FreeBSD/Linux Kernel Cross Reference
sys/xen/interface/io/blkif.h

     /******************************************************************************
      * blkif.h
      *
      * Unified block-device I/O interface for Xen guest OSes.
      *
      * Permission is hereby granted, free of charge, to any person obtaining a copy
      * of this software and associated documentation files (the "Software"), to
      * deal in the Software without restriction, including without limitation the
      * rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
     * sell copies of the Software, and to permit persons to whom the Software is
     * furnished to do so, subject to the following conditions:
     *
     * The above copyright notice and this permission notice shall be included in
     * all copies or substantial portions of the Software.
     *
     * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
     * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
     * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
     * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
     * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
     * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
     * DEALINGS IN THE SOFTWARE.
     *
     * Copyright (c) 2003-2004, Keir Fraser
     * Copyright (c) 2012, Spectra Logic Corporation
     */
    
    #ifndef __XEN_PUBLIC_IO_BLKIF_H__
    #define __XEN_PUBLIC_IO_BLKIF_H__
    
    #include "ring.h"
    #include "../grant_table.h"
    
    /*
     * Front->back notifications: When enqueuing a new request, sending a
     * notification can be made conditional on req_event (i.e., the generic
     * hold-off mechanism provided by the ring macros). Backends must set
     * req_event appropriately (e.g., using RING_FINAL_CHECK_FOR_REQUESTS()).
     *
     * Back->front notifications: When enqueuing a new response, sending a
     * notification can be made conditional on rsp_event (i.e., the generic
     * hold-off mechanism provided by the ring macros). Frontends must set
     * rsp_event appropriately (e.g., using RING_FINAL_CHECK_FOR_RESPONSES()).
     */
    
    #ifndef blkif_vdev_t
    #define blkif_vdev_t   uint16_t
    #endif
    #define blkif_sector_t uint64_t
    
    /*
     * Feature and Parameter Negotiation
     * =================================
     * The two halves of a Xen block driver utilize nodes within the XenStore to
     * communicate capabilities and to negotiate operating parameters.  This
     * section enumerates these nodes which reside in the respective front and
     * backend portions of the XenStore, following the XenBus convention.
     *
     * All data in the XenStore is stored as strings.  Nodes specifying numeric
     * values are encoded in decimal.  Integer value ranges listed below are
     * expressed as fixed sized integer types capable of storing the conversion
     * of a properly formated node string, without loss of information.
     *
     * Any specified default value is in effect if the corresponding XenBus node
     * is not present in the XenStore.
     *
     * XenStore nodes in sections marked "PRIVATE" are solely for use by the
     * driver side whose XenBus tree contains them.
     *
     * XenStore nodes marked "DEPRECATED" in their notes section should only be
     * used to provide interoperability with legacy implementations.
     *
     * See the XenBus state transition diagram below for details on when XenBus
     * nodes must be published and when they can be queried.
     *
     *****************************************************************************
     *                            Backend XenBus Nodes
     *****************************************************************************
     *
     *------------------ Backend Device Identification (PRIVATE) ------------------
     *
     * mode
     *      Values:         "r" (read only), "w" (writable)
     *
     *      The read or write access permissions to the backing store to be
     *      granted to the frontend.
     *
     * params
     *      Values:         string
     *
     *      A free formatted string providing sufficient information for the
     *      hotplug script to attach the device and provide a suitable
     *      handler (ie: a block device) for blkback to use.
     *
     * physical-device
     *      Values:         "MAJOR:MINOR"
     *      Notes: 11
     *
     *      MAJOR and MINOR are the major number and minor number of the
    *      backing device respectively.
    *
    * physical-device-path
    *      Values:         path string
    *
    *      A string that contains the absolute path to the disk image. On
    *      NetBSD and Linux this is always a block device, while on FreeBSD
    *      it can be either a block device or a regular file.
    *
    * type
    *      Values:         "file", "phy", "tap"
    *
    *      The type of the backing device/object.
    *
    *
    * direct-io-safe
    *      Values:         0/1 (boolean)
    *      Default Value:  0
    *
    *      The underlying storage is not affected by the direct IO memory
    *      lifetime bug.  See:
    *        https://lists.xen.org/archives/html/xen-devel/2012-12/msg01154.html
    *
    *      Therefore this option gives the backend permission to use
    *      O_DIRECT, notwithstanding that bug.
    *
    *      That is, if this option is enabled, use of O_DIRECT is safe,
    *      in circumstances where we would normally have avoided it as a
    *      workaround for that bug.  This option is not relevant for all
    *      backends, and even not necessarily supported for those for
    *      which it is relevant.  A backend which knows that it is not
    *      affected by the bug can ignore this option.
    *
    *      This option doesn't require a backend to use O_DIRECT, so it
    *      should not be used to try to control the caching behaviour.
    *
    *--------------------------------- Features ---------------------------------
    *
    * feature-barrier
    *      Values:         0/1 (boolean)
    *      Default Value:  0
    *
    *      A value of "1" indicates that the backend can process requests
    *      containing the BLKIF_OP_WRITE_BARRIER request opcode.  Requests
    *      of this type may still be returned at any time with the
    *      BLKIF_RSP_EOPNOTSUPP result code.
    *
    * feature-flush-cache
    *      Values:         0/1 (boolean)
    *      Default Value:  0
    *
    *      A value of "1" indicates that the backend can process requests
    *      containing the BLKIF_OP_FLUSH_DISKCACHE request opcode.  Requests
    *      of this type may still be returned at any time with the
    *      BLKIF_RSP_EOPNOTSUPP result code.
    *
    * feature-discard
    *      Values:         0/1 (boolean)
    *      Default Value:  0
    *
    *      A value of "1" indicates that the backend can process requests
    *      containing the BLKIF_OP_DISCARD request opcode.  Requests
    *      of this type may still be returned at any time with the
    *      BLKIF_RSP_EOPNOTSUPP result code.
    *
    * feature-persistent
    *      Values:         0/1 (boolean)
    *      Default Value:  0
    *      Notes: 7
    *
    *      A value of "1" indicates that the backend can keep the grants used
    *      by the frontend driver mapped, so the same set of grants should be
    *      used in all transactions. The maximum number of grants the backend
    *      can map persistently depends on the implementation, but ideally it
    *      should be RING_SIZE * BLKIF_MAX_SEGMENTS_PER_REQUEST. Using this
    *      feature the backend doesn't need to unmap each grant, preventing
    *      costly TLB flushes. The backend driver should only map grants
    *      persistently if the frontend supports it. If a backend driver chooses
    *      to use the persistent protocol when the frontend doesn't support it,
    *      it will probably hit the maximum number of persistently mapped grants
    *      (due to the fact that the frontend won't be reusing the same grants),
    *      and fall back to non-persistent mode. Backend implementations may
    *      shrink or expand the number of persistently mapped grants without
    *      notifying the frontend depending on memory constraints (this might
    *      cause a performance degradation).
    *
    *      If a backend driver wants to limit the maximum number of persistently
    *      mapped grants to a value less than RING_SIZE *
    *      BLKIF_MAX_SEGMENTS_PER_REQUEST a LRU strategy should be used to
    *      discard the grants that are less commonly used. Using a LRU in the
    *      backend driver paired with a LIFO queue in the frontend will
    *      allow us to have better performance in this scenario.
    *
    *----------------------- Request Transport Parameters ------------------------
    *
    * max-ring-page-order
    *      Values:         <uint32_t>
    *      Default Value:  0
    *      Notes:          1, 3
    *
    *      The maximum supported size of the request ring buffer in units of
    *      lb(machine pages). (e.g. 0 == 1 page,  1 = 2 pages, 2 == 4 pages,
    *      etc.).
    *
    * max-ring-pages
    *      Values:         <uint32_t>
    *      Default Value:  1
    *      Notes:          DEPRECATED, 2, 3
    *
    *      The maximum supported size of the request ring buffer in units of
    *      machine pages.  The value must be a power of 2.
    *
    *------------------------- Backend Device Properties -------------------------
    *
    * discard-enable
    *      Values:         0/1 (boolean)
    *      Default Value:  1
    *
    *      This optional property, set by the toolstack, instructs the backend
    *      to offer (or not to offer) discard to the frontend. If the property
    *      is missing the backend should offer discard if the backing storage
    *      actually supports it.
    *
    * discard-alignment
    *      Values:         <uint32_t>
    *      Default Value:  0
    *      Notes:          4, 5
    *
    *      The offset, in bytes from the beginning of the virtual block device,
    *      to the first, addressable, discard extent on the underlying device.
    *
    * discard-granularity
    *      Values:         <uint32_t>
    *      Default Value:  <"sector-size">
    *      Notes:          4
    *
    *      The size, in bytes, of the individually addressable discard extents
    *      of the underlying device.
    *
    * discard-secure
    *      Values:         0/1 (boolean)
    *      Default Value:  0
    *      Notes:          10
    *
    *      A value of "1" indicates that the backend can process BLKIF_OP_DISCARD
    *      requests with the BLKIF_DISCARD_SECURE flag set.
    *
    * info
    *      Values:         <uint32_t> (bitmap)
    *
    *      A collection of bit flags describing attributes of the backing
    *      device.  The VDISK_* macros define the meaning of each bit
    *      location.
    *
    * sector-size
    *      Values:         <uint32_t>
    *
    *      The logical block size, in bytes, of the underlying storage. This
    *      must be a power of two with a minimum value of 512.
    *
    *      NOTE: Because of implementation bugs in some frontends this must be
    *            set to 512, unless the frontend advertizes a non-zero value
    *            in its "feature-large-sector-size" xenbus node. (See below).
    *
    * physical-sector-size
    *      Values:         <uint32_t>
    *      Default Value:  <"sector-size">
    *
    *      The physical block size, in bytes, of the backend storage. This
    *      must be an integer multiple of "sector-size".
    *
    * sectors
    *      Values:         <uint64_t>
    *
    *      The size of the backend device, expressed in units of "sector-size".
    *      The product of "sector-size" and "sectors" must also be an integer
    *      multiple of "physical-sector-size", if that node is present.
    *
    *****************************************************************************
    *                            Frontend XenBus Nodes
    *****************************************************************************
    *
    *----------------------- Request Transport Parameters -----------------------
    *
    * event-channel
    *      Values:         <uint32_t>
    *
    *      The identifier of the Xen event channel used to signal activity
    *      in the ring buffer.
    *
    * ring-ref
    *      Values:         <uint32_t>
    *      Notes:          6
    *
    *      The Xen grant reference granting permission for the backend to map
    *      the sole page in a single page sized ring buffer.
    *
    * ring-ref%u
    *      Values:         <uint32_t>
    *      Notes:          6
    *
    *      For a frontend providing a multi-page ring, a "number of ring pages"
    *      sized list of nodes, each containing a Xen grant reference granting
    *      permission for the backend to map the page of the ring located
    *      at page index "%u".  Page indexes are zero based.
    *
    * protocol
    *      Values:         string (XEN_IO_PROTO_ABI_*)
    *      Default Value:  XEN_IO_PROTO_ABI_NATIVE
    *
    *      The machine ABI rules governing the format of all ring request and
    *      response structures.
    *
    * ring-page-order
    *      Values:         <uint32_t>
    *      Default Value:  0
    *      Maximum Value:  MAX(ffs(max-ring-pages) - 1, max-ring-page-order)
    *      Notes:          1, 3
    *
    *      The size of the frontend allocated request ring buffer in units
    *      of lb(machine pages). (e.g. 0 == 1 page, 1 = 2 pages, 2 == 4 pages,
    *      etc.).
    *
    * num-ring-pages
    *      Values:         <uint32_t>
    *      Default Value:  1
    *      Maximum Value:  MAX(max-ring-pages,(0x1 << max-ring-page-order))
    *      Notes:          DEPRECATED, 2, 3
    *
    *      The size of the frontend allocated request ring buffer in units of
    *      machine pages.  The value must be a power of 2.
    *
    *--------------------------------- Features ---------------------------------
    *
    * feature-persistent
    *      Values:         0/1 (boolean)
    *      Default Value:  0
    *      Notes: 7, 8, 9
    *
    *      A value of "1" indicates that the frontend will reuse the same grants
    *      for all transactions, allowing the backend to map them with write
    *      access (even when it should be read-only). If the frontend hits the
    *      maximum number of allowed persistently mapped grants, it can fallback
    *      to non persistent mode. This will cause a performance degradation,
    *      since the the backend driver will still try to map those grants
    *      persistently. Since the persistent grants protocol is compatible with
    *      the previous protocol, a frontend driver can choose to work in
    *      persistent mode even when the backend doesn't support it.
    *
    *      It is recommended that the frontend driver stores the persistently
    *      mapped grants in a LIFO queue, so a subset of all persistently mapped
    *      grants gets used commonly. This is done in case the backend driver
    *      decides to limit the maximum number of persistently mapped grants
    *      to a value less than RING_SIZE * BLKIF_MAX_SEGMENTS_PER_REQUEST.
    *
    * feature-large-sector-size
    *      Values:         0/1 (boolean)
    *      Default Value:  0
    *
    *      A value of "1" indicates that the frontend will correctly supply and
    *      interpret all sector-based quantities in terms of the "sector-size"
    *      value supplied in the backend info, whatever that may be set to.
    *      If this node is not present or its value is "" then it is assumed
    *      that the frontend requires that the logical block size is 512 as it
    *      is hardcoded (which is the case in some frontend implementations).
    *
    *------------------------- Virtual Device Properties -------------------------
    *
    * device-type
    *      Values:         "disk", "cdrom", "floppy", etc.
    *
    * virtual-device
    *      Values:         <uint32_t>
    *
    *      A value indicating the physical device to virtualize within the
    *      frontend's domain.  (e.g. "The first ATA disk", "The third SCSI
    *      disk", etc.)
    *
    *      See docs/misc/vbd-interface.txt for details on the format of this
    *      value.
    *
    * Notes
    * -----
    * (1) Multi-page ring buffer scheme first developed in the Citrix XenServer
    *     PV drivers.
    * (2) Multi-page ring buffer scheme first used in some RedHat distributions
    *     including a distribution deployed on certain nodes of the Amazon
    *     EC2 cluster.
    * (3) Support for multi-page ring buffers was implemented independently,
    *     in slightly different forms, by both Citrix and RedHat/Amazon.
    *     For full interoperability, block front and backends should publish
    *     identical ring parameters, adjusted for unit differences, to the
    *     XenStore nodes used in both schemes.
    * (4) Devices that support discard functionality may internally allocate space
    *     (discardable extents) in units that are larger than the exported logical
    *     block size. If the backing device has such discardable extents the
    *     backend should provide both discard-granularity and discard-alignment.
    *     Providing just one of the two may be considered an error by the frontend.
    *     Backends supporting discard should include discard-granularity and
    *     discard-alignment even if it supports discarding individual sectors.
    *     Frontends should assume discard-alignment == 0 and discard-granularity
    *     == sector size if these keys are missing.
    * (5) The discard-alignment parameter allows a physical device to be
    *     partitioned into virtual devices that do not necessarily begin or
    *     end on a discardable extent boundary.
    * (6) When there is only a single page allocated to the request ring,
    *     'ring-ref' is used to communicate the grant reference for this
    *     page to the backend.  When using a multi-page ring, the 'ring-ref'
    *     node is not created.  Instead 'ring-ref0' - 'ring-refN' are used.
    * (7) When using persistent grants data has to be copied from/to the page
    *     where the grant is currently mapped. The overhead of doing this copy
    *     however doesn't suppress the speed improvement of not having to unmap
    *     the grants.
    * (8) The frontend driver has to allow the backend driver to map all grants
    *     with write access, even when they should be mapped read-only, since
    *     further requests may reuse these grants and require write permissions.
    * (9) Linux implementation doesn't have a limit on the maximum number of
    *     grants that can be persistently mapped in the frontend driver, but
    *     due to the frontent driver implementation it should never be bigger
    *     than RING_SIZE * BLKIF_MAX_SEGMENTS_PER_REQUEST.
    *(10) The discard-secure property may be present and will be set to 1 if the
    *     backing device supports secure discard.
    *(11) Only used by Linux and NetBSD.
    */
   
   /*
    * Multiple hardware queues/rings:
    * If supported, the backend will write the key "multi-queue-max-queues" to
    * the directory for that vbd, and set its value to the maximum supported
    * number of queues.
    * Frontends that are aware of this feature and wish to use it can write the
    * key "multi-queue-num-queues" with the number they wish to use, which must be
    * greater than zero, and no more than the value reported by the backend in
    * "multi-queue-max-queues".
    *
    * For frontends requesting just one queue, the usual event-channel and
    * ring-ref keys are written as before, simplifying the backend processing
    * to avoid distinguishing between a frontend that doesn't understand the
    * multi-queue feature, and one that does, but requested only one queue.
    *
    * Frontends requesting two or more queues must not write the toplevel
    * event-channel and ring-ref keys, instead writing those keys under sub-keys
    * having the name "queue-N" where N is the integer ID of the queue/ring for
    * which those keys belong. Queues are indexed from zero.
    * For example, a frontend with two queues must write the following set of
    * queue-related keys:
    *
    * /local/domain/1/device/vbd/0/multi-queue-num-queues = "2"
    * /local/domain/1/device/vbd/0/queue-0 = ""
    * /local/domain/1/device/vbd/0/queue-0/ring-ref = "<ring-ref#0>"
    * /local/domain/1/device/vbd/0/queue-0/event-channel = "<evtchn#0>"
    * /local/domain/1/device/vbd/0/queue-1 = ""
    * /local/domain/1/device/vbd/0/queue-1/ring-ref = "<ring-ref#1>"
    * /local/domain/1/device/vbd/0/queue-1/event-channel = "<evtchn#1>"
    *
    * It is also possible to use multiple queues/rings together with
    * feature multi-page ring buffer.
    * For example, a frontend requests two queues/rings and the size of each ring
    * buffer is two pages must write the following set of related keys:
    *
    * /local/domain/1/device/vbd/0/multi-queue-num-queues = "2"
    * /local/domain/1/device/vbd/0/ring-page-order = "1"
    * /local/domain/1/device/vbd/0/queue-0 = ""
    * /local/domain/1/device/vbd/0/queue-0/ring-ref0 = "<ring-ref#0>"
    * /local/domain/1/device/vbd/0/queue-0/ring-ref1 = "<ring-ref#1>"
    * /local/domain/1/device/vbd/0/queue-0/event-channel = "<evtchn#0>"
    * /local/domain/1/device/vbd/0/queue-1 = ""
    * /local/domain/1/device/vbd/0/queue-1/ring-ref0 = "<ring-ref#2>"
    * /local/domain/1/device/vbd/0/queue-1/ring-ref1 = "<ring-ref#3>"
    * /local/domain/1/device/vbd/0/queue-1/event-channel = "<evtchn#1>"
    *
    */
   
   /*
    * STATE DIAGRAMS
    *
    *****************************************************************************
    *                                   Startup                                 *
    *****************************************************************************
    *
    * Tool stack creates front and back nodes with state XenbusStateInitialising.
    *
    * Front                                Back
    * =================================    =====================================
    * XenbusStateInitialising              XenbusStateInitialising
    *  o Query virtual device               o Query backend device identification
    *    properties.                          data.
    *  o Setup OS device instance.          o Open and validate backend device.
    *                                       o Publish backend features and
    *                                         transport parameters.
    *                                                      |
    *                                                      |
    *                                                      V
    *                                      XenbusStateInitWait
    *
    * o Query backend features and
    *   transport parameters.
    * o Allocate and initialize the
    *   request ring.
    * o Publish transport parameters
    *   that will be in effect during
    *   this connection.
    *              |
    *              |
    *              V
    * XenbusStateInitialised
    *
    *                                       o Query frontend transport parameters.
    *                                       o Connect to the request ring and
    *                                         event channel.
    *                                       o Publish backend device properties.
    *                                                      |
    *                                                      |
    *                                                      V
    *                                      XenbusStateConnected
    *
    *  o Query backend device properties.
    *  o Finalize OS virtual device
    *    instance.
    *              |
    *              |
    *              V
    * XenbusStateConnected
    *
    * Note: Drivers that do not support any optional features, or the negotiation
    *       of transport parameters, can skip certain states in the state machine:
    *
    *       o A frontend may transition to XenbusStateInitialised without
    *         waiting for the backend to enter XenbusStateInitWait.  In this
    *         case, default transport parameters are in effect and any
    *         transport parameters published by the frontend must contain
    *         their default values.
    *
    *       o A backend may transition to XenbusStateInitialised, bypassing
    *         XenbusStateInitWait, without waiting for the frontend to first
    *         enter the XenbusStateInitialised state.  In this case, default
    *         transport parameters are in effect and any transport parameters
    *         published by the backend must contain their default values.
    *
    *       Drivers that support optional features and/or transport parameter
    *       negotiation must tolerate these additional state transition paths.
    *       In general this means performing the work of any skipped state
    *       transition, if it has not already been performed, in addition to the
    *       work associated with entry into the current state.
    */
   
   /*
    * REQUEST CODES.
    */
   #define BLKIF_OP_READ              0
   #define BLKIF_OP_WRITE             1
   /*
    * All writes issued prior to a request with the BLKIF_OP_WRITE_BARRIER
    * operation code ("barrier request") must be completed prior to the
    * execution of the barrier request.  All writes issued after the barrier
    * request must not execute until after the completion of the barrier request.
    *
    * Optional.  See "feature-barrier" XenBus node documentation above.
    */
   #define BLKIF_OP_WRITE_BARRIER     2
   /*
    * Commit any uncommitted contents of the backing device's volatile cache
    * to stable storage.
    *
    * Optional.  See "feature-flush-cache" XenBus node documentation above.
    */
   #define BLKIF_OP_FLUSH_DISKCACHE   3
   /*
    * Used in SLES sources for device specific command packet
    * contained within the request. Reserved for that purpose.
    */
   #define BLKIF_OP_RESERVED_1        4
   /*
    * Indicate to the backend device that a region of storage is no longer in
    * use, and may be discarded at any time without impact to the client.  If
    * the BLKIF_DISCARD_SECURE flag is set on the request, all copies of the
    * discarded region on the device must be rendered unrecoverable before the
    * command returns.
    *
    * This operation is analogous to performing a trim (ATA) or unamp (SCSI),
    * command on a native device.
    *
    * More information about trim/unmap operations can be found at:
    * http://t13.org/Documents/UploadedDocuments/docs2008/
    *     e07154r6-Data_Set_Management_Proposal_for_ATA-ACS2.doc
    * http://www.seagate.com/staticfiles/support/disc/manuals/
    *     Interface%20manuals/100293068c.pdf
    *
    * Optional.  See "feature-discard", "discard-alignment",
    * "discard-granularity", and "discard-secure" in the XenBus node
    * documentation above.
    */
   #define BLKIF_OP_DISCARD           5
   
   /*
    * Recognized if "feature-max-indirect-segments" in present in the backend
    * xenbus info. The "feature-max-indirect-segments" node contains the maximum
    * number of segments allowed by the backend per request. If the node is
    * present, the frontend might use blkif_request_indirect structs in order to
    * issue requests with more than BLKIF_MAX_SEGMENTS_PER_REQUEST (11). The
    * maximum number of indirect segments is fixed by the backend, but the
    * frontend can issue requests with any number of indirect segments as long as
    * it's less than the number provided by the backend. The indirect_grefs field
    * in blkif_request_indirect should be filled by the frontend with the
    * grant references of the pages that are holding the indirect segments.
    * These pages are filled with an array of blkif_request_segment that hold the
    * information about the segments. The number of indirect pages to use is
    * determined by the number of segments an indirect request contains. Every
    * indirect page can contain a maximum of
    * (PAGE_SIZE / sizeof(struct blkif_request_segment)) segments, so to
    * calculate the number of indirect pages to use we have to do
    * ceil(indirect_segments / (PAGE_SIZE / sizeof(struct blkif_request_segment))).
    *
    * If a backend does not recognize BLKIF_OP_INDIRECT, it should *not*
    * create the "feature-max-indirect-segments" node!
    */
   #define BLKIF_OP_INDIRECT          6
   
   /*
    * Maximum scatter/gather segments per request.
    * This is carefully chosen so that sizeof(blkif_ring_t) <= PAGE_SIZE.
    * NB. This could be 12 if the ring indexes weren't stored in the same page.
    */
   #define BLKIF_MAX_SEGMENTS_PER_REQUEST 11
   
   /*
    * Maximum number of indirect pages to use per request.
    */
   #define BLKIF_MAX_INDIRECT_PAGES_PER_REQUEST 8
   
   /*
    * NB. 'first_sect' and 'last_sect' in blkif_request_segment, as well as
    * 'sector_number' in blkif_request, blkif_request_discard and
    * blkif_request_indirect are sector-based quantities. See the description
    * of the "feature-large-sector-size" frontend xenbus node above for
    * more information.
    */
   struct blkif_request_segment {
       grant_ref_t gref;        /* reference to I/O buffer frame        */
       /* @first_sect: first sector in frame to transfer (inclusive).   */
       /* @last_sect: last sector in frame to transfer (inclusive).     */
       uint8_t     first_sect, last_sect;
   };
   
   /*
    * Starting ring element for any I/O request.
    */
   struct blkif_request {
       uint8_t        operation;    /* BLKIF_OP_???                         */
       uint8_t        nr_segments;  /* number of segments                   */
       blkif_vdev_t   handle;       /* only for read/write requests         */
       uint64_t       id;           /* private guest value, echoed in resp  */
       blkif_sector_t sector_number;/* start sector idx on disk (r/w only)  */
       struct blkif_request_segment seg[BLKIF_MAX_SEGMENTS_PER_REQUEST];
   };
   typedef struct blkif_request blkif_request_t;
   
   /*
    * Cast to this structure when blkif_request.operation == BLKIF_OP_DISCARD
    * sizeof(struct blkif_request_discard) <= sizeof(struct blkif_request)
    */
   struct blkif_request_discard {
       uint8_t        operation;    /* BLKIF_OP_DISCARD                     */
       uint8_t        flag;         /* BLKIF_DISCARD_SECURE or zero         */
   #define BLKIF_DISCARD_SECURE (1<<0)  /* ignored if discard-secure=0      */
       blkif_vdev_t   handle;       /* same as for read/write requests      */
       uint64_t       id;           /* private guest value, echoed in resp  */
       blkif_sector_t sector_number;/* start sector idx on disk             */
       uint64_t       nr_sectors;   /* number of contiguous sectors to discard*/
   };
   typedef struct blkif_request_discard blkif_request_discard_t;
   
   struct blkif_request_indirect {
       uint8_t        operation;    /* BLKIF_OP_INDIRECT                    */
       uint8_t        indirect_op;  /* BLKIF_OP_{READ/WRITE}                */
       uint16_t       nr_segments;  /* number of segments                   */
       uint64_t       id;           /* private guest value, echoed in resp  */
       blkif_sector_t sector_number;/* start sector idx on disk (r/w only)  */
       blkif_vdev_t   handle;       /* same as for read/write requests      */
       grant_ref_t    indirect_grefs[BLKIF_MAX_INDIRECT_PAGES_PER_REQUEST];
   #ifdef __i386__
       uint64_t       pad;          /* Make it 64 byte aligned on i386      */
   #endif
   };
   typedef struct blkif_request_indirect blkif_request_indirect_t;
   
   struct blkif_response {
       uint64_t        id;              /* copied from request */
       uint8_t         operation;       /* copied from request */
       int16_t         status;          /* BLKIF_RSP_???       */
   };
   typedef struct blkif_response blkif_response_t;
   
   /*
    * STATUS RETURN CODES.
    */
    /* Operation not supported (only happens on barrier writes). */
   #define BLKIF_RSP_EOPNOTSUPP  -2
    /* Operation failed for some unspecified reason (-EIO). */
   #define BLKIF_RSP_ERROR       -1
    /* Operation completed successfully. */
   #define BLKIF_RSP_OKAY         0
   
   /*
    * Generate blkif ring structures and types.
    */
   DEFINE_RING_TYPES(blkif, struct blkif_request, struct blkif_response);
   
   #define VDISK_CDROM        0x1
   #define VDISK_REMOVABLE    0x2
   #define VDISK_READONLY     0x4
   
   #endif /* __XEN_PUBLIC_IO_BLKIF_H__ */
   
   /*
    * Local variables:
    * mode: C
    * c-file-style: "BSD"
    * c-basic-offset: 4
    * tab-width: 4
    * indent-tabs-mode: nil
    * End:
    */

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