[RFC PATCH 02/60] hyper_dmabuf: added a doc for hyper_dmabuf sharing

[Dongwon Kim]

High-level description of hyper_dmabuf driver has been added
to "Documentation" directory.

Signed-off-by: Dongwon Kim <dongwon.kim@xxxxxxxxx>
---
 Documentation/hyper-dmabuf-sharing.txt | 734 +++++++++++++++++++++++++++++++++
 1 file changed, 734 insertions(+)
 create mode 100644 Documentation/hyper-dmabuf-sharing.txt

diff --git a/Documentation/hyper-dmabuf-sharing.txt b/Documentation/hyper-dmabuf-sharing.txt
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+Linux Hyper DMABUF Driver
+
+------------------------------------------------------------------------------
+Section 1. Overview
+------------------------------------------------------------------------------
+
+Hyper_DMABUF driver is a Linux device driver running on multiple Virtual
+achines (VMs), which expands DMA-BUF sharing capability to the VM environment
+where multiple different OS instances need to share same physical data without
+data-copy across VMs.
+
+To share a DMA_BUF across VMs, an instance of the Hyper_DMABUF drv on the
+exporting VM (so called, âexporterâ) imports a local DMA_BUF from the original
+producer of the buffer, then re-exports it with an unique ID, hyper_dmabuf_id
+for the buffer to the importing VM (so called, âimporterâ).
+
+Another instance of the Hyper_DMABUF driver on importer registers
+a hyper_dmabuf_id together with reference information for the shared physical
+pages associated with the DMA_BUF to its database when the export happens.
+
+The actual mapping of the DMA_BUF on the importerâs side is done by
+the Hyper_DMABUF driver when user space issues the IOCTL command to access
+the shared DMA_BUF. The Hyper_DMABUF driver works as both an importing and
+exporting driver as is, that is, no special configuration is required.
+Consequently, only a single module per VM is needed to enable cross-VM DMA_BUF
+exchange.
+
+------------------------------------------------------------------------------
+Section 2. Architecture
+------------------------------------------------------------------------------
+
+1. Hyper_DMABUF ID
+
+hyper_dmabuf_id is a global handle for shared DMA BUFs, which is compatible
+across VMs. It is a key used by the importer to retrieve information about
+shared Kernel pages behind the DMA_BUF structure from the IMPORT list. When
+a DMA_BUF is exported to another domain, its hyper_dmabuf_id and META data
+are also kept in the EXPORT list by the exporter for further synchronization
+of control over the DMA_BUF.
+
+hyper_dmabuf_id is âtargetedâ, meaning it is valid only in exporting (owner of
+the buffer) and importing VMs, where the corresponding hyper_dmabuf_id is
+stored in their database (EXPORT and IMPORT lists).
+
+A user-space application specifies the targeted VM id in the user parameter
+when it calls the IOCTL command to export shared DMA_BUF to another VM.
+
+hyper_dmabuf_id_t is a data type for hyper_dmabuf_id. It is defined as 16-byte
+data structure, and it contains id and rng_key[3] as elements for
+the structure.
+
+typedef struct {
+        int id;
+        int rng_key[3]; /* 12bytes long random number */
+} hyper_dmabuf_id_t;
+
+The first element in the hyper_dmabuf_id structure, int id is combined data of
+a count number generated by the driver running on the exporter and
+the exporterâs ID. The VMâs ID is a one byte value and located at the fieldâs
+SB in int id. The remaining three bytes in int id are reserved for a count
+number.
+
+However, there is a limit related to this count number, which is 1000.
+Therefore, only little more than a byte starting from the LSB is actually used
+for storing this count number.
+
+#define HYPER_DMABUF_ID_CREATE(domid, id) \
+        ((((domid) & 0xFF) << 24) | ((id) & 0xFFFFFF))
+
+This limit on the count number directly means the maximum number of DMA BUFs
+that  can be shared simultaneously by one VM. The second element of
+hyper_dmabuf_id, that is int rng_key[3], is an array of three integers. These
+numbers are generated by Linuxâs native random number generation mechanism.
+This field is added to enhance the security of the Hyper DMABUF driver by
+maximizing the entropy of hyper_dmabuf_id (that is, preventing it from being
+guessed by a security attacker).
+
+Once DMA_BUF is no longer shared, the hyper_dmabuf_id associated with
+the DMA_BUF is released, but the count number in hyper_dmabuf_id is saved in
+the ID list for reuse. However, random keys stored in int rng_key[3] are not
+reused. Instead, those keys are always filled with freshly generated random
+keys for security.
+
+2. IOCTLs
+
+a. IOCTL_HYPER_DMABUF_TX_CH_SETUP
+
+This type of IOCTL is used for initialization of a one-directional transmit
+communication channel with a remote domain.
+
+The user space argument for this type of IOCTL is defined as:
+
+struct ioctl_hyper_dmabuf_tx_ch_setup {
+    /* IN parameters */
+    /* Remote domain id */
+    int remote_domain;
+};
+
+b. IOCTL_HYPER_DMABUF_RX_CH_SETUP
+
+This type of IOCTL is used for initialization of a one-directional receive
+communication channel with a remote domain.
+
+The user space argument for this type of IOCTL is defined as:
+
+struct ioctl_hyper_dmabuf_rx_ch_setup {
+    /* IN parameters */
+    /* Source domain id */
+    int source_domain;
+};
+
+c. IOCTL_HYPER_DMABUF_EXPORT_REMOTE
+
+This type of IOCTL is used to export a DMA BUF to another VM. When a user
+space application makes this call to the driver, it extracts Kernel pages
+associated with the DMA_BUF, then makes those shared with the importing VM.
+
+All reference information for this shared pages and hyper_dmabuf_id is
+created, then passed to the importing domain through a communications
+channel for synchronous registration. In the meantime, the hyper_dmabuf_id
+for the shared DMA_BUF is also returned to user-space application.
+
+This IOCTL can accept a reference to âuser-definedâ data as well as a FD
+for the DMA BUF. This private data is then attached to the DMA BUF and
+exported together with it.
+
+ore details regarding this private data can be found in chapter for
+âHyper_DMABUF Private Dataâ.
+
+The user space argument for this type of IOCTL is defined as:
+
+struct ioctl_hyper_dmabuf_export_remote {
+    /* IN parameters */
+    /* DMA buf fd to be exported */
+    int dmabuf_fd;
+    /* Domain id to which buffer should be exported */
+    int remote_domain;
+    /* exported dma buf id */
+    hyper_dmabuf_id_t hid;
+    /* size of private data */
+    int sz_priv;
+    /* ptr to the private data for Hyper_DMABUF */
+    char *priv;
+};
+
+d. IOCTL_HYPER_DMABUF_EXPORT_FD
+
+The importing VM uses this IOCTL to import and re-export a shared DMA_BUF
+locally to the end-consumer using the standard Linux DMA_BUF framework.
+Upon IOCTL call, the Hyper_DMABUF driver finds the reference information
+of the shared DMA_BUF with the given hyper_dmabuf_id, then maps all shared
+pages in its own Kernel space. The driver then constructs a scatter-gather
+list with those mapped pages and creates a brand-new DMA_BUF with the list,
+which is eventually exported with a file descriptor to the local consumer.
+
+The user space argument for this type of IOCTL is defined as:
+
+struct ioctl_hyper_dmabuf_export_fd {
+    /* IN parameters */
+    /* hyper dmabuf id to be imported */
+    int hyper_dmabuf_id;
+    /* flags */
+    int flags;
+    /* OUT parameters */
+    /* exported dma buf fd */
+    int fd;
+};
+
+e. IOCTL_HYPER_DMABUF_UNEXPORT
+
+This type of IOCTL is used when it is necessary to terminate the current
+sharing of a DMA_BUF. When called, the driver first checks if there are any
+consumers actively using the DMA_BUF. Then, it unexports it if it is not
+mapped or used by any consumers. Otherwise, it postpones unexporting, but
+makes the buffer invalid to prevent any further import of the same DMA_BUF.
+DMA_BUF is completely unexported after the last consumer releases it.
+
+âUnexportâ means remove all reference information about the DMA_BUF from the
+LISTs and make all pages private again.
+
+The user space argument for this type of IOCTL is defined as:
+
+struct ioctl_hyper_dmabuf_unexport {
+    /* IN parameters */
+    /* hyper dmabuf id to be unexported */
+    int hyper_dmabuf_id;
+    /* delay in ms by which unexport processing will be postponed */
+    int delay_ms;
+    /* OUT parameters */
+    /* Status of request */
+    int status;
+};
+
+f. IOCTL_HYPER_DMABUF_QUERY
+
+This IOCTL is used to retrieve specific information about a DMA_BUF that
+is being shared.
+
+The user space argument for this type of IOCTL is defined as:
+
+struct ioctl_hyper_dmabuf_query {
+    /* in parameters */
+    /* hyper dmabuf id to be queried */
+    int hyper_dmabuf_id;
+    /* item to be queried */
+    int item;
+    /* OUT parameters */
+    /* output of query */
+    /* info can be either value or reference */
+    unsigned long info;
+};
+
+<Available Queries>
+
+HYPER_DMABUF_QUERY_TYPE
+ - Return the type of DMA_BUF from the current domain, Exported or Imported.
+
+HYPER_DMABUF_QUERY_EXPORTER
+ - Return the exporting domainâs ID of a shared DMA_BUF.
+
+HYPER_DMABUF_QUERY_IMPORTER
+ - Return the importing domainâs ID of a shared DMA_BUF.
+
+HYPER_DMABUF_QUERY_SIZE
+ - Return the size of a shared DMA_BUF in bytes.
+
+HYPER_DMABUF_QUERY_BUSY
+ - Return âtrueâ if a shared DMA_BUF is currently used
+   (mapped by the end-consumer).
+
+HYPER_DMABUF_QUERY_UNEXPORTED
+ - Return âtrueâ if a shared DMA_BUF is not valid anymore
+   (so it does not allow a new consumer to map it).
+
+HYPER_DMABUF_QUERY_DELAYED_UNEXPORTED
+ - Return âtrueâ if a shared DMA_BUF is scheduled to be unexported
+   (but is still valid) within a fixed time.
+
+HYPER_DMABUF_QUERY_PRIV_INFO
+ - Return âprivateâ data attached to shared DMA_BUF to the user space.
+   âunsigned long infoâ is the user space pointer for the buffer, where
+   private data will be copied to.
+
+HYPER_DMABUF_QUERY_PRIV_INFO_SIZE
+ - Return the size of the private data attached to the shared DMA_BUF.
+
+3. Event Polling
+
+Event-polling can be enabled optionally by selecting the Kernel config option,
+Enable event-generation and polling operation under xen/hypervisor in Kernelâs
+menuconfig. The event-polling mechanism includes the generation of
+an import-event, adding it to the event-queue and providing a notification to
+the application so that it can retrieve the event data from the queue.
+
+For this mechanism, âPollâ and âReadâ operations are added to the Hyper_DMABUF
+driver. A user application that polls the driver goes into a sleep state until
+there is a new event added to the queue. An application uses âReadâ to retrieve
+event data from the event queue. Event data contains the hyper_dmabuf_id and
+the private data of the buffer that has been received by the importer.
+
+For more information on private data, refer to Section 3.5).
+Using this method, it is possible to lower the risk of the hyper_dmabuf_id and
+other sensitive information about the shared buffer (for example, meta-data
+for shared images) being leaked while being transferred to the importer because
+all of this data is shared as âprivate infoâ at the driver level. However,
+please note there should be a way for the importer to find the correct DMA_BUF
+in this case when there are multiple Hyper_DMABUFs being shared simultaneously.
+For example, the surface name or the surface ID of a specific rendering surface
+needs to be sent to the importer in advance before it is exported in a surface-
+sharing use-case.
+
+Each event data given to the user-space consists of a header and the private
+information of the buffer. The data type is defined as follows:
+
+struct hyper_dmabuf_event_hdr {
+        int event_type; /* one type only for now - new import */
+        hyper_dmabuf_id_t hid; /* hyper_dmabuf_id of specific hyper_dmabuf */
+        int size; /* size of data */
+};
+
+struct hyper_dmabuf_event_data {
+        struct hyper_dmabuf_event_hdr hdr;
+        void *data; /* private data */
+};
+
+4. Hyper_DMABUF Private Data
+
+Each Hyper_DMABUF can come with private data, the size of which can be up to
+AX_SIZE_PRIV_DATA (currently 192 byte). This private data is just a chunk of
+plain data attached to every Hyper_DMABUF. It is guaranteed to be synchronized
+across VMs, exporter and importer. This private data does not have any specific
+structure defined at the driver level, so any âuser-definedâ format or
+structure can be used. In addition, there is no dedicated use-case for this
+data. It can be used virtually for any purpose. For example, it can be used to
+share meta-data such as dimension and color formats for shared images in
+a surface sharing model. Another example is when we share protected media
+contents.
+
+This private data can be used to transfer flags related to content protection
+information on streamed media to the importer.
+
+Private data is initially generated when a buffer is exported for the first
+time. Then, it is updated whenever the same buffer is re-exported. During the
+re-exporting process, the Hyper_DMABUF driver only updates private data on
+both sides with new data from user-space since the same buffer already exists
+on both the IMPORT LIST and EXPORT LIST.
+
+There are two different ways to retrieve this private data from user-space.
+The first way is to use âReadâ on the Hyper_DMABUF driver. âReadâ returns the
+data of events containing private data of the buffer. The second way is to
+make a query to Hyper_DMABUF. There are two query items,
+HYPER_DMABUF_QUERY_PRIV_INFO and HYPER_DMABUF_QUERY_PRIV_INFO_SIZE available
+for retrieving private data and its size.
+
+5. Scatter-Gather List Table (SGT) Management
+
+SGT management is the core part of the Hyper_DMABUF driver that manages an
+SGT, a representation of the group of kernel pages associated with a DMA_BUF.
+This block includes four different sub-blocks:
+
+a. Hyper_DMABUF_id Manager
+
+This ID manager is responsible for generating a hyper_dmabuf_id for an
+exported DMA_BUF. When an ID is requested, the ID Manager first checks if
+there are any reusable IDs left in the list and returns one of those,
+if available. Otherwise, it creates the next count number and returns it
+to the caller.
+
+b. SGT Creator
+
+The SGT (struct sg_table) contains information about the DMA_BUF such as
+references to all kernel pages for the buffer and their connections. The
+SGT Creator creates a new SGT on the importer side with pages shared by
+the hypervisor.
+
+c. Kernel Page Extractor
+
+The Page Extractor extracts pages from a given SGT before those pages
+are shared.
+
+d. List Manager Interface
+
+The SGT manger also interacts with export and import list managers. It
+sends out information (for example, hyper_dmabuf_id, reference, and
+DMA_BUF information) about the exported or imported DMA_BUFs to the
+list manager. Also, on IOCTL request, it asks the list manager to find
+and return the information for a corresponding DMA_BUF in the list.
+
+6. DMA-BUF Interface
+
+The DMA-BUF interface provides standard methods to manage DMA_BUFs
+reconstructed by the Hyper_DMABUF driver from shared pages. All of the
+relevant operations are listed in struct dma_buf_ops. These operations
+are standard DMA_BUF operations, therefore they follow standard DMA BUF
+protocols.
+
+Each DMA_BUF operation communicates with the exporter at the end of the
+routine for âindirect DMA_BUF synchronizationâ.
+
+7. Export/Import List Management
+
+Whenever a DMA_BUF is shared and exported, its information is added to the
+database (EXPORT-list) on the exporting VM. Similarly, information about an
+imported DMA_BUF is added to the importing database (IMPORT list) on the
+importing VM, when the export happens.
+
+All of the entries in the lists are needed to manage the exported/imported
+DMA_BUF more efficiently. Both lists are implemented as Linux hash tables.
+The key to the list is hyper_dmabuf_id and the output is the information of
+the DMA_BUF. The List Manager manages all requests from other blocks and
+transactions within lists to ensure that all entries are up-to-date and
+that the list structure is consistent.
+
+The List Manager provides basic functionality, such as:
+
+- Adding to the List
+- Removal from the List
+- Finding information about a DMA_BUF, given the hyper_dmabuf_id
+
+8. Page Sharing by Hypercalls
+
+The Hyper_DMABUF driver assumes that there is a native page-by-page memory
+sharing mechanism available on the hypervisor. Referencing a group of pages
+that are being shared is what the driver expects from âbackendâ APIs or the
+hypervisor itself.
+
+For the example, xen backend integrated in current code base utilizes Xenâs
+grant-table interface for sharing the underlying kernel pages (struct *page).
+
+ore details about grant-table interface can be found at the following locations:
+
+https://wiki.xen.org/wiki/Grant_Table
+https://xenbits.xen.org/docs/4.6-testing/misc/grant-tables.txt
+
+9. Message Management
+
+The exporter and importer can each create a message that consists of an opcode
+(command) and operands (parameters) and send it to each other.
+
+The message format is defined as:
+
+struct hyper_dmabuf_req {
+        unsigned int req_id; /* Sequence number. Used for RING BUF
+                                synchronization */
+        unsigned int stat; /* Status.Response from receiver. */
+        unsigned int cmd;  /* Opcode */
+        unsigned int op[MAX_NUMBER_OF_OPERANDS]; /* Operands */
+};
+
+The following table gives the list of opcodes:
+
+<Opcodes in Message to Exporter/Importer>
+
+HYPER_DMABUF_EXPORT (exporter --> importer)
+ - Export a DMA_BUF to the importer. The importer registers the corresponding
+   DMA_BUF in its IMPORT LIST when the message is received.
+
+HYPER_DMABUF_EXPORT_FD (importer --> exporter)
+ - Locally exported as FD. The importer sends out this command to the exporter
+   to notify that the buffer is now locally exported (mapped and used).
+
+HYPER_DMABUF_EXPORT_FD_FAILED (importer --> exporter)
+ - Failed while exporting locally. The importer sends out this command to the
+   exporter to notify the exporter that the EXPORT_FD failed.
+
+HYPER_DMABUF_NOTIFY_UNEXPORT (exporter --> importer)
+ - Termination of sharing. The exporter notifies the importer that the DMA_BUF
+   has been unexported.
+
+HYPER_DMABUF_OPS_TO_REMOTE (importer --> exporter)
+ - Not implemented yet.
+
+HYPER_DMABUF_OPS_TO_SOURCE (exporter --> importer)
+ - DMA_BUF ops to the exporter, for DMA_BUF upstream synchronization.
+   Note: Implemented but it is done asynchronously due to performance issues.
+
+The following table shows the list of operands for each opcode.
+
+<Operands in Message to Exporter/Importer>
+
+- HYPER_DMABUF_EXPORT
+
+op0 to op3 â hyper_dmabuf_id
+op4 â number of pages to be shared
+op5 â offset of data in the first page
+op6 â length of data in the last page
+op7 â reference number for the group of shared pages
+op8 â size of private data
+op9 to (op9+op8)  â private data
+
+- HYPER_DMABUF_EXPORT_FD
+
+op0 to op3 â hyper_dmabuf_id
+
+- HYPER_DMABUF_EXPORT_FD_FAILED
+
+op0 to op3 â hyper_dmabuf_id
+
+- HYPER_DMABUF_NOTIFY_UNEXPORT
+
+op0 to op3 â hyper_dmabuf_id
+
+- HYPER_DMABUF_OPS_TO_REMOTE(Not implemented)
+
+- HYPER_DMABUF_OPS_TO_SOURCE
+
+op0 to op3 â hyper_dmabuf_id
+op4 â type of DMA_BUF operation
+
+9. Inter VM (Domain) Communication
+
+Two different types of inter-domain communication channels are required,
+one in kernel space and the other in user space. The communication channel
+in user space is for transmitting or receiving the hyper_dmabuf_id. Since
+there is no specific security (for example, encryption) involved in the
+generation of a global id at the driver level, it is highly recommended that
+the customerâs user application set up a very secure channel for exchanging
+hyper_dmabuf_id between VMs.
+
+The communication channel in kernel space is required for exchanging messages
+from âmessage managementâ block between two VMs. In the current reference
+backend for Xen hypervisor, Xen ring-buffer and event-channel mechanisms are
+used for message exchange between impoter and exporter.
+
+10. What are required in hypervisor
+
+emory sharing and message communication between VMs
+
+------------------------------------------------------------------------------
+Section 3. Hyper DMABUF Sharing Flow
+------------------------------------------------------------------------------
+
+1. Exporting
+
+To export a DMA_BUF to another VM, user space has to call an IOCTL
+(IOCTL_HYPER_DMABUF_EXPORT_REMOTE) with a file descriptor for the buffer given
+by the original exporter. The Hyper_DMABUF driver maps a DMA_BUF locally, then
+issues a hyper_dmabuf_id and SGT for the DMA_BUF, which is registered to the
+EXPORT list. Then, all pages for the SGT are extracted and each individual
+page is shared via a hypervisor-specific memory sharing mechanism
+(for example, in Xen this is grant-table).
+
+One important requirement on this memory sharing method is that it needs to
+create a single integer value that represents the list of pages, which can
+then be used by the importer for retrieving the group of shared pages.  For
+this, the âBackendâ in the reference driver utilizes the multiple level
+addressing mechanism.
+
+Once the integer reference to the list of pages is created, the exporter
+builds the âexportâ command and sends it to the importer, then notifies the
+importer.
+
+2. Importing
+
+The Import process is divided into two sections. One is the registration
+of DMA_BUF from the exporter. The other is the actual mapping of the buffer
+before accessing the data in the buffer. The former (termed âRegistrationâ)
+happens on an export event (that is, the export command with an interrupt)
+in the exporter.
+
+The latter (termed âMappingâ) is done asynchronously when the driver gets the
+IOCTL call from user space. When the importer gets an interrupt from the
+exporter, it checks the command in the receiving queue and if it is an
+âexportâ command, the registration process is started. It first finds
+hyper_dmabuf_id and the integer reference for the shared pages, then stores
+all of that information together with the âdomain idâ of the exporting domain
+in the IMPORT LIST.
+
+In the case where âevent-pollingâ is enabled (Kernel Config - Enable event-
+generation and polling operation), a ânew sharing availableâ event is
+generated right after the reference info for the new shared DMA_BUF is
+registered to the IMPORT LIST. This event is added to the event-queue.
+
+The user process that polls Hyper_DMABUF driver wakes up when this event-queue
+is not empty and is able to read back event data from the queue using the
+driverâs âReadâ function. Once the user-application calls EXPORT_FD IOCTL with
+the proper parameters including hyper_dmabuf_id, the Hyper_DMABUF driver
+retrieves information about the matched DMA_BUF from the IMPORT LIST. Then, it
+maps all pages shared (referenced by the integer reference) in its kernel
+space and creates its own DMA_BUF referencing the same shared pages. After
+this, it exports this new DMA_BUF to the other drivers with a file descriptor.
+DMA_BUF can then be used just in the same way a local DMA_BUF is.
+
+3. Indirect Synchronization of DMA_BUF
+
+Synchronization of a DMA_BUF within a single OS is automatically achieved
+because all of importerâs DMA_BUF operations are done using functions defined
+on the exporterâs side, which means there is one central place that has full
+control over the DMA_BUF. In other words, any primary activities such as
+attaching/detaching and mapping/un-mapping are all captured by the exporter,
+meaning that the exporter knows basic information such as who is using the
+DMA_BUF and how it is being used. This, however, is not applicable if this
+sharing is done beyond a single OS because kernel space (where the exporterâs
+DMA_BUF operations reside) is simply not visible to the importing VM.
+
+Therefore, âindirect synchronizationâ was introduced as an alternative solution,
+which is now implemented in the Hyper_DMABUF driver. This technique makes
+the exporter create a shadow DMA_BUF when the end-consumer of the buffer maps
+the DMA_BUF, then duplicates any DMA_BUF operations performed on
+the importerâs side. Through this âindirect synchronizationâ, the exporter is
+able to virtually track all activities done by the consumer (mostly reference
+counter) as if those are done in exporterâs local system.
+
+------------------------------------------------------------------------------
+Section 4. Hypervisor Backend Interface
+------------------------------------------------------------------------------
+
+The Hyper_DMABUF driver has a standard âBackendâ structure that contains
+mappings to various functions designed for a specific Hypervisor. Most of
+these API functions should provide a low-level implementation of communication
+and memory sharing capability that utilize a Hypervisorâs native mechanisms.
+
+struct hyper_dmabuf_backend_ops {
+        /* retreiving id of current virtual machine */
+        int (*get_vm_id)(void);
+        /* get pages shared via hypervisor-specific method */
+        int (*share_pages)(struct page **, int, int, void **);
+        /* make shared pages unshared via hypervisor specific method */
+        int (*unshare_pages)(void **, int);
+        /* map remotely shared pages on importer's side via
+         *  hypervisor-specific method
+         */
+        struct page ** (*map_shared_pages)(int, int, int, void **);
+        /* unmap and free shared pages on importer's side via
+         *  hypervisor-specific method
+         */
+        int (*unmap_shared_pages)(void **, int);
+        /* initialize communication environment */
+        int (*init_comm_env)(void);
+        /* destroy communication channel */
+        void (*destroy_comm)(void);
+        /* upstream ch setup (receiving and responding) */
+        int (*init_rx_ch)(int);
+        /* downstream ch setup (transmitting and parsing responses) */
+        int (*init_tx_ch)(int);
+        /* send msg via communication ch */
+        int (*send_req)(int, struct hyper_dmabuf_req *, int);
+};
+
+<Hypervisor-specific Backend Structure>
+
+1. get_vm_id
+
+	Returns the VM (domain) ID
+
+	Input:
+
+		-ID of the current domain
+
+	Output:
+
+		None
+
+2. share_pages
+
+	Get pages shared via hypervisor-specific method and return one reference
+	ID that represents the complete list of shared pages
+
+	Input:
+
+		-Array of pages
+		-ID of importing VM
+		-Number of pages
+		-Hypervisor specific Representation of reference info of shared
+		 pages
+
+	Output:
+
+		-Hypervisor specific integer value that represents all of
+		 the shared pages
+
+3. unshare_pages
+
+	Stop sharing pages
+
+	Input:
+
+		-Hypervisor specific Representation of reference info of shared
+		 pages
+		-Number of shared pages
+
+	Output:
+
+		0
+
+4. map_shared_pages
+
+	Map shared pages locally using a hypervisor-specific method
+
+	Input:
+
+		-Reference number that represents all of shared pages
+		-ID of exporting VM, Number of pages
+		-Reference information for any purpose
+
+	Output:
+
+		-An array of shared pages (struct page**)
+
+5. unmap_shared_pages
+
+	Unmap shared pages
+
+	Input:
+
+		-Hypervisor specific Representation of reference info of shared pages
+
+	Output:
+
+		-0 (successful) or one of Standard Kernel errors
+
+6. init_comm_env
+
+	Setup infrastructure needed for communication channel
+
+	Input:
+
+		None
+
+	Output:
+
+		None
+
+7. destroy_comm
+
+	Cleanup everything done via init_comm_env
+
+	Input:
+
+		None
+
+	Output:
+
+		None
+
+8. init_rx_ch
+
+	Configure receive channel
+
+	Input:
+
+		-ID of VM on the other side of the channel
+
+	Output:
+
+		-0 (successful) or one of Standard Kernel errors
+
+9. init_tx_ch
+
+	Configure transmit channel
+
+	Input:
+
+		-ID of VM on the other side of the channel
+
+	Output:
+
+		-0 (success) or one of Standard Kernel errors
+
+10. send_req
+
+	Send message to other VM
+
+	Input:
+
+		-ID of VM that receives the message
+		-Message
+
+	Output:
+
+		-0 (success) or one of Standard Kernel errors
+
+-------------------------------------------------------------------------------
+-------------------------------------------------------------------------------
-- 
2.7.4