Xen project Mailing List

[Xen-changelog] [xen staging] docs/designs: Add a design document for migration of xenstore data

Date: Fri, 27 Mar 2020 17:22:15 +0000

Delivery-date: Fri, 27 Mar 2020 17:22:18 +0000

List-id: "Change log for Mercurial $receive only$" <xen-changelog.lists.xenproject.org>

commit a87676c4a32f94d79fcaf5b4e0eb59e880e0f032 Author: Paul Durrant <pdurrant@xxxxxxxxxx> AuthorDate: Fri Mar 27 13:46:10 2020 +0000 Commit: Julien Grall <jgrall@xxxxxxxxxx> CommitDate: Fri Mar 27 17:12:21 2020 +0000 docs/designs: Add a design document for migration of xenstore data 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 amended to replace the <mfn> term for the INTRODUCE operation with the <gfn>, since this is what it actually is. Signed-off-by: Paul Durrant <paul@xxxxxxx> Acked-by: Julien Grall <jgrall@xxxxxxxxxx> --- docs/designs/xenstore-migration.md | 256 +++++++++++++++++++++++++++++++++++++ docs/misc/xenstore.txt | 6 +- 2 files changed, 259 insertions(+), 3 deletions(-) diff --git a/docs/designs/xenstore-migration.md b/docs/designs/xenstore-migration.md new file mode 100644 index 0000000000..6ab351e8fe --- /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 ++-------+-------+-------+-------+ +| NODE_DATA | ++-------------------------------+ +| 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). 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 ++-------+-------+-------+-------+ +| WATCH_DATA | ++-------------------------------+ +| 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 ++-------+-------+-------+-------+ +| TRANSACTION_DATA | ++-------------------------------+ +| 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 performed, as modifications made under +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 issues 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..7f2b388dd5 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 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>| -- generated by git-patchbot for /home/xen/git/xen.git#staging _______________________________________________ Xen-changelog mailing list Xen-changelog@xxxxxxxxxxxxxxxxxxxx https://lists.xenproject.org/xen-changelog

©2013 Xen Project, A Linux Foundation Collaborative Project. All Rights Reserved.
Linux Foundation is a registered trademark of The Linux Foundation.
Xen Project is a trademark of The Linux Foundation.