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

Hi Paul,

On 05/03/2020 17:30, pdurrant@xxxxxxxx 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
       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>

  - Addressed comments from Julien

  - Make semantics of <index> in GET_DOMAIN_WATCHES more clear

  - Drop the restrictions on special paths

  - 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 
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:
+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

NIT: missing full stop.

+**node data**
In the xenstore spec, | means a 'nul (zero) byte'. Are you using the same terminology here? If so, it may be worth mentioning it.

+`<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

What is the size of <perm-count>? Also, we may want perm-count to be aligned to its size so we don't have to worry of unaligned access.

How about moving <perm-count> at the beginning of the data blob?

+**watch data**
+`<path>` again is absolute and, together with `<token>`, should
+be suitable to formulate an `ADD_DOMAIN_WATCHES` operation (see below).
+**transaction data**
+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.

What is the size of <transid-count>?. Also the | suggests we would have a NUL byte between the two numbers. This would mean the <transid> will not be aligned to its size.

+### 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
+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.

The transaction ID are per connection. As the toolstack will issue the command on restore, you would reserve the ID for the wrong domain. So I think you want to introduce a new command maybe RESERVE_DOMAIN_TRANSACTIONS?


Julien Grall

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