1 Non-Cooperative Migration of Guests on Xen

1.1 Background

The normal model of migration in Xen is driven by the guest because it was originally implemented for PV guests, where the guest must be aware it is running under Xen and is hence expected to co-operate. This model dates from an era when it was assumed that the host administrator had control of at least the privileged software running in the guest (i.e. the guest kernel) which may still be true in an enterprise deployment but is not generally true in a cloud environment. The aim of this design is to provide a model which is purely host driven, requiring no co-operation from the software running in the guest, and is thus suitable for cloud scenarios.

PV guests are out of scope for this project because, as is outlined above, they have a symbiotic relationship with the hypervisor and therefore a certain level of co-operation is required.

x86 HVM guests can already be migrated on Xen without guest co-operation but only if they don’t have PV drivers installed[1] or are not in ACPI power state S0. The reason for not expecting co-operation if the guest is any sort of suspended state is obvious, but the reason co-operation is expected if PV drivers are installed is due to the nature of PV protocols.

1.2 Xenstore Nodes and Domain ID

The PV driver model consists of a frontend and a backend. The frontend runs inside the guest domain and the backend runs inside a service domain which may or may not be domain 0. The frontend and backend typically pass data via memory pages which are shared between the two domains, but this channel of communication is generally established using xenstore (the store protocol itself being an exception to this for obvious chicken-and-egg reasons).

Typical protocol establishment is based on use of two separate xenstore areas. If we consider PV drivers for the netif protocol (i.e. class vif) and assume the guest has domid X, the service domain has domid Y, and the vif has index Z then the frontend area will reside under the parent node:


All backends, by convention, typically reside under parent node:


and the normal backend area for vif Z would be:


but this should not be assumed.

The toolstack will place two nodes in the frontend area to explicitly locate the backend:

* `backend`: the fully qualified xenstore path of the backend area
* `backend-id`: the domid of the service domain

and similarly two nodes in the backend area to locate the frontend area:

* `frontend`: the fully qualified xenstore path of the frontend area
* `frontend-id`: the domid of the guest domain

The guest domain only has write permission to the frontend area and similarly the service domain only has write permission to the backend area, but both ends have read permission to both areas.

Under both frontend and backend areas is a node called state. This is key to protocol establishment. Upon PV device creation the toolstack will set the value of both state nodes to 1 (XenbusStateInitialising[2]). This should cause enumeration of appropriate devices in both the guest and service domains. The backend device, once it has written any necessary protocol specific information into the xenstore backend area (to be read by the frontend driver) will update the backend state node to 2 (XenbusStateInitWait). From this point on PV protocols differ slightly; the following illustration is true of the netif protocol.

Upon seeing a backend state value of 2, the frontend driver will then read the protocol specific information, write details of grant references (for shared pages) and event channel ports (for signalling) that it has created, and set the state node in the frontend area to 4 (XenbusStateConnected). Upon see this frontend state, the backend driver will then read the grant references (mapping the shared pages) and event channel ports (opening its end of them) and set the state node in the backend area to 4. Protocol establishment is now complete and the frontend and backend start to pass data.

Because the domid of both ends of a PV protocol forms a key part of negotiating the data plane for that protocol (because it is encoded into both xenstore nodes and node paths), and because guest’s own domid and the domid of the service domain are visible to the guest in xenstore (and hence ay cached internally), and neither are necessarily preserved during migration, it is hence necessary to have the co-operation of the frontend in re-negotiating the protocol using the new domid after migration.

Moreover the backend-id value will be used by the frontend driver in setting up grant table entries and event channels to communicate with the service domain, so the co-operation of the guest is required to re-establish these in the new host environment after migration.

Thus if we are to change the model and support migration of a guest with PV drivers, without the co-operation of the frontend driver code, the paths and values in both the frontend and backend xenstore areas must remain unchanged and valid in the new host environment, and the grant table entries and event channels must be preserved (and remain operational once guest execution is resumed).

Because the service domain’s domid is used directly by the guest in setting up grant entries and event channels, the backend drivers in the new host environment must be provided by service domain with the same domid. Also, because the guest can sample its own domid from the frontend area and use it in hypercalls (e.g. HVMOP_set_param) rather than DOMID_SELF, the guest domid must also be preserved to maintain the ABI.

Furthermore, it will necessary to modify backend drivers to re-establish communication with frontend drivers without perturbing the content of the backend area or requiring any changes to the values of the xenstore state nodes.

1.3 Other Para-Virtual State

1.3.1 Shared Rings

Because the console and store protocol shared pages are actually part of the guest memory image (in an E820 reserved region just below 4G in x86 VMs) then the content will get migrated as part of the guest memory image. Hence no additional code is require to prevent any guest visible change in the content.

1.3.2 Shared Info

There is already a record defined in libxenctrl Domain Image Format [3] called SHARED_INFO which simply contains a complete copy of the domain’s shared info page. It is not currently incuded in an HVM (type 0x0002) migration stream. It may be feasible to include it as an optional record but it is not clear that the content of the shared info page ever needs to be preserved for an HVM guest.

For a PV guest the arch_shared_info sub-structure contains important information about the guest’s P2M, but this information is not relevant for an HVM guest where the P2M is not directly manipulated via the guest. The other state contained in the shared_info structure relates the domain wall-clock (the state of which should already be transferred by the RTC HVM context information which contained in the HVM_CONTEXT save record) and some event channel state (particularly if using the 2l protocol). Event channel state will need to be fully transferred if we are not going to require the guest co-operation to re-open the channels and so it should be possible to re-build a shared info page for an HVM guest from such other state.

Note that the shared info page also contains an array of XEN_LEGACY_MAX_VCPUS (32 for x86) vcpu_info structures. A domain may nominate a different guest physical address to use for the vcpu info. This is mandatory if a domain wants to use more than XEN_LEGACY_MAX_VCPUS vCPUs and optional otherwise. This mapping is not currently transferred in the migration state so this will either need to be added into an existing save record, or an additional type of save record will be needed.

1.3.3 Xenstore Watches

As mentioned above, no domain Xenstore state is currently transferred in the migration stream. There is a record defined in libxenlight Domain Image Format [4] called EMULATOR_XENSTORE_DATA for transferring Xenstore nodes relating to emulators but no record type is defined for nodes relating to the domain itself, nor for registered watches. A XenStore watch is a mechanism used by PV frontend and backend drivers to request a notification if the value of a particular node (e.g. the other end’s state node) changes, so it is important that watches continue to function after a migration. One or more new save records will therefore be required to transfer Xenstore state. It will also be necessary to extend the store protocol[5] with mechanisms to allow the toolstack to acquire the list of watches that the guest has registered and for the toolstack to register a watch on behalf of a domain.

1.3.4 Event channels

Event channels are essentially the para-virtual equivalent of interrupts. They are an important part of post PV protocols. Normally a frontend driver creates an inter-domain event channel between its own domain and the domain running the backend, which it discovers using the backend-id node in Xenstore (see above), by making a EVTCHNOP_alloc_unbound hypercall. This hypercall allocates an event channel object in the hypervisor and assigns a local port number which is then written into the frontend area in Xenstore. The backend driver then reads this port number and binds to the event channel by specifying it, and the value of frontend-id, as remote domain and remote port (respectively) to a EVTCHNOP_bind_interdomain hypercall. Once connection is established in this fashion frontend and backend drivers can use the event channel as a mailbox to notify each other when a shared ring has been updated with new requests or response structures.

Currently no event channel state is preserved on migration, requiring frontend and backend drivers to create and bind a complete new set of event channels in order to re-establish a protocol connection. Hence, one or more new save records will be required to transfer event channel state in order to avoid the need for explicit action by frontend drivers running in the guest. Note that the local port numbers need to preserved in this state as they are the only context the guest has to refer to the hypervisor event channel objects.

Note also that the PV store (Xenstore access) and console protocols also rely on event channels which are set up by the toolstack. Normally, early in migration, the toolstack running on the remote host would set up a new pair of event channels for these protocols in the destination domain. These may not be assigned the same local port numbers as the protocols running in the source domain. For non-cooperative migration these channels must either be created with fixed port numbers, or their creation must be avoided and instead be included in the general event channel state record(s).

1.3.5 Grant table

The grant table is essentially the para-virtual equivalent of an IOMMU. For example, the shared rings of a PV protocol are granted by a frontend driver to the backend driver by allocating grant entries in the guest’s table, filling in details of the memory pages and then writing the grant references (the index values of the grant entries) into Xenstore. The grant references of the protocol buffers themselves are typically written directly into the request structures passed via a shared ring.

The guest is responsible for managing its own grant table. No hypercall is required to grant a memory page to another domain. It is sufficient to find an unused grant entry and set bits in the entry to give read and/or write access to a remote domain also specified in the entry along with the page frame number. Thus the layout and content of the grant table logically forms part of the guest state.

Currently no grant table state is migrated, requiring a guest to separately maintain any state that it wishes to persist elsewhere in its memory image and then restore it after migration. Thus to avoid the need for such explicit action by the guest, one or more new save records will be required to migrate the contents of the grant table.

2 Outline Proposal

[1] PV drivers are deemed to be installed if the HVM parameter HVM_PARAM_CALLBACK_IRQ has been set to a non-zero value.

[2] See https://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=xen/include/public/io/xenbus.h

[3] See https://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=docs/specs/libxc-migration-stream.pandoc

[4] See https://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=docs/specs/libxl-migration-stream.pandoc

[5] See https://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=docs/misc/xenstore.txt

[6] xen-blkback and xen-netback have already been modified in Linux to do this.