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[Xen-devel] [PATCH v4 18/19] OvmfPkg/XenPvBlkDxe: Add BlockFront client.



This is the code that will do the actual communication between OVMF and
a PV block backend, where the block device lives. The protocol used is
describe in the blkif.h header.

This implementation originally comes from Mini-OS, a part of the Xen
Project.

License: This patch adds some files which are under the MIT license.

Signed-off-by: Samuel Thibault <samuel.thibault@xxxxxxxxxxxxx>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Anthony PERARD <anthony.perard@xxxxxxxxxx>

---
Change in V4:
- add file header to BlockFront.h (license, copyright, brief desc)

Change in V3:
- Improve comment of XenBusReadUint64.
- Moving blkif.h to this patch
  with the necessary #pragma pack(4) applied for Ia32.
- Add a note about the license in the commit message
- Add "The protocol used is describe in the blkif.h header." in the
  commit message
- Have a mandatory sector-size multiple of 512 or fail to initialize.
- use Sector instead of Offset for IO request.
  with Sector been 512-byte unit.
- print something if EventChannelNotify return an error.

Change in V2:
- trigger CoW is probably not needed on OVMF (as opposed to Mini-OS),
  removed the test.
- comments
- renamed XenbusReadInteger to XenBusReadUint64
- remove callback from IoData, use simple status instead
- return a status from the synchronus io
- Close protocol if blockfront init fail.
- fix few debug print
- Rename XenbusIo to XenBusIo
- XenPvBlkWaitForBackendState will return an error if the new backend
  states is not the expected state.
- Add the license
---
 OvmfPkg/Include/IndustryStandard/Xen/io/blkif.h | 619 +++++++++++++++++++++++
 OvmfPkg/XenPvBlkDxe/BlockFront.c                | 647 ++++++++++++++++++++++++
 OvmfPkg/XenPvBlkDxe/BlockFront.h                | 101 ++++
 OvmfPkg/XenPvBlkDxe/XenPvBlkDxe.c               |  13 +
 OvmfPkg/XenPvBlkDxe/XenPvBlkDxe.inf             |   2 +
 5 files changed, 1382 insertions(+)
 create mode 100644 OvmfPkg/Include/IndustryStandard/Xen/io/blkif.h
 create mode 100644 OvmfPkg/XenPvBlkDxe/BlockFront.c
 create mode 100644 OvmfPkg/XenPvBlkDxe/BlockFront.h

diff --git a/OvmfPkg/Include/IndustryStandard/Xen/io/blkif.h 
b/OvmfPkg/Include/IndustryStandard/Xen/io/blkif.h
new file mode 100644
index 0000000..2cfc1ab
--- /dev/null
+++ b/OvmfPkg/Include/IndustryStandard/Xen/io/blkif.h
@@ -0,0 +1,619 @@
+/******************************************************************************
+ * 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
+#endif
+#define blkif_sector_t UINT64
+
+/*
+ * 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
+ *      backend driver to open the backing device.  (e.g. the path to the
+ *      file or block device representing the backing store.)
+ *
+ * type
+ *      Values:         "file", "phy", "tap"
+ *
+ *      The type of the backing device/object.
+ *
+ *--------------------------------- 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>
+ *      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>
+ *      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-alignment
+ *      Values:         <UINT32>
+ *      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>
+ *      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> (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>
+ *
+ *      The logical sector size, in bytes, of the backend device.
+ *
+ * physical-sector-size
+ *      Values:         <UINT32>
+ *
+ *      The physical sector size, in bytes, of the backend device.
+ *
+ * sectors
+ *      Values:         <UINT64>
+ *
+ *      The size of the backend device, expressed in units of its logical
+ *      sector size ("sector-size").
+ *
+ *****************************************************************************
+ *                            Frontend XenBus Nodes
+ *****************************************************************************
+ *
+ *----------------------- Request Transport Parameters -----------------------
+ *
+ * event-channel
+ *      Values:         <UINT32>
+ *
+ *      The identifier of the Xen event channel used to signal activity
+ *      in the ring buffer.
+ *
+ * ring-ref
+ *      Values:         <UINT32>
+ *      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>
+ *      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>
+ *      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>
+ *      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.
+ *
+ * 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.
+ *
+ *------------------------- Virtual Device Properties -------------------------
+ *
+ * device-type
+ *      Values:         "disk", "cdrom", "floppy", etc.
+ *
+ * virtual-device
+ *      Values:         <UINT32>
+ *
+ *      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.
+ */
+
+/*
+ * 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, are always expressed in 512-byte units.
+ * However they must be properly aligned to the real sector size of the
+ * physical disk, which is reported in the "physical-sector-size" node in
+ * the backend xenbus info. Also the xenbus "sectors" node is expressed in
+ * 512-byte units.
+ */
+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     first_sect, last_sect;
+};
+
+/*
+ * Starting ring element for any I/O request.
+ */
+#if defined(__i386__)
+//
+// pack(4) is necessary when these structs are compiled for Ia32.
+// Without it, the struct will have a different alignment than the one
+// a backend expect for a 32bit guest.
+//
+#pragma pack(4)
+#endif
+struct blkif_request {
+    UINT8        operation;    /* BLKIF_OP_???                         */
+    UINT8        nr_segments;  /* number of segments                   */
+    blkif_vdev_t   handle;       /* only for read/write requests         */
+    UINT64       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        operation;    /* BLKIF_OP_DISCARD                     */
+    UINT8        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       id;           /* private guest value, echoed in resp  */
+    blkif_sector_t sector_number;/* start sector idx on disk             */
+    UINT64       nr_sectors;   /* number of contiguous sectors to discard*/
+};
+typedef struct blkif_request_discard blkif_request_discard_t;
+
+struct blkif_request_indirect {
+    UINT8        operation;    /* BLKIF_OP_INDIRECT                    */
+    UINT8        indirect_op;  /* BLKIF_OP_{READ/WRITE}                */
+    UINT16       nr_segments;  /* number of segments                   */
+    UINT64       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       pad;          /* Make it 64 byte aligned on i386      */
+#endif
+};
+typedef struct blkif_request_indirect blkif_request_indirect_t;
+
+struct blkif_response {
+    UINT64        id;              /* copied from request */
+    UINT8         operation;       /* copied from request */
+    INT16         status;          /* BLKIF_RSP_???       */
+};
+typedef struct blkif_response blkif_response_t;
+#if defined(__i386__)
+#pragma pack()
+#endif
+
+/*
+ * 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:
+ */
diff --git a/OvmfPkg/XenPvBlkDxe/BlockFront.c b/OvmfPkg/XenPvBlkDxe/BlockFront.c
new file mode 100644
index 0000000..a3e1555
--- /dev/null
+++ b/OvmfPkg/XenPvBlkDxe/BlockFront.c
@@ -0,0 +1,647 @@
+/** @file
+  Minimal block driver for Mini-OS.
+
+  Copyright (c) 2007-2008 Samuel Thibault.
+  Copyright (C) 2014, Citrix Ltd.
+
+  Redistribution and use in source and binary forms, with or without
+  modification, are permitted provided that the following conditions
+  are met:
+  1. Redistributions of source code must retain the above copyright
+     notice, this list of conditions and the following disclaimer.
+  2. Redistributions in binary form must reproduce the above copyright
+     notice, this list of conditions and the following disclaimer in the
+     documentation and/or other materials provided with the distribution.
+
+  THIS SOFTWARE IS PROVIDED BY AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+  ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+  IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+  ARE DISCLAIMED.  IN NO EVENT SHALL AUTHOR OR CONTRIBUTORS BE LIABLE
+  FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+  DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+  OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+  HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+  LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+  OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+  SUCH DAMAGE.
+**/
+
+#include <Library/PrintLib.h>
+#include <Library/DebugLib.h>
+
+#include "BlockFront.h"
+
+#include <IndustryStandard/Xen/io/protocols.h>
+#include <IndustryStandard/Xen/io/xenbus.h>
+
+//
+// Header used for UINT32_MAX and UINT16_MAX
+//
+#include "inttypes.h"
+
+/**
+  Helper to read an integer from XenStore.
+
+  If the number overflows according to the range defined by UINT64,
+  then ASSERT().
+
+  @param This         A pointer to a XENBUS_PROTOCOL instance.
+  @param Node         The XenStore node to read from.
+  @param FromBackend  Read frontend or backend value.
+  @param ValuePtr     Where to put the value.
+
+  @retval XENSTORE_STATUS_SUCCESS  If succefull, will update ValuePtr.
+  @return                          Any other return value indicate the error,
+                                   ValuePtr is not updated in this case.
+**/
+STATIC
+XENSTORE_STATUS
+XenBusReadUint64 (
+  IN  XENBUS_PROTOCOL *This,
+  IN  CONST CHAR8     *Node,
+  IN  BOOLEAN         FromBackend,
+  OUT UINT64          *ValuePtr
+  )
+{
+  XENSTORE_STATUS Status;
+  CHAR8 *Ptr;
+
+  if (!FromBackend) {
+    Status = This->XsRead (This, XST_NIL, Node, (VOID**)&Ptr);
+  } else {
+    Status = This->XsBackendRead (This, XST_NIL, Node, (VOID**)&Ptr);
+  }
+  if (Status != XENSTORE_STATUS_SUCCESS) {
+    return Status;
+  }
+  // AsciiStrDecimalToUint64 will ASSERT if Ptr overflow UINT64.
+  *ValuePtr = AsciiStrDecimalToUint64 (Ptr);
+  FreePool (Ptr);
+  return Status;
+}
+
+/**
+  Free an instance of XEN_BLOCK_FRONT_DEVICE.
+
+  @param Dev  The instance to free.
+**/
+STATIC
+VOID
+XenPvBlockFree (
+  IN XEN_BLOCK_FRONT_DEVICE *Dev
+  )
+{
+  XENBUS_PROTOCOL *XenBusIo = Dev->XenBusIo;
+
+  if (Dev->RingRef != 0) {
+    XenBusIo->GrantEndAccess (XenBusIo, Dev->RingRef);
+  }
+  if (Dev->Ring.sring != NULL) {
+    FreePages (Dev->Ring.sring, 1);
+  }
+  if (Dev->EventChannel != 0) {
+    XenBusIo->EventChannelClose (XenBusIo, Dev->EventChannel);
+  }
+  FreePool (Dev);
+}
+
+/**
+  Wait until until the backend has reached the ExpectedState.
+
+  @param Dev            A XEN_BLOCK_FRONT_DEVICE instance.
+  @param ExpectedState  The backend state expected.
+  @param LastStatePtr   An optional pointer where to right the final state.
+
+  @return Return XENSTORE_STATUS_SUCCESS if the new backend state is 
ExpectedState
+          or return an error otherwise.
+**/
+STATIC
+XENSTORE_STATUS
+XenPvBlkWaitForBackendState (
+  IN  XEN_BLOCK_FRONT_DEVICE *Dev,
+  IN  XenbusState            ExpectedState,
+  OUT XenbusState            *LastStatePtr OPTIONAL
+  )
+{
+  XENBUS_PROTOCOL *XenBusIo = Dev->XenBusIo;
+  XenbusState State;
+  UINT64 Value;
+  XENSTORE_STATUS Status = XENSTORE_STATUS_SUCCESS;
+
+  while (TRUE) {
+    Status = XenBusReadUint64 (XenBusIo, "state", TRUE, &Value);
+    if (Status != XENSTORE_STATUS_SUCCESS) {
+      return Status;
+    }
+    if (Value > XenbusStateReconfigured) {
+      //
+      // Value is not a State value.
+      //
+      return XENSTORE_STATUS_EIO;
+    }
+    State = Value;
+    if (State == ExpectedState) {
+      break;
+    } else if (State > ExpectedState) {
+      Status = XENSTORE_STATUS_FAIL;
+      break;
+    }
+    DEBUG ((EFI_D_INFO,
+            "XenPvBlk: waiting backend state %d, current: %d\n",
+            ExpectedState, State));
+    XenBusIo->WaitForWatch (XenBusIo, Dev->StateWatchToken);
+  }
+
+  if (LastStatePtr != NULL) {
+    *LastStatePtr = State;
+  }
+
+  return Status;
+}
+
+EFI_STATUS
+XenPvBlockFrontInitialization (
+  IN  XENBUS_PROTOCOL         *XenBusIo,
+  IN  CONST CHAR8             *NodeName,
+  OUT XEN_BLOCK_FRONT_DEVICE  **DevPtr
+  )
+{
+  XENSTORE_TRANSACTION xbt;
+  CHAR8 *DeviceType;
+  blkif_sring_t *SharedRing;
+  XENSTORE_STATUS Status;
+  XEN_BLOCK_FRONT_DEVICE *Dev;
+  XenbusState State;
+  UINT64 Value;
+
+  ASSERT (NodeName != NULL);
+
+  Dev = AllocateZeroPool (sizeof (XEN_BLOCK_FRONT_DEVICE));
+  Dev->Signature = XEN_BLOCK_FRONT_SIGNATURE;
+  Dev->NodeName = NodeName;
+  Dev->XenBusIo = XenBusIo;
+  Dev->DeviceId = XenBusIo->DeviceId;
+
+  XenBusIo->XsRead (XenBusIo, XST_NIL, "device-type", (VOID**)&DeviceType);
+  if (AsciiStrCmp (DeviceType, "cdrom") == 0) {
+    Dev->MediaInfo.CdRom = TRUE;
+  } else {
+    Dev->MediaInfo.CdRom = FALSE;
+  }
+  FreePool (DeviceType);
+
+  Status = XenBusReadUint64 (XenBusIo, "backend-id", FALSE, &Value);
+  if (Status != XENSTORE_STATUS_SUCCESS || Value > UINT16_MAX) {
+    DEBUG ((EFI_D_ERROR, "XenPvBlk: Failed to get backend-id (%d)\n",
+            Status));
+    goto Error;
+  }
+  Dev->DomainId = Value;
+  XenBusIo->EventChannelAllocate (XenBusIo, Dev->DomainId, &Dev->EventChannel);
+
+  SharedRing = (blkif_sring_t*) AllocatePages (1);
+  SHARED_RING_INIT (SharedRing);
+  FRONT_RING_INIT (&Dev->Ring, SharedRing, EFI_PAGE_SIZE);
+  XenBusIo->GrantAccess (XenBusIo,
+                         Dev->DomainId,
+                         (INTN) SharedRing >> EFI_PAGE_SHIFT,
+                         FALSE,
+                         &Dev->RingRef);
+
+Again:
+  Status = XenBusIo->XsTransactionStart (XenBusIo, &xbt);
+  if (Status != XENSTORE_STATUS_SUCCESS) {
+    DEBUG ((EFI_D_WARN, "XenPvBlk: Failed to start transaction, %d\n", 
Status));
+    goto Error;
+  }
+
+  Status = XenBusIo->XsPrintf (XenBusIo, xbt, NodeName, "ring-ref", "%d",
+                               Dev->RingRef);
+  if (Status != XENSTORE_STATUS_SUCCESS) {
+    DEBUG ((EFI_D_ERROR, "XenPvBlk: Failed to write ring-ref.\n"));
+    goto AbortTransaction;
+  }
+  Status = XenBusIo->XsPrintf (XenBusIo, xbt, NodeName,
+                               "event-channel", "%d", Dev->EventChannel);
+  if (Status != XENSTORE_STATUS_SUCCESS) {
+    DEBUG ((EFI_D_ERROR, "XenPvBlk: Failed to write event-channel.\n"));
+    goto AbortTransaction;
+  }
+  Status = XenBusIo->XsPrintf (XenBusIo, xbt, NodeName,
+                               "protocol", "%a", XEN_IO_PROTO_ABI_NATIVE);
+  if (Status != XENSTORE_STATUS_SUCCESS) {
+    DEBUG ((EFI_D_ERROR, "XenPvBlk: Failed to write protocol.\n"));
+    goto AbortTransaction;
+  }
+
+  Status = XenBusIo->SetState (XenBusIo, xbt, XenbusStateConnected);
+  if (Status != XENSTORE_STATUS_SUCCESS) {
+    DEBUG ((EFI_D_ERROR, "XenPvBlk: Failed to switch state.\n"));
+    goto AbortTransaction;
+  }
+
+  Status = XenBusIo->XsTransactionEnd (XenBusIo, xbt, FALSE);
+  if (Status == XENSTORE_STATUS_EAGAIN) {
+    goto Again;
+  }
+
+  XenBusIo->RegisterWatchBackend (XenBusIo, "state", &Dev->StateWatchToken);
+
+  //
+  // Waiting for backend
+  //
+  Status = XenPvBlkWaitForBackendState (Dev, XenbusStateConnected, &State);
+  if (Status != XENSTORE_STATUS_SUCCESS) {
+    DEBUG ((EFI_D_ERROR,
+            "XenPvBlk: backend for %a/%d not available, rc=%d state=%d\n",
+            XenBusIo->Type, XenBusIo->DeviceId, Status, State));
+    goto Error2;
+  }
+
+  Status = XenBusReadUint64 (XenBusIo, "info", TRUE, &Value);
+  if (Status != XENSTORE_STATUS_SUCCESS || Value > UINT32_MAX) {
+    goto Error2;
+  }
+  Dev->MediaInfo.VDiskInfo = Value;
+  if (Dev->MediaInfo.VDiskInfo & VDISK_READONLY) {
+    Dev->MediaInfo.ReadWrite = FALSE;
+  } else {
+    Dev->MediaInfo.ReadWrite = TRUE;
+  }
+
+  Status = XenBusReadUint64 (XenBusIo, "sectors", TRUE, 
&Dev->MediaInfo.Sectors);
+  if (Status != XENSTORE_STATUS_SUCCESS) {
+    goto Error2;
+  }
+
+  Status = XenBusReadUint64 (XenBusIo, "sector-size", TRUE, &Value);
+  if (Status != XENSTORE_STATUS_SUCCESS || Value > UINT32_MAX) {
+    goto Error2;
+  }
+  if (Value % 512 != 0) {
+    //
+    // This is not supported by the driver.
+    //
+    DEBUG ((EFI_D_ERROR, "XenPvBlk: Unsupported sector-size value %d, "
+            "it must be a multiple of 512\n", Value));
+    goto Error2;
+  }
+  Dev->MediaInfo.SectorSize = Value;
+
+  // Default value
+  Value = 0;
+  XenBusReadUint64 (XenBusIo, "feature-barrier", TRUE, &Value);
+  if (Value == 1) {
+    Dev->MediaInfo.FeatureBarrier = TRUE;
+  } else {
+    Dev->MediaInfo.FeatureBarrier = FALSE;
+  }
+
+  // Default value
+  Value = 0;
+  XenBusReadUint64 (XenBusIo, "feature-flush-cache", TRUE, &Value);
+  if (Value == 1) {
+    Dev->MediaInfo.FeatureFlushCache = TRUE;
+  } else {
+    Dev->MediaInfo.FeatureFlushCache = FALSE;
+  }
+
+  DEBUG ((EFI_D_INFO, "XenPvBlk: New disk with %ld sectors of %d bytes\n",
+          Dev->MediaInfo.Sectors, Dev->MediaInfo.SectorSize));
+
+  *DevPtr = Dev;
+  return EFI_SUCCESS;
+
+Error2:
+  XenBusIo->UnregisterWatch (XenBusIo, Dev->StateWatchToken);
+  XenBusIo->XsRemove (XenBusIo, XST_NIL, "ring-ref");
+  XenBusIo->XsRemove (XenBusIo, XST_NIL, "event-channel");
+  XenBusIo->XsRemove (XenBusIo, XST_NIL, "protocol");
+  goto Error;
+AbortTransaction:
+  XenBusIo->XsTransactionEnd (XenBusIo, xbt, TRUE);
+Error:
+  XenPvBlockFree (Dev);
+  return EFI_DEVICE_ERROR;
+}
+
+VOID
+XenPvBlockFrontShutdown (
+  IN XEN_BLOCK_FRONT_DEVICE *Dev
+  )
+{
+  XENBUS_PROTOCOL *XenBusIo = Dev->XenBusIo;
+  XENSTORE_STATUS Status;
+  UINT64 Value;
+
+  XenPvBlockSync (Dev);
+
+  Status = XenBusIo->SetState (XenBusIo, XST_NIL, XenbusStateClosing);
+  if (Status != XENSTORE_STATUS_SUCCESS) {
+    DEBUG ((EFI_D_ERROR,
+            "XenPvBlk: error while changing state to Closing: %d\n",
+            Status));
+    goto Close;
+  }
+
+  Status = XenPvBlkWaitForBackendState (Dev, XenbusStateClosing, NULL);
+  if (Status != XENSTORE_STATUS_SUCCESS) {
+    DEBUG ((EFI_D_ERROR,
+            "XenPvBlk: error while waiting for closing backend state: %d\n",
+            Status));
+    goto Close;
+  }
+
+  Status = XenBusIo->SetState (XenBusIo, XST_NIL, XenbusStateClosed);
+  if (Status != XENSTORE_STATUS_SUCCESS) {
+    DEBUG ((EFI_D_ERROR,
+            "XenPvBlk: error while changing state to Closed: %d\n",
+            Status));
+    goto Close;
+  }
+
+  Status = XenPvBlkWaitForBackendState (Dev, XenbusStateClosed, NULL);
+  if (Status != XENSTORE_STATUS_SUCCESS) {
+    DEBUG ((EFI_D_ERROR,
+            "XenPvBlk: error while waiting for closed backend state: %d\n",
+            Status));
+    goto Close;
+  }
+
+  Status = XenBusIo->SetState (XenBusIo, XST_NIL, XenbusStateInitialising);
+  if (Status != XENSTORE_STATUS_SUCCESS) {
+    DEBUG ((EFI_D_ERROR,
+            "XenPvBlk: error while changing state to initialising: %d\n",
+            Status));
+    goto Close;
+  }
+
+  while (TRUE) {
+    Status = XenBusReadUint64 (XenBusIo, "state", TRUE, &Value);
+    if (Status != XENSTORE_STATUS_SUCCESS) {
+      DEBUG ((EFI_D_ERROR,
+              "XenPvBlk: error while waiting for new backend state: %d\n",
+              Status));
+      goto Close;
+    }
+    if (Value <= XenbusStateInitWait || Value >= XenbusStateClosed) {
+      break;
+    }
+    DEBUG ((EFI_D_INFO,
+            "XenPvBlk: waiting backend state %d, current: %d\n",
+            XenbusStateInitWait, Value));
+    XenBusIo->WaitForWatch (XenBusIo, Dev->StateWatchToken);
+  }
+
+Close:
+  XenBusIo->UnregisterWatch (XenBusIo, Dev->StateWatchToken);
+  XenBusIo->XsRemove (XenBusIo, XST_NIL, "ring-ref");
+  XenBusIo->XsRemove (XenBusIo, XST_NIL, "event-channel");
+  XenBusIo->XsRemove (XenBusIo, XST_NIL, "protocol");
+
+  XenPvBlockFree (Dev);
+}
+
+STATIC
+VOID
+XenPvBlockWaitSlot (
+  IN XEN_BLOCK_FRONT_DEVICE *Dev
+  )
+{
+  /* Wait for a slot */
+  if (RING_FULL (&Dev->Ring)) {
+    while (TRUE) {
+      XenPvBlockAsyncIoPoll (Dev);
+      if (!RING_FULL (&Dev->Ring)) {
+        break;
+      }
+      /* Really no slot, could wait for an event on Dev->EventChannel. */
+    }
+  }
+}
+
+VOID
+XenPvBlockAsyncIo (
+  IN OUT XEN_BLOCK_FRONT_IO *IoData,
+  IN     BOOLEAN            IsWrite
+  )
+{
+  XEN_BLOCK_FRONT_DEVICE *Dev = IoData->Dev;
+  XENBUS_PROTOCOL *XenBusIo = Dev->XenBusIo;
+  blkif_request_t *Request;
+  RING_IDX RingIndex;
+  BOOLEAN Notify;
+  INT32 NumSegments, Index;
+  UINTN Start, End;
+
+  // Can't io at non-sector-aligned location
+  ASSERT(!(IoData->Sector & ((Dev->MediaInfo.SectorSize / 512) - 1)));
+  // Can't io non-sector-sized amounts
+  ASSERT(!(IoData->Size & (Dev->MediaInfo.SectorSize - 1)));
+  // Can't io non-sector-aligned buffer
+  ASSERT(!((UINTN) IoData->Buffer & (Dev->MediaInfo.SectorSize - 1)));
+
+  Start = (UINTN) IoData->Buffer & ~EFI_PAGE_MASK;
+  End = ((UINTN) IoData->Buffer + IoData->Size + EFI_PAGE_SIZE - 1) & 
~EFI_PAGE_MASK;
+  IoData->NumRef = NumSegments = (End - Start) / EFI_PAGE_SIZE;
+
+  ASSERT (NumSegments <= BLKIF_MAX_SEGMENTS_PER_REQUEST);
+
+  XenPvBlockWaitSlot (Dev);
+  RingIndex = Dev->Ring.req_prod_pvt;
+  Request = RING_GET_REQUEST (&Dev->Ring, RingIndex);
+
+  Request->operation = IsWrite ? BLKIF_OP_WRITE : BLKIF_OP_READ;
+  Request->nr_segments = NumSegments;
+  Request->handle = Dev->DeviceId;
+  Request->id = (UINTN) IoData;
+  Request->sector_number = IoData->Sector;
+
+  for (Index = 0; Index < NumSegments; Index++) {
+    Request->seg[Index].first_sect = 0;
+    Request->seg[Index].last_sect = EFI_PAGE_SIZE / 512 - 1;
+  }
+  Request->seg[0].first_sect = ((UINTN) IoData->Buffer & EFI_PAGE_MASK) / 512;
+  Request->seg[NumSegments - 1].last_sect =
+      (((UINTN) IoData->Buffer + IoData->Size - 1) & EFI_PAGE_MASK) / 512;
+  for (Index = 0; Index < NumSegments; Index++) {
+    UINTN Data = Start + Index * EFI_PAGE_SIZE;
+    XenBusIo->GrantAccess (XenBusIo, Dev->DomainId,
+                           Data >> EFI_PAGE_SHIFT, IsWrite,
+                           &Request->seg[Index].gref);
+    IoData->GrantRef[Index] = Request->seg[Index].gref;
+  }
+
+  Dev->Ring.req_prod_pvt = RingIndex + 1;
+
+  MemoryFence ();
+  RING_PUSH_REQUESTS_AND_CHECK_NOTIFY (&Dev->Ring, Notify);
+
+  if (Notify) {
+    UINT32 ReturnCode;
+    ReturnCode = XenBusIo->EventChannelNotify (XenBusIo, Dev->EventChannel);
+    if (ReturnCode != 0) {
+      DEBUG ((EFI_D_ERROR,
+              "XenPvBlk: Unexpected return value from EventChannelNotify: 
%d\n",
+              ReturnCode));
+    }
+  }
+}
+
+EFI_STATUS
+XenPvBlockIo (
+  IN OUT XEN_BLOCK_FRONT_IO *IoData,
+  IN     BOOLEAN            IsWrite
+  )
+{
+  //
+  // Status value that correspond to an IO in progress.
+  //
+  IoData->Status = EFI_ALREADY_STARTED;
+  XenPvBlockAsyncIo (IoData, IsWrite);
+
+  while (IoData->Status == EFI_ALREADY_STARTED) {
+    XenPvBlockAsyncIoPoll (IoData->Dev);
+  }
+
+  return IoData->Status;
+}
+
+STATIC
+VOID
+XenPvBlockPushOperation (
+  IN XEN_BLOCK_FRONT_DEVICE *Dev,
+  IN UINT8                  Operation,
+  IN UINT64                 Id
+  )
+{
+  INT32 Index;
+  blkif_request_t *Request;
+  BOOLEAN Notify;
+
+  XenPvBlockWaitSlot (Dev);
+  Index = Dev->Ring.req_prod_pvt;
+  Request = RING_GET_REQUEST(&Dev->Ring, Index);
+  Request->operation = Operation;
+  Request->nr_segments = 0;
+  Request->handle = Dev->DeviceId;
+  Request->id = Id;
+  /* Not needed anyway, but the backend will check it */
+  Request->sector_number = 0;
+  Dev->Ring.req_prod_pvt = Index + 1;
+  MemoryFence ();
+  RING_PUSH_REQUESTS_AND_CHECK_NOTIFY (&Dev->Ring, Notify);
+  if (Notify) {
+    XENBUS_PROTOCOL *XenBusIo = Dev->XenBusIo;
+    UINT32 ReturnCode;
+    ReturnCode = XenBusIo->EventChannelNotify (XenBusIo, Dev->EventChannel);
+    if (ReturnCode != 0) {
+      DEBUG ((EFI_D_ERROR,
+              "XenPvBlk: Unexpected return value from EventChannelNotify: 
%d\n",
+              ReturnCode));
+    }
+  }
+}
+
+VOID
+XenPvBlockSync (
+  IN XEN_BLOCK_FRONT_DEVICE *Dev
+  )
+{
+  if (Dev->MediaInfo.ReadWrite) {
+    if (Dev->MediaInfo.FeatureBarrier) {
+      XenPvBlockPushOperation (Dev, BLKIF_OP_WRITE_BARRIER, 0);
+    }
+
+    if (Dev->MediaInfo.FeatureFlushCache) {
+      XenPvBlockPushOperation (Dev, BLKIF_OP_FLUSH_DISKCACHE, 0);
+    }
+  }
+
+  /* Note: This won't finish if another thread enqueues requests.  */
+  while (TRUE) {
+    XenPvBlockAsyncIoPoll (Dev);
+    if (RING_FREE_REQUESTS (&Dev->Ring) == RING_SIZE (&Dev->Ring)) {
+      break;
+    }
+  }
+}
+
+VOID
+XenPvBlockAsyncIoPoll (
+  IN XEN_BLOCK_FRONT_DEVICE *Dev
+  )
+{
+  RING_IDX ProducerIndex, ConsumerIndex;
+  blkif_response_t *Response;
+  INT32 More;
+
+  do {
+    ProducerIndex = Dev->Ring.sring->rsp_prod;
+    /* Ensure we see queued responses up to 'ProducerIndex'. */
+    MemoryFence ();
+    ConsumerIndex = Dev->Ring.rsp_cons;
+
+    while (ConsumerIndex != ProducerIndex) {
+      XEN_BLOCK_FRONT_IO *IoData = NULL;
+      INT16 Status;
+
+      Response = RING_GET_RESPONSE (&Dev->Ring, ConsumerIndex);
+
+      IoData = (VOID *) (UINTN) Response->id;
+      Status = Response->status;
+
+      switch (Response->operation) {
+      case BLKIF_OP_READ:
+      case BLKIF_OP_WRITE:
+        {
+          INT32 Index;
+
+          if (Status != BLKIF_RSP_OKAY) {
+            DEBUG ((EFI_D_ERROR,
+                    "XenPvBlk: "
+                    "%a error %d on %a at sector %p, num bytes %p\n",
+                    Response->operation == BLKIF_OP_READ ? "read" : "write",
+                    Status, IoData->Dev->NodeName,
+                    IoData->Sector,
+                    IoData->Size));
+          }
+
+          for (Index = 0; Index < IoData->NumRef; Index++) {
+            Dev->XenBusIo->GrantEndAccess (Dev->XenBusIo, 
IoData->GrantRef[Index]);
+          }
+
+          break;
+        }
+
+      case BLKIF_OP_WRITE_BARRIER:
+        if (Status != BLKIF_RSP_OKAY) {
+          DEBUG ((EFI_D_ERROR, "XenPvBlk: write barrier error %d\n", Status));
+        }
+        break;
+      case BLKIF_OP_FLUSH_DISKCACHE:
+        if (Status != BLKIF_RSP_OKAY) {
+          DEBUG ((EFI_D_ERROR, "XenPvBlk: flush error %d\n", Status));
+        }
+        break;
+
+      default:
+        DEBUG ((EFI_D_ERROR,
+                "XenPvBlk: unrecognized block operation %d response (status 
%d)\n",
+                Response->operation, Status));
+        break;
+      }
+
+      Dev->Ring.rsp_cons = ++ConsumerIndex;
+      if (IoData != NULL) {
+        IoData->Status = Status ? EFI_DEVICE_ERROR : EFI_SUCCESS;
+      }
+      if (Dev->Ring.rsp_cons != ConsumerIndex) {
+        /* We reentered, we must not continue here */
+        break;
+      }
+    }
+
+    RING_FINAL_CHECK_FOR_RESPONSES (&Dev->Ring, More);
+  } while (More != 0);
+}
diff --git a/OvmfPkg/XenPvBlkDxe/BlockFront.h b/OvmfPkg/XenPvBlkDxe/BlockFront.h
new file mode 100644
index 0000000..348f074
--- /dev/null
+++ b/OvmfPkg/XenPvBlkDxe/BlockFront.h
@@ -0,0 +1,101 @@
+/** @file
+  BlockFront functions and types declarations.
+
+  Copyright (C) 2014, Citrix Ltd.
+
+  This program and the accompanying materials
+  are licensed and made available under the terms and conditions of the BSD 
License
+  which accompanies this distribution.  The full text of the license may be 
found at
+  http://opensource.org/licenses/bsd-license.php
+
+  THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS,
+  WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
+
+**/
+#include "XenPvBlkDxe.h"
+
+#include <IndustryStandard/Xen/event_channel.h>
+#include <IndustryStandard/Xen/io/blkif.h>
+
+typedef struct _XEN_BLOCK_FRONT_DEVICE XEN_BLOCK_FRONT_DEVICE;
+typedef struct _XEN_BLOCK_FRONT_IO XEN_BLOCK_FRONT_IO;
+
+struct _XEN_BLOCK_FRONT_IO
+{
+  XEN_BLOCK_FRONT_DEVICE  *Dev;
+  UINT8                   *Buffer;
+  UINTN                   Size;
+  UINTN                   Sector; ///< 512 bytes sector.
+
+  grant_ref_t             GrantRef[BLKIF_MAX_SEGMENTS_PER_REQUEST];
+  INT32                   NumRef;
+
+  EFI_STATUS              Status;
+};
+
+typedef struct
+{
+  UINT64    Sectors;
+  UINT32    SectorSize;
+  UINT32    VDiskInfo;
+  BOOLEAN   ReadWrite;
+  BOOLEAN   CdRom;
+  BOOLEAN   FeatureBarrier;
+  BOOLEAN   FeatureFlushCache;
+} XEN_BLOCK_FRONT_MEDIA_INFO;
+
+#define XEN_BLOCK_FRONT_SIGNATURE SIGNATURE_32 ('X', 'p', 'v', 'B')
+struct _XEN_BLOCK_FRONT_DEVICE {
+  UINT32                      Signature;
+  EFI_BLOCK_IO_PROTOCOL       BlockIo;
+  domid_t                     DomainId;
+
+  blkif_front_ring_t          Ring;
+  grant_ref_t                 RingRef;
+  evtchn_port_t               EventChannel;
+  blkif_vdev_t                DeviceId;
+
+  CONST CHAR8                 *NodeName;
+  XEN_BLOCK_FRONT_MEDIA_INFO  MediaInfo;
+
+  VOID                        *StateWatchToken;
+
+  XENBUS_PROTOCOL             *XenBusIo;
+};
+
+#define XEN_BLOCK_FRONT_FROM_BLOCK_IO(b) \
+  CR (b, XEN_BLOCK_FRONT_DEVICE, BlockIo, XEN_BLOCK_FRONT_SIGNATURE)
+
+EFI_STATUS
+XenPvBlockFrontInitialization (
+  IN  XENBUS_PROTOCOL  *XenBusIo,
+  IN  CONST CHAR8      *NodeName,
+  OUT XEN_BLOCK_FRONT_DEVICE **DevPtr
+  );
+
+VOID
+XenPvBlockFrontShutdown (
+  IN XEN_BLOCK_FRONT_DEVICE *Dev
+  );
+
+VOID
+XenPvBlockAsyncIo (
+  IN OUT XEN_BLOCK_FRONT_IO *IoData,
+  IN     BOOLEAN            IsWrite
+  );
+
+EFI_STATUS
+XenPvBlockIo (
+  IN OUT XEN_BLOCK_FRONT_IO *IoData,
+  IN     BOOLEAN            IsWrite
+  );
+
+VOID
+XenPvBlockAsyncIoPoll (
+  IN XEN_BLOCK_FRONT_DEVICE *Dev
+  );
+
+VOID
+XenPvBlockSync (
+  IN XEN_BLOCK_FRONT_DEVICE *Dev
+  );
diff --git a/OvmfPkg/XenPvBlkDxe/XenPvBlkDxe.c 
b/OvmfPkg/XenPvBlkDxe/XenPvBlkDxe.c
index a5a6953..e755a43 100644
--- a/OvmfPkg/XenPvBlkDxe/XenPvBlkDxe.c
+++ b/OvmfPkg/XenPvBlkDxe/XenPvBlkDxe.c
@@ -20,6 +20,8 @@
 
 #include "XenPvBlkDxe.h"
 
+#include "BlockFront.h"
+
 
 ///
 /// Driver Binding Protocol instance
@@ -258,6 +260,7 @@ XenPvBlkDxeDriverBindingStart (
 {
   EFI_STATUS Status;
   XENBUS_PROTOCOL *XenBusIo;
+  XEN_BLOCK_FRONT_DEVICE *Dev;
 
   Status = gBS->OpenProtocol (
                 ControllerHandle,
@@ -271,7 +274,17 @@ XenPvBlkDxeDriverBindingStart (
     return Status;
   }
 
+  Status = XenPvBlockFrontInitialization (XenBusIo, XenBusIo->Node, &Dev);
+  if (EFI_ERROR (Status)) {
+    goto CloseProtocol;
+  }
+
   return EFI_SUCCESS;
+
+CloseProtocol:
+  gBS->CloseProtocol (ControllerHandle, &gXenBusProtocolGuid,
+                      This->DriverBindingHandle, ControllerHandle);
+  return Status;
 }
 
 /**
diff --git a/OvmfPkg/XenPvBlkDxe/XenPvBlkDxe.inf 
b/OvmfPkg/XenPvBlkDxe/XenPvBlkDxe.inf
index 32ae952..af0ed2e 100644
--- a/OvmfPkg/XenPvBlkDxe/XenPvBlkDxe.inf
+++ b/OvmfPkg/XenPvBlkDxe/XenPvBlkDxe.inf
@@ -33,6 +33,8 @@
   XenPvBlkDxe.c
   ComponentName.c
   ComponentName.h
+  BlockFront.c
+  BlockFront.h
 
 
 [LibraryClasses]
-- 
Anthony PERARD


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