Re: [Xen-devel] [PATCH v7 2/2] docs/designs: Add a design document for migration of xenstore data

[Julien Grall <julien@xxxxxxx>]

Hi Paul,

On 18/03/2020 11:11, Paul Durrant wrote:
> From: Paul Durrant <pdurrant@xxxxxxxxxx>
>
> This patch details proposes extra migration data and xenstore protocol
> extensions to support non-cooperative live migration of guests.
>
> NOTE: doc/misc/xenstore.txt is also amened to replace the <mfn> term
NIT: s/amened/amended/

>        for the INTRODUCE operation with the <gfn>, since this is what
>        it actually is.
>
> Signed-off-by: Paul Durrant <paul@xxxxxxx>
> ---
> Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Cc: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>
> Cc: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
> Cc: Jan Beulich <jbeulich@xxxxxxxx>
> Cc: Julien Grall <julien@xxxxxxx>
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> Cc: Wei Liu <wl@xxxxxxx>
>
> v7:
>   - Addressed further comments from Julien
>   - Switched migration records to defined structures instead of tuples
>
> v6:
>   - Addressed comments from Julien
>
> v5:
>   - Add QUIESCE
>   - Make semantics of <index> in GET_DOMAIN_WATCHES more clear
>
> v4:
>   - Drop the restrictions on special paths
>
> v3:
>   - New in v3
> ---
>   docs/designs/xenstore-migration.md | 256 +++++++++++++++++++++++++++++
>   docs/misc/xenstore.txt             |   6 +-
>   2 files changed, 259 insertions(+), 3 deletions(-)
>   create mode 100644 docs/designs/xenstore-migration.md
>
> diff --git a/docs/designs/xenstore-migration.md 
> b/docs/designs/xenstore-migration.md
> new file mode 100644
> index 0000000000..e7e6593002
> --- /dev/null
> +++ b/docs/designs/xenstore-migration.md
> @@ -0,0 +1,256 @@
> +# Xenstore Migration
> +
> +## Background
> +
> +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 xenstore protocol[2] will also be required.
> +
> +The *libxenlight Domain Image Format* specification[3] already defines a
> +record type `EMULATOR_XENSTORE_DATA` but this is not suitable for
> +transferring xenstore data pertaining to the domain directly as it is
> +specified such that keys are relative to the path
> +`/local/domain/$dm_domid/device-model/$domid`. Thus it is necessary to
> +define at least one new save record type.
> +
> +## Proposal
> +
> +### New Save Record
> +
> +A new mandatory record type should be defined within the libxenlight Domain
> +Image Format:
> +
> +`0x00000007: DOMAIN_XENSTORE_DATA`
> +
> +An arbitrary number of these records may be present in the migration
> +stream and may appear in any order. The format of each record should be as
> +follows:
> +
> +
> +```
> +    0       1       2       3       4       5       6       7    octet
> ++-------+-------+-------+-------+-------+-------+-------+-------+
> +| type                          | record specific data          |
> ++-------------------------------+                               |
> +...
> ++---------------------------------------------------------------+
> +```
> +
> +where type is one of the following values
> +
> +
> +| Field  | Description                                      |
> +|--------|--------------------------------------------------|
> +| `type` | 0x00000000: invalid                              |
> +|        | 0x00000001: NODE_DATA                            |
> +|        | 0x00000002: WATCH_DATA                           |
> +|        | 0x00000003: TRANSACTION_DATA                     |
> +|        | 0x00000004 - 0xFFFFFFFF: reserved for future use |
> +
> +
> +and data is one of the record data formats described in the following
> +sections.
> +
> +
> +NOTE: The record data does not contain an overall length because the
> +libxenlight record header specifies the length.
> +
> +
> +**NODE_DATA**
> +
> +
> +Each NODE_DATA record specifies a single node in xenstore and is formatted
> +as follows:
> +
> +
> +```
> +    0       1       2       3     octet
> ++-------+-------+-------+-------+
> +|   1   |   0   |   0   |   0   |

The stream may either be big-endian or little-endian. This will be 
specified in the header of the stream.
But here, this suggests that the type will always be described in 
little-endian.
I think it would be best to only describe the "record specific data".

> ++-------------------------------+
> +| path length                   |
> ++-------------------------------+
> +| path data                     |
> +...
> +| pad (0 to 3 octets)           |
> ++-------------------------------+
> +| perm count (N)                |
> ++-------------------------------+
> +| perm0                         |
> ++-------------------------------+
> +...
> ++-------------------------------+
> +| permN                         |
> ++-------------------------------+
> +| value length                  |
> ++-------------------------------+
> +| value data                    |
> +...
> +| pad (0 to 3 octets)           |
> ++-------------------------------+
> +```
> +
> +where perm0..N are formatted as follows:
> +
> +
> +```
> +    0       1       2       3     octet
> ++-------+-------+-------+-------+
> +| perm  | pad   | domid         |
> ++-------------------------------+
> +```
> +
> +
> +path length and value length are specified in octets (excluding the NUL
> +terminator of the path).
IIUC, this means the "path data" would not contain a NUL at the end.am I 
correct?
> perm should be one of the ASCII values `w`, `r`,
> +`b` or `n` as described in [2]. All pad values should be 0.
> +All paths should be absolute (i.e. start with `/`) and as described in
> +[2].
> +
> +
> +**WATCH_DATA**
> +
> +
> +Each WATCH_DATA record specifies a registered watch and is formatted as
> +follows:
> +
> +
> +```
> +    0       1       2       3     octet
> ++-------+-------+-------+-------+
> +|   2   |   0   |   0   |   0   |
Same remark here about the endianess.

> ++-------------------------------+
> +| wpath length                  |
> ++-------------------------------+
> +| wpath data                    |
> +...
> +| pad (0 to 3 octets)           |
> ++-------------------------------+
> +...
> ++-------------------------------+
> +| token length                  |
> ++-------------------------------+
> +| token data                    |
> +...
> +| pad (0 to 3 octets)           |
> ++-------------------------------+
> +```
> +
> +wpath length and token length are specified in octets (excluding the NUL
> +terminator). The wpath should be as described for the `WATCH` operation in
> +[2]. The token is an arbitrary string of octets not containing any NUL
> +values.
> +
> +
> +**TRANSACTION_DATA**
> +
> +
> +Each TRANSACTION_DATA record specifies an open transaction and is formatted
> +as follows:
> +
> +
> +```
> +    0       1       2       3     octet
> ++-------+-------+-------+-------+
> +|   3   |   0   |   0   |   0   |
Same remark here about the endianess.

> ++-------------------------------+
> +| tx_id                         |
> ++-------------------------------+
> +```
> +
> +where tx_id is the non-zero identifier values of an open transaction.
> +
> +
> +### Protocol Extension
> +
> +Before xenstore state is migrated it is necessary to wait for any pending
> +reads, writes, watch registrations etc. to complete, and also to make sure
> +that xenstored does not start processing any new requests (so that new
> +requests remain pending on the shared ring for subsequent processing on the
> +new host). Hence the following operation is needed:
> +
> +```
> +QUIESCE                 <domid>|
> +
> +Complete processing of any request issued by the specified domain, and
> +do not process any further requests from the shared ring.
> +```
> +
> +The `WATCH` operation does not allow specification of a `<domid>`; it is
> +assumed that the watch pertains to the domain that owns the shared ring
> +over which the operation is passed. Hence, for the tool-stack to be able
> +to register a watch on behalf of a domain a new operation is needed:
> +
> +```
> +ADD_DOMAIN_WATCHES      <domid>|<watch>|+
> +
> +Adds watches on behalf of the specified domain.
> +
> +<watch> is a NUL separated tuple of <path>|<token>. The semantics of this
> +operation are identical to the domain issuing WATCH <path>|<token>| for
> +each <watch>.
> +```
> +
> +The watch information for a domain also needs to be extracted from the
> +sending xenstored so the following operation is also needed:
> +
> +```
> +GET_DOMAIN_WATCHES      <domid>|<index>   <gencnt>|<watch>|*
> +
> +Gets the list of watches that are currently registered for the domain.
> +
> +<watch> is a NUL separated tuple of <path>|<token>. The sub-list returned
> +will start at <index> items into the the overall list of watches and may
> +be truncated (at a <watch> boundary) such that the returned data fits
> +within XENSTORE_PAYLOAD_MAX.
> +
> +If <index> is beyond the end of the overall list then the returned sub-
> +list will be empty. If the value of <gencnt> changes then it indicates
> +that the overall watch list has changed and thus it may be necessary
> +to re-issue the operation for previous values of <index>.
> +```
> +
> +To deal with transactions that were pending when the domain is migrated
> +it is necessary to start transactions with the same tx_id on behalf of the
> +domain in the receiving xenstored.
> +
> +NOTE: For safety each such transaction should result in an `EAGAIN` when
> +the `TRANSACTION_END` operation is peformed, as modifications made under
NIT: s/peformed/performed/

> +the tx_id will not be part of the migration stream.
> +
> +The `TRANSACTION_START` operation does not allow specification of a
> +`<domid>`; it is assumed that the transaction pertains to the domain that
> +owns the shared ring over which the operation is passed. Neither does it
> +allow a `<transid>` to be specified; it is always chosen by xenstored.
> +Hence, for the tool-stack to be able to open a transaction on behalf of a
> +domain a new operation is needed:
> +
> +```
> +START_DOMAIN_TRANSACTION    <domid>|<transid>|
> +
> +Starts a transaction on behalf of a domain.
> +
> +The semantics of this are similar to the domain issuing
> +TRANSACTION_START and receiving the specified <transid> as the response.
> +The main difference is that the transaction will be immediately marked as
> +'conflicting' such that when the domain isses TRANSACTION_END T|, it will
> +result in EAGAIN.
> +```
> +
> +It may also be desirable to state in the protocol specification that
> +the `INTRODUCE` operation should not clear the `<gfn>` specified such that
> +a `RELEASE` operation followed by an `INTRODUCE` operation form an
> +idempotent pair. The current implementation of *C xentored* does this
> +(in the `domain_conn_reset()` function) but this could be dropped as this
> +behaviour is not currently specified and the page will always be zeroed
> +for a newly created domain.
> +
> +
> +* * *
> +
> +[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=docs/specs/libxl-migration-stream.pandoc
> diff --git a/docs/misc/xenstore.txt b/docs/misc/xenstore.txt
> index 6f8569d576..51e6b12931 100644
> --- a/docs/misc/xenstore.txt
> +++ b/docs/misc/xenstore.txt
> @@ -254,7 +254,7 @@ TRANSACTION_END             F|
>   
>   ---------- Domain management and xenstored communications ----------
>   
> -INTRODUCE		<domid>|<mfn>|<evtchn>|?
> +INTRODUCE              <domid>|<gfn>|<evtchn>|?
>         Notifies xenstored to communicate with this domain.
>   
>   	INTRODUCE is currently only used by xend (during domain
> @@ -262,12 +262,12 @@ INTRODUCE         <domid>|<mfn>|<evtchn>|?
>         xenstored prevents its use other than by dom0.
>   
>   	<domid> must be a real domain id (not 0 and not a special
> -       DOMID_... value).  <mfn> must be a machine page in that domain
> +       DOMID_... value).  <gfn> must be a machine page in that domain
Now that you are using "gfn", do you also want to update "machine page"?

>         represented in signed decimal (!).  <evtchn> must be event
>         channel is an unbound event channel in <domid> (likewise in
>         decimal), on which xenstored will call bind_interdomain.
>         Violations of these rules may result in undefined behaviour;
> -       for example passing a high-bit-set 32-bit mfn as an unsigned
> +       for example passing a high-bit-set 32-bit gfn as an unsigned
>         decimal will attempt to use 0x7fffffff instead (!).
>   
>   RELEASE			<domid>|
Cheeers,

--
Julien Grall

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