] otherwise | |
evtchn |
The port number of the interdomain channel used |
by xenstored to communicate with domid |
|
The GFN of the shared page is not preserved because the ABI reserves entry 1 in domid’s grant table to point to the xenstore shared page. Note there is no guarantee the page will still be valid at the time of the restore because a domain can revoke the permission.
For socket connections it is as follows:
+---------------+---------------+---------------+---------------+
| socket-fd | pad |
+-------------------------------+-------------------------------+| Field | Description |
|---|---|
socket-fd |
The file descriptor of the connected socket |
This type of connection is only relevant for live update, where the xenstored resumes in the original process context. Hence socket-fd simply specify the file descriptor of the socket connection.
The image format will contain a WATCH_DATA record for each watch registered by a connection for which there is CONNECTION_DATA record previously present.
0 1 2 3 octet
+-------+-------+-------+-------+
| conn-id |
+---------------+---------------+
| wpath-len | token-len |
+---------------+---------------+
| wpath
...
| token
...| Field | Description |
|---|---|
conn-id |
The connection that issued the WATCH |
| operation [2] | |
wpath-len |
The length (in octets) of wpath including the |
| NUL terminator | |
token-len |
The length (in octets) of token including the |
| NUL terminator | |
wpath |
The watch path, as specified in the WATCH |
| operation | |
token |
The watch identifier token, as specified in the |
WATCH operation |
The image format will contain a TRANSACTION_DATA record for each transaction that is pending on a connection for which there is CONNECTION_DATA record previously present.
0 1 2 3 octet
+-------+-------+-------+-------+
| conn-id |
+-------------------------------+
| tx-id |
+-------------------------------+| Field | Description |
|---|---|
conn-id |
The connection that issued the |
TRANSACTION_START operation [2] |
|
tx-id |
The transaction id passed back to the domain |
by the TRANSACTION_START operation |
For live update the image format will contain a NODE_DATA record for each node in xenstore. For migration it will only contain a record for the nodes relating to the domain being migrated. The NODE_DATA may be related to a committed node (globally visible in xenstored) or a pending node (created or modified by a transaction for which there is also a TRANSACTION_DATA record previously present).
0 1 2 3 octet
+-------+-------+-------+-------+
| conn-id |
+-------------------------------+
| tx-id |
+---------------+---------------+
| path-len | value-len |
+---------------+---------------+
| access | perm-count |
+---------------+---------------+
| perm1 |
+-------------------------------+
...
+-------------------------------+
| permN |
+---------------+---------------+
| path
...
| value
...| Field | Description |
|---|---|
conn-id |
If this value is non-zero then this record |
| related to a pending transaction | |
tx-id |
This value should be ignored if conn-id is |
| zero. Otherwise it specifies the id of the | |
| pending transaction | |
path-len |
The length (in octets) of path including the |
| NUL terminator | |
value-len |
The length (in octets) of value (which will |
| be zero for a deleted node) | |
access |
This value should be ignored if this record |
| does not relate to a pending transaction, | |
| otherwise it specifies the accesses made to | |
| the node and hence is a bitwise OR of: | |
| 0x0001: read | |
| 0x0002: written | |
| The value will be zero for a deleted node | |
perm-count |
The number (N) of node permission specifiers |
| (which will be 0 for a node deleted in a | |
| pending transaction) | |
perm1..N |
A list of zero or more node permission |
| specifiers (see below) | |
path |
The absolute path of the node |
value |
The node value (which may be empty or contain |
| NUL octets) |
A node permission specifier has the following format:
0 1 2 3 octet
+-------+-------+-------+-------+
| perm | flags | domid |
+-------+-------+---------------+| Field | Description |
|---|---|
perm |
One of the ASCII values w, r, b or n as |
specified for the SET_PERMS operation [2] |
|
flags |
A bit-wise OR of: |
| 0x01: stale permission, ignore when checking | |
| permissions | |
domid |
The domain-id to which the permission relates |
Note that perm1 defines the domain owning the node. See [4] for more explanation of node permissions.
[1] See https://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=docs/designs/non-cooperative-migration.md
[2] See https://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=docs/misc/xenstore.txt
[3] See https://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=xen/include/public/xen.h;hb=HEAD#l612
[4] https://wiki.xen.org/wiki/XenBus