[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

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

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>

  - Make semantics of <index> in GET_DOMAIN_WATCHES more clear

  - Drop the restrictions on special paths

  - 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 
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:
+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?

+| `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.

+where data is always in the form of a NUL separated and terminated tuple
+as follows
+**node data**

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?

+`<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>` 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 @...)?

+### 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>.
+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.

+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 
+[2] See 
+[3] See 


Julien Grall

Xen-devel mailing list



Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.