[Xen-devel] [PATCH v6 2/2] docs/designs: Add a design document for migration of xenstore data

[<pdurrant@xxxxxxxx>]

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
      for the INTRODUCE operation with the <gfn>, since this is what
      it actually is.

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>

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 | 171 +++++++++++++++++++++++++++++
 docs/misc/xenstore.txt             |   6 +-
 2 files changed, 174 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..7e61f462f0
--- /dev/null
+++ b/docs/designs/xenstore-migration.md
@@ -0,0 +1,171 @@
+# 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   |
++------------------------+                        |
+...
++-------------------------------------------------+
+```
+
+NB: The record data does not contain a length because the libxenlight record
+header specifies the length.
+
+
+| Field  | Description                                      |
+|--------|--------------------------------------------------|
+| `type` | 0x00000000: invalid                              |
+|        | 0x00000001: node data                            |
+|        | 0x00000002: watch data                           |
+|        | 0x00000003: transaction data                     |
+|        | 0x00000004 - 0xFFFFFFFF: reserved for future use |
+
+
+where data is always in the form of a tuple as follows
+
+
+**node data**
+
+
+`<path>|<perm-count>|<perm-as-string>|+<value|>`
+
+
+`<path>` and `<value|>` should be suitable to formulate a `WRITE` operation
+to the receiving xenstored and the `<perm-as-string>|+` list should be
+similarly suitable to formulate a subsequent `SET_PERMS` operation.
+`<perm-count>` specifies the number of entries in the `<perm-as-string>|+`
+list and `<value|>` must be placed at the end because it may contain NUL
+octets.
+
+
+**watch data**
+
+
+`<path>|<token>|`
+
+`<path>` again is absolute and, together with `<token>`, should
+be suitable to formulate an `ADD_DOMAIN_WATCHES` operation (see below).
+
+
+**transaction data**
+
+
+`<transid-count>|<transid>|+`
+
+Each `<transid>` should be a uint32_t value represented as unsigned decimal
+suitable for passing as a *tx_id* to the re-defined `TRANSACTION_START`
+operation (see below). `<transid-count>` is the number of entries in the
+`<transid>|+` list.
+
+
+### 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 `<trans-id>` in the
+receiving xenstored but for them to result in an `EAGAIN` when the
+`TRANSACTION_END` operation is peformed. Thus the `TRANSACTION_START`
+operation needs to be re-defined as follows:
+
+```
+TRANSACTION_START      |                       <transid>|
+       <transid> is an opaque uint32_t represented as unsigned decimal.
+    If tx_id is 0 for this operation then a new transaction will be started
+    with a tx_id allocated by xenstored. If a non-0 tx_id is specified then
+    a transaction with that tx_id will be started and automatically marked
+    `conflicting'. The tx_id will always be passed back in <transid>.
+    After this, the tx_id may be used in the request header field for
+    other operations.
+    When a transaction is started whole db is copied; reads and writes
+    happen on the copy.
+```
+
+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
        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>|
-- 
2.20.1


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