Re: [Xen-devel] [PATCH v5 2/2] docs/designs: Add a design document for migration of xenstore data

["Durrant, Paul" <pdurrant@xxxxxxxxxxxx>]

> -----Original Message-----
> From: Julien Grall <julien@xxxxxxx>
> Sent: 04 March 2020 18:32
> To: Durrant, Paul <pdurrant@xxxxxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx
> Cc: 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: [EXTERNAL][PATCH v5 2/2] docs/designs: Add a design document for 
> migration of xenstore
> data
> 
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> 
> 
> Hi Paul,
> 
> On 13/02/2020 10:53, Paul Durrant wrote:
> > This patch details proposes extra migration data and xenstore protocol
> > extensions to support non-cooperative live migration of guests.
> >
> > Signed-off-by: Paul Durrant <pdurrant@xxxxxxxxxx>
> > ---
> > 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>
> >
> > 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 | 136 +++++++++++++++++++++++++++++
> >   1 file changed, 136 insertions(+)
> >   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..5cfe2d9a7d
> > --- /dev/null
> > +++ b/docs/designs/xenstore-migration.md
> > @@ -0,0 +1,136 @@
> > +# 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`
> > +
> > +The format of each of these new records should be as follows:
> > +
> > +
> > +```
> > +0     1     2     3     4     5     6     7 octet
> > ++------------------------+------------------------+
> > +| type                   | record specific data   |
> > ++------------------------+                        |
> > +...
> > ++-------------------------------------------------+
> > +```
> > +
> > +
> > +| Field | Description |
> > +|---|---|
> 
> Did you indend to add more - so | is on the same column as the onter lines?
> 

Yep, cut'n'paste error.

> > +| `type` | 0x00000000: invalid |
> > +|        | 0x00000001: node data |
> > +|        | 0x00000002: watch data |
> 
> Should not the last | be some of the columns on all the lines?
> 
> > +|        | 0x00000003 - 0xFFFFFFFF: reserved for future use |
> 
> Looking at the spec, the command TRANSACTION_END *must* be used with an
> existing transaction. As a guest would be migrate to a new domain, the
> transaction ID would now be invalid.
> 
> I understand that xenstored is able to cope with it, but such behavior
> is not described in the spec. So I am not sure we can expect a guest to
> cope with an error value other than the ones described for the command.
> 

And (as we discussed offline) there would be an issue if the migrated guest 
started a new transaction before completing one that was started pre-migration, 
as the ids may clash. So, we are going to need a record to transfer open 
transaction ids so that we can reserve them in the receiving xenstored.

> > +
> > +
> > +where data is always in the form of a NUL separated and terminated tuple
> > +as follows
> > +
> > +
> > +**node data**
> > +
> > +
> > +`<path>|<value>|<perm-as-string>|`
> 
> I don't think this would work. From the spec, <value> is a binary data
> and therefore it can contain zero or nul. So you would not be able to
> find out where the <perm-as-string> starts.
> 
> Regarding the <perm-as-string>, it is only describing the permission for
> one domain. If multiple domains can access the node, then you would have
> multiple <perm-as-string>. Do we want to transfer all the permissions,
> if not how do we define which permissions should be transferred?

Yes this should cope with multiple perms and binary data, even though I think 
we don't necessarily need it in the normal case.

> 
> > +
> > +
> > +`<path>` is considered relative to the domain path `/local/domain/$domid`
> > +and hence must not begin with `/`.
> > +`<path>` and `<value>` should be suitable to formulate a `WRITE` operation
> > +to the receiving xenstore and `<perm-as-string>` should be similarly 
> > suitable
> > +to formulate a subsequent `SET_PERMS` operation.
> > +
> > +**watch data**
> > +
> > +
> > +`<path>|<token>|`
> > +
> > +`<path>` again is considered relative and, together with `<token>`, should
> > +be suitable to formulate an `ADD_DOMAIN_WATCHES` operation (see below).
> 
> AFAICT, a guest is allowed to watch /. So is it a sensible thing to only
> transfer relative watch?
> 
> Also, how about special watch (i.e @...)?

I guess we need to cope with whatever a guest is allowed to register... which 
appears to be anything.

> 
> > +
> > +
> > +### 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>.
> > +```
> > +
> > +It may also be desirable to state in the protocol specification that
> > +the `INTRODUCE` operation should not clear the `<mfn>` specified such that
> 
> Not directly related to this patch, the '<mfn>' is slightly confusing
> because, AFAICT, this will actually hold an GFN. To avoid spreading more
> misuse, it would make sense to update the xenstore accordingly and use
> the new term here.
> 

Ok, I can add a small patch to modify the doc.

  Paul

> > +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
> >
> 
> Cheers,
> 
> --
> Julien Grall
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