[Xen-devel] [PATCH v8 0/2] docs: Migration design documents

[Xen-devel] [PATCH v8 0/2] docs: Migration design documents

[Paul Durrant]

Paul Durrant (2):
  docs/designs: Add a design document for non-cooperative live migration
  docs/designs: Add a design document for migration of xenstore data

 docs/designs/non-cooperative-migration.md | 280 ++++++++++++++++++++++
 docs/designs/xenstore-migration.md        | 256 ++++++++++++++++++++
 docs/misc/xenstore.txt                    |   6 +-
 3 files changed, 539 insertions(+), 3 deletions(-)
 create mode 100644 docs/designs/non-cooperative-migration.md
 create mode 100644 docs/designs/xenstore-migration.md

-- 
2.20.1

[Xen-devel] [PATCH v8 1/2] docs/designs: Add a design document for non-cooperative live migration

[Paul Durrant]

From: Paul Durrant <pdurrant@amazon.com>

It has become apparent to some large cloud providers that the current
model of cooperative migration of guests under Xen is not usable as it
relies on software running inside the guest, which is likely beyond the
provider's control.
This patch introduces a proposal for non-cooperative live migration,
designed not to rely on any guest-side software.

Signed-off-by: Paul Durrant <paul@xen.org>
---
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: George Dunlap <George.Dunlap@eu.citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Julien Grall <julien@xen.org>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Wei Liu <wl@xen.org>

v8:
 - Addressed comments from Julien on v6 that I missed

v6:
 - Addressed comments from Julien

v5:
 - Note that PV domain are not just expected to co-operate, they are
   required to

v4:
 - Fix issues raised by Wei

v2:
 - Use the term 'non-cooperative' instead of 'transparent'
 - Replace 'trust in' with 'reliance on' when referring to guest-side
   software
---
 docs/designs/non-cooperative-migration.md | 280 ++++++++++++++++++++++
 1 file changed, 280 insertions(+)
 create mode 100644 docs/designs/non-cooperative-migration.md

diff --git a/docs/designs/non-cooperative-migration.md b/docs/designs/non-cooperative-migration.md
new file mode 100644
index 0000000000..4b876d809f
--- /dev/null
+++ b/docs/designs/non-cooperative-migration.md
@@ -0,0 +1,280 @@
+# Non-Cooperative Migration of Guests on Xen
+
+## 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.
+
+## 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:
+
+`/local/domain/Y/device/vif/Z`
+
+All backends, by convention, typically reside under parent node:
+
+`/local/domain/X/backend`
+
+and the normal backend area for vif Z would be:
+
+`/local/domain/X/backend/vif/Y/Z`
+
+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.
+
+## Other Para-Virtual State
+
+### 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.
+
+### 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.
+
+### 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.
+
+### 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).
+
+### 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.
+
+# Outline Proposal
+
+* PV backend drivers will be modified to unilaterally re-establish
+connection to a frontend if the backend state node is restored with value 4
+(XenbusStateConnected)[6].
+
+* The toolstack choose a randomized domid for initial creation or default
+migration, but preserve the source domid non-cooperative migration.
+Non-Cooperative migration will have to be denied if the domid is
+unavailable on the target host, but randomization of domid on creation
+should hopefully minimize the likelihood of this. Non-Cooperative migration
+to localhost will clearly not be possible.
+
+* `xenstored` should be modified to implement the new mechanisms needed.
+See *Other Para-Virtual State* above. A further design document will
+propose additional protocol messages.
+
+* Within the migration stream extra save records will be defined as
+required. See *Other Para-Virtual State* above. A further design document
+will propose modifications to the libxenlight and libxenctrl Domain Image
+Formats.
+
+* An option should be added to the toolstack to initiate a non-cooperative
+migration, instead of the (default) potentially co-operative migration.
+Essentially this should skip the check to see if PV drivers and migrate as
+if there are none present, but also enabling the extra save records. Note
+that at least some of the extra records should only form part of a
+non-cooperative migration stream. For example, migrating event channel
+state would be counter productive in a normal migration as this will
+essentially leak event channel objects at the receiving end. Others, such
+as grant table state, could potentially harmlessly form part of a normal
+migration stream.
+
+* * *
+[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.
-- 
2.20.1

Re: [Xen-devel] [PATCH v8 1/2] docs/designs: Add a design document for non-cooperative live migration

[Julien Grall]

Hi Paul,

On 27/03/2020 13:46, Paul Durrant wrote:
> From: Paul Durrant <pdurrant@amazon.com>
> 
> It has become apparent to some large cloud providers that the current
> model of cooperative migration of guests under Xen is not usable as it
> relies on software running inside the guest, which is likely beyond the
> provider's control.
> This patch introduces a proposal for non-cooperative live migration,
> designed not to rely on any guest-side software.
> 
> Signed-off-by: Paul Durrant <paul@xen.org>

Acked-by: Julien Grall <jgrall@amazon.com>

Cheers,

> ---
> Cc: Andrew Cooper <andrew.cooper3@citrix.com>
> Cc: George Dunlap <George.Dunlap@eu.citrix.com>
> Cc: Ian Jackson <ian.jackson@eu.citrix.com>
> Cc: Jan Beulich <jbeulich@suse.com>
> Cc: Julien Grall <julien@xen.org>
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> Cc: Stefano Stabellini <sstabellini@kernel.org>
> Cc: Wei Liu <wl@xen.org>
> 
> v8:
>   - Addressed comments from Julien on v6 that I missed
> 
> v6:
>   - Addressed comments from Julien
> 
> v5:
>   - Note that PV domain are not just expected to co-operate, they are
>     required to
> 
> v4:
>   - Fix issues raised by Wei
> 
> v2:
>   - Use the term 'non-cooperative' instead of 'transparent'
>   - Replace 'trust in' with 'reliance on' when referring to guest-side
>     software
> ---
>   docs/designs/non-cooperative-migration.md | 280 ++++++++++++++++++++++
>   1 file changed, 280 insertions(+)
>   create mode 100644 docs/designs/non-cooperative-migration.md
> 
> diff --git a/docs/designs/non-cooperative-migration.md b/docs/designs/non-cooperative-migration.md
> new file mode 100644
> index 0000000000..4b876d809f
> --- /dev/null
> +++ b/docs/designs/non-cooperative-migration.md
> @@ -0,0 +1,280 @@
> +# Non-Cooperative Migration of Guests on Xen
> +
> +## 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.
> +
> +## 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:
> +
> +`/local/domain/Y/device/vif/Z`
> +
> +All backends, by convention, typically reside under parent node:
> +
> +`/local/domain/X/backend`
> +
> +and the normal backend area for vif Z would be:
> +
> +`/local/domain/X/backend/vif/Y/Z`
> +
> +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.
> +
> +## Other Para-Virtual State
> +
> +### 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.
> +
> +### 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.
> +
> +### 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.
> +
> +### 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).
> +
> +### 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.
> +
> +# Outline Proposal
> +
> +* PV backend drivers will be modified to unilaterally re-establish
> +connection to a frontend if the backend state node is restored with value 4
> +(XenbusStateConnected)[6].
> +
> +* The toolstack choose a randomized domid for initial creation or default
> +migration, but preserve the source domid non-cooperative migration.
> +Non-Cooperative migration will have to be denied if the domid is
> +unavailable on the target host, but randomization of domid on creation
> +should hopefully minimize the likelihood of this. Non-Cooperative migration
> +to localhost will clearly not be possible.
> +
> +* `xenstored` should be modified to implement the new mechanisms needed.
> +See *Other Para-Virtual State* above. A further design document will
> +propose additional protocol messages.
> +
> +* Within the migration stream extra save records will be defined as
> +required. See *Other Para-Virtual State* above. A further design document
> +will propose modifications to the libxenlight and libxenctrl Domain Image
> +Formats.
> +
> +* An option should be added to the toolstack to initiate a non-cooperative
> +migration, instead of the (default) potentially co-operative migration.
> +Essentially this should skip the check to see if PV drivers and migrate as
> +if there are none present, but also enabling the extra save records. Note
> +that at least some of the extra records should only form part of a
> +non-cooperative migration stream. For example, migrating event channel
> +state would be counter productive in a normal migration as this will
> +essentially leak event channel objects at the receiving end. Others, such
> +as grant table state, could potentially harmlessly form part of a normal
> +migration stream.
> +
> +* * *
> +[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.
> 

-- 
Julien Grall

[Xen-devel] [PATCH v8 2/2] docs/designs: Add a design document for migration of xenstore data

[Paul Durrant]

From: Paul Durrant <pdurrant@amazon.com>

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@xen.org>
---
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: George Dunlap <George.Dunlap@eu.citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Julien Grall <julien@xen.org>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Wei Liu <wl@xen.org>

v8:
 - Addressed further comments form Julien

v7:
 - Addressed further comments from Julien
 - Switched migration records to defined structures instead of tuples

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 | 256 +++++++++++++++++++++++++++++
 docs/misc/xenstore.txt             |   6 +-
 2 files changed, 259 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..97695f3ec9
--- /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 isses 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>|
-- 
2.20.1

Re: [Xen-devel] [PATCH v8 2/2] docs/designs: Add a design document for migration of xenstore data

[Julien Grall]

On 27/03/2020 13:46, Paul Durrant wrote:
> +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 isses TRANSACTION_END T|, it will

NIT: s/isses/issues/

Acked-by: Julien Grall <jgrall@amazon.com>

I can fix the typo while committing the patch series.

Cheers,

-- 
Julien Grall

Re: [Xen-devel] [PATCH v8 2/2] docs/designs: Add a design document for migration of xenstore data

[Paul Durrant]

> -----Original Message-----
> From: Julien Grall <julien@xen.org>
> Sent: 27 March 2020 16:58
> To: Paul Durrant <paul@xen.org>; xen-devel@lists.xenproject.org
> Cc: Paul Durrant <pdurrant@amazon.com>; Andrew Cooper <andrew.cooper3@citrix.com>; George Dunlap
> <George.Dunlap@eu.citrix.com>; Ian Jackson <ian.jackson@eu.citrix.com>; Jan Beulich
> <jbeulich@suse.com>; Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>; Stefano Stabellini
> <sstabellini@kernel.org>; Wei Liu <wl@xen.org>
> Subject: Re: [PATCH v8 2/2] docs/designs: Add a design document for migration of xenstore data
> 
> 
> 
> On 27/03/2020 13:46, Paul Durrant wrote:
> > +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 isses TRANSACTION_END T|, it will
> 
> NIT: s/isses/issues/

Oh yes.

> 
> Acked-by: Julien Grall <jgrall@amazon.com>
> 
> I can fix the typo while committing the patch series.
> 

Thanks,

  Paul

> Cheers,
> 
> --
> Julien Grall