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

[Paul Durrant <xadimgnik@xxxxxxxxx>]

> -----Original Message-----
> From: Julien Grall <julien@xxxxxxx>
> Sent: 23 March 2020 11:34
> To: Paul Durrant <paul@xxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx
> Cc: Paul Durrant <pdurrant@xxxxxxxxxx>; Andrew Cooper 
> <andrew.cooper3@xxxxxxxxxx>; George Dunlap
> <George.Dunlap@xxxxxxxxxxxxx>; Ian Jackson <ian.jackson@xxxxxxxxxxxxx>; Jan 
> Beulich
> <jbeulich@xxxxxxxx>; Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>; Stefano 
> Stabellini
> <sstabellini@xxxxxxxxxx>; Wei Liu <wl@xxxxxxx>
> Subject: Re: [PATCH v7 2/2] docs/designs: Add a design document for migration 
> of xenstore data
> 
> 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/
> 

Yes.

> >        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".
> 

True, I'll just say 'NODE_DATA' (or similar) in the first line.

> > ++-------------------------------+
> > +| 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?

Yes, it is supposed to apply to both.

> 
> > 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/
> 

Ok.

> > +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"?
> 

Oops, yes.

 Paul

> >     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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