xen: add virtual block device driver.

The block device frontend driver allows the kernel to access block
devices exported exported by a virtual machine containing a physical
block device driver.

Signed-off-by: Ian Pratt <ian.pratt@xensource.com>
Signed-off-by: Christian Limpach <Christian.Limpach@cl.cam.ac.uk>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Cc: Arjan van de Ven <arjan@infradead.org>
Cc: Greg KH <greg@kroah.com>
Cc: Jens Axboe <axboe@kernel.dk>

4 files changed, 1000 insertions, 0 deletions

diff --git a/drivers/block/Kconfig b/drivers/block/Kconfig
index 8f65b88cf711..a4a311992408 100644
--- a/drivers/block/Kconfig
+++ b/drivers/block/Kconfig
@@ -427,4 +427,13 @@ config XILINX_SYSACE
         help
           Include support for the Xilinx SystemACE CompactFlash interface
 
+config XEN_BLKDEV_FRONTEND
+        tristate "Xen virtual block device support"
+        depends on XEN
+        default y
+        help
+          This driver implements the front-end of the Xen virtual
+          block device driver.  It communicates with a back-end driver
+          in another domain which drives the actual block device.
+
 endif # BLK_DEV
diff --git a/drivers/block/Makefile b/drivers/block/Makefile
index 9ee08ab4ffa8..3e31532df0ed 100644
--- a/drivers/block/Makefile
+++ b/drivers/block/Makefile
@@ -29,3 +29,4 @@ obj-$(CONFIG_VIODASD)		+= viodasd.o
 obj-$(CONFIG_BLK_DEV_SX8)       += sx8.o
 obj-$(CONFIG_BLK_DEV_UB)        += ub.o
 
+obj-$(CONFIG_XEN_BLKDEV_FRONTEND)       += xen-blkfront.o
diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
new file mode 100644
index 000000000000..6746c29181f8
--- /dev/null
+++ b/drivers/block/xen-blkfront.c
@@ -0,0 +1,988 @@
+/*
+ * blkfront.c
+ *
+ * XenLinux virtual block device driver.
+ *
+ * Copyright (c) 2003-2004, Keir Fraser & Steve Hand
+ * Modifications by Mark A. Williamson are (c) Intel Research Cambridge
+ * Copyright (c) 2004, Christian Limpach
+ * Copyright (c) 2004, Andrew Warfield
+ * Copyright (c) 2005, Christopher Clark
+ * Copyright (c) 2005, XenSource Ltd
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License version 2
+ * as published by the Free Software Foundation; or, when distributed
+ * separately from the Linux kernel or incorporated into other
+ * software packages, subject to the following license:
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this source file (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.
+ */
+
+#include <linux/interrupt.h>
+#include <linux/blkdev.h>
+#include <linux/module.h>
+
+#include <xen/xenbus.h>
+#include <xen/grant_table.h>
+#include <xen/events.h>
+#include <xen/page.h>
+
+#include <xen/interface/grant_table.h>
+#include <xen/interface/io/blkif.h>
+
+#include <asm/xen/hypervisor.h>
+
+enum blkif_state {
+        BLKIF_STATE_DISCONNECTED,
+        BLKIF_STATE_CONNECTED,
+        BLKIF_STATE_SUSPENDED,
+};
+
+struct blk_shadow {
+        struct blkif_request req;
+        unsigned long request;
+        unsigned long frame[BLKIF_MAX_SEGMENTS_PER_REQUEST];
+};
+
+static struct block_device_operations xlvbd_block_fops;
+
+#define BLK_RING_SIZE __RING_SIZE((struct blkif_sring *)0, PAGE_SIZE)
+
+/*
+ * We have one of these per vbd, whether ide, scsi or 'other'.  They
+ * hang in private_data off the gendisk structure. We may end up
+ * putting all kinds of interesting stuff here :-)
+ */
+struct blkfront_info
+{
+        struct xenbus_device *xbdev;
+        dev_t dev;
+        struct gendisk *gd;
+        int vdevice;
+        blkif_vdev_t handle;
+        enum blkif_state connected;
+        int ring_ref;
+        struct blkif_front_ring ring;
+        unsigned int evtchn, irq;
+        struct request_queue *rq;
+        struct work_struct work;
+        struct gnttab_free_callback callback;
+        struct blk_shadow shadow[BLK_RING_SIZE];
+        unsigned long shadow_free;
+        int feature_barrier;
+
+        /**
+         * The number of people holding this device open.  We won't allow a
+         * hot-unplug unless this is 0.
+         */
+        int users;
+};
+
+static DEFINE_SPINLOCK(blkif_io_lock);
+
+#define MAXIMUM_OUTSTANDING_BLOCK_REQS \
+        (BLKIF_MAX_SEGMENTS_PER_REQUEST * BLK_RING_SIZE)
+#define GRANT_INVALID_REF       0
+
+#define PARTS_PER_DISK          16
+
+#define BLKIF_MAJOR(dev) ((dev)>>8)
+#define BLKIF_MINOR(dev) ((dev) & 0xff)
+
+#define DEV_NAME        "xvd"   /* name in /dev */
+
+/* Information about our VBDs. */
+#define MAX_VBDS 64
+static LIST_HEAD(vbds_list);
+
+static int get_id_from_freelist(struct blkfront_info *info)
+{
+        unsigned long free = info->shadow_free;
+        BUG_ON(free > BLK_RING_SIZE);
+        info->shadow_free = info->shadow[free].req.id;
+        info->shadow[free].req.id = 0x0fffffee; /* debug */
+        return free;
+}
+
+static void add_id_to_freelist(struct blkfront_info *info,
+                               unsigned long id)
+{
+        info->shadow[id].req.id  = info->shadow_free;
+        info->shadow[id].request = 0;
+        info->shadow_free = id;
+}
+
+static void blkif_restart_queue_callback(void *arg)
+{
+        struct blkfront_info *info = (struct blkfront_info *)arg;
+        schedule_work(&info->work);
+}
+
+/*
+ * blkif_queue_request
+ *
+ * request block io
+ *
+ * id: for guest use only.
+ * operation: BLKIF_OP_{READ,WRITE,PROBE}
+ * buffer: buffer to read/write into. this should be a
+ *   virtual address in the guest os.
+ */
+static int blkif_queue_request(struct request *req)
+{
+        struct blkfront_info *info = req->rq_disk->private_data;
+        unsigned long buffer_mfn;
+        struct blkif_request *ring_req;
+        struct bio *bio;
+        struct bio_vec *bvec;
+        int idx;
+        unsigned long id;
+        unsigned int fsect, lsect;
+        int ref;
+        grant_ref_t gref_head;
+
+        if (unlikely(info->connected != BLKIF_STATE_CONNECTED))
+                return 1;
+
+        if (gnttab_alloc_grant_references(
+                BLKIF_MAX_SEGMENTS_PER_REQUEST, &gref_head) < 0) {
+                gnttab_request_free_callback(
+                        &info->callback,
+                        blkif_restart_queue_callback,
+                        info,
+                        BLKIF_MAX_SEGMENTS_PER_REQUEST);
+                return 1;
+        }
+
+        /* Fill out a communications ring structure. */
+        ring_req = RING_GET_REQUEST(&info->ring, info->ring.req_prod_pvt);
+        id = get_id_from_freelist(info);
+        info->shadow[id].request = (unsigned long)req;
+
+        ring_req->id = id;
+        ring_req->sector_number = (blkif_sector_t)req->sector;
+        ring_req->handle = info->handle;
+
+        ring_req->operation = rq_data_dir(req) ?
+                BLKIF_OP_WRITE : BLKIF_OP_READ;
+        if (blk_barrier_rq(req))
+                ring_req->operation = BLKIF_OP_WRITE_BARRIER;
+
+        ring_req->nr_segments = 0;
+        rq_for_each_bio (bio, req) {
+                bio_for_each_segment (bvec, bio, idx) {
+                        BUG_ON(ring_req->nr_segments
+                               == BLKIF_MAX_SEGMENTS_PER_REQUEST);
+                        buffer_mfn = pfn_to_mfn(page_to_pfn(bvec->bv_page));
+                        fsect = bvec->bv_offset >> 9;
+                        lsect = fsect + (bvec->bv_len >> 9) - 1;
+                        /* install a grant reference. */
+                        ref = gnttab_claim_grant_reference(&gref_head);
+                        BUG_ON(ref == -ENOSPC);
+
+                        gnttab_grant_foreign_access_ref(
+                                ref,
+                                info->xbdev->otherend_id,
+                                buffer_mfn,
+                                rq_data_dir(req) );
+
+                        info->shadow[id].frame[ring_req->nr_segments] =
+                                mfn_to_pfn(buffer_mfn);
+
+                        ring_req->seg[ring_req->nr_segments] =
+                                (struct blkif_request_segment) {
+                                        .gref       = ref,
+                                        .first_sect = fsect,
+                                        .last_sect  = lsect };
+
+                        ring_req->nr_segments++;
+                }
+        }
+
+        info->ring.req_prod_pvt++;
+
+        /* Keep a private copy so we can reissue requests when recovering. */
+        info->shadow[id].req = *ring_req;
+
+        gnttab_free_grant_references(gref_head);
+
+        return 0;
+}
+
+
+static inline void flush_requests(struct blkfront_info *info)
+{
+        int notify;
+
+        RING_PUSH_REQUESTS_AND_CHECK_NOTIFY(&info->ring, notify);
+
+        if (notify)
+                notify_remote_via_irq(info->irq);
+}
+
+/*
+ * do_blkif_request
+ *  read a block; request is in a request queue
+ */
+static void do_blkif_request(request_queue_t *rq)
+{
+        struct blkfront_info *info = NULL;
+        struct request *req;
+        int queued;
+
+        pr_debug("Entered do_blkif_request\n");
+
+        queued = 0;
+
+        while ((req = elv_next_request(rq)) != NULL) {
+                info = req->rq_disk->private_data;
+                if (!blk_fs_request(req)) {
+                        end_request(req, 0);
+                        continue;
+                }
+
+                if (RING_FULL(&info->ring))
+                        goto wait;
+
+                pr_debug("do_blk_req %p: cmd %p, sec %lx, "
+                         "(%u/%li) buffer:%p [%s]\n",
+                         req, req->cmd, (unsigned long)req->sector,
+                         req->current_nr_sectors,
+                         req->nr_sectors, req->buffer,
+                         rq_data_dir(req) ? "write" : "read");
+
+
+                blkdev_dequeue_request(req);
+                if (blkif_queue_request(req)) {
+                        blk_requeue_request(rq, req);
+wait:
+                        /* Avoid pointless unplugs. */
+                        blk_stop_queue(rq);
+                        break;
+                }
+
+                queued++;
+        }
+
+        if (queued != 0)
+                flush_requests(info);
+}
+
+static int xlvbd_init_blk_queue(struct gendisk *gd, u16 sector_size)
+{
+        request_queue_t *rq;
+
+        rq = blk_init_queue(do_blkif_request, &blkif_io_lock);
+        if (rq == NULL)
+                return -1;
+
+        elevator_init(rq, "noop");
+
+        /* Hard sector size and max sectors impersonate the equiv. hardware. */
+        blk_queue_hardsect_size(rq, sector_size);
+        blk_queue_max_sectors(rq, 512);
+
+        /* Each segment in a request is up to an aligned page in size. */
+        blk_queue_segment_boundary(rq, PAGE_SIZE - 1);
+        blk_queue_max_segment_size(rq, PAGE_SIZE);
+
+        /* Ensure a merged request will fit in a single I/O ring slot. */
+        blk_queue_max_phys_segments(rq, BLKIF_MAX_SEGMENTS_PER_REQUEST);
+        blk_queue_max_hw_segments(rq, BLKIF_MAX_SEGMENTS_PER_REQUEST);
+
+        /* Make sure buffer addresses are sector-aligned. */
+        blk_queue_dma_alignment(rq, 511);
+
+        gd->queue = rq;
+
+        return 0;
+}
+
+
+static int xlvbd_barrier(struct blkfront_info *info)
+{
+        int err;
+
+        err = blk_queue_ordered(info->rq,
+                                info->feature_barrier ? QUEUE_ORDERED_DRAIN : QUEUE_ORDERED_NONE,
+                                NULL);
+
+        if (err)
+                return err;
+
+        printk(KERN_INFO "blkfront: %s: barriers %s\n",
+               info->gd->disk_name,
+               info->feature_barrier ? "enabled" : "disabled");
+        return 0;
+}
+
+
+static int xlvbd_alloc_gendisk(int minor, blkif_sector_t capacity,
+                               int vdevice, u16 vdisk_info, u16 sector_size,
+                               struct blkfront_info *info)
+{
+        struct gendisk *gd;
+        int nr_minors = 1;
+        int err = -ENODEV;
+
+        BUG_ON(info->gd != NULL);
+        BUG_ON(info->rq != NULL);
+
+        if ((minor % PARTS_PER_DISK) == 0)
+                nr_minors = PARTS_PER_DISK;
+
+        gd = alloc_disk(nr_minors);
+        if (gd == NULL)
+                goto out;
+
+        if (nr_minors > 1)
+                sprintf(gd->disk_name, "%s%c", DEV_NAME,
+                        'a' + minor / PARTS_PER_DISK);
+        else
+                sprintf(gd->disk_name, "%s%c%d", DEV_NAME,
+                        'a' + minor / PARTS_PER_DISK,
+                        minor % PARTS_PER_DISK);
+
+        gd->major = XENVBD_MAJOR;
+        gd->first_minor = minor;
+        gd->fops = &xlvbd_block_fops;
+        gd->private_data = info;
+        gd->driverfs_dev = &(info->xbdev->dev);
+        set_capacity(gd, capacity);
+
+        if (xlvbd_init_blk_queue(gd, sector_size)) {
+                del_gendisk(gd);
+                goto out;
+        }
+
+        info->rq = gd->queue;
+        info->gd = gd;
+
+        if (info->feature_barrier)
+                xlvbd_barrier(info);
+
+        if (vdisk_info & VDISK_READONLY)
+                set_disk_ro(gd, 1);
+
+        if (vdisk_info & VDISK_REMOVABLE)
+                gd->flags |= GENHD_FL_REMOVABLE;
+
+        if (vdisk_info & VDISK_CDROM)
+                gd->flags |= GENHD_FL_CD;
+
+        return 0;
+
+ out:
+        return err;
+}
+
+static void kick_pending_request_queues(struct blkfront_info *info)
+{
+        if (!RING_FULL(&info->ring)) {
+                /* Re-enable calldowns. */
+                blk_start_queue(info->rq);
+                /* Kick things off immediately. */
+                do_blkif_request(info->rq);
+        }
+}
+
+static void blkif_restart_queue(struct work_struct *work)
+{
+        struct blkfront_info *info = container_of(work, struct blkfront_info, work);
+
+        spin_lock_irq(&blkif_io_lock);
+        if (info->connected == BLKIF_STATE_CONNECTED)
+                kick_pending_request_queues(info);
+        spin_unlock_irq(&blkif_io_lock);
+}
+
+static void blkif_free(struct blkfront_info *info, int suspend)
+{
+        /* Prevent new requests being issued until we fix things up. */
+        spin_lock_irq(&blkif_io_lock);
+        info->connected = suspend ?
+                BLKIF_STATE_SUSPENDED : BLKIF_STATE_DISCONNECTED;
+        /* No more blkif_request(). */
+        if (info->rq)
+                blk_stop_queue(info->rq);
+        /* No more gnttab callback work. */
+        gnttab_cancel_free_callback(&info->callback);
+        spin_unlock_irq(&blkif_io_lock);
+
+        /* Flush gnttab callback work. Must be done with no locks held. */
+        flush_scheduled_work();
+
+        /* Free resources associated with old device channel. */
+        if (info->ring_ref != GRANT_INVALID_REF) {
+                gnttab_end_foreign_access(info->ring_ref, 0,
+                                          (unsigned long)info->ring.sring);
+                info->ring_ref = GRANT_INVALID_REF;
+                info->ring.sring = NULL;
+        }
+        if (info->irq)
+                unbind_from_irqhandler(info->irq, info);
+        info->evtchn = info->irq = 0;
+
+}
+
+static void blkif_completion(struct blk_shadow *s)
+{
+        int i;
+        for (i = 0; i < s->req.nr_segments; i++)
+                gnttab_end_foreign_access(s->req.seg[i].gref, 0, 0UL);
+}
+
+static irqreturn_t blkif_interrupt(int irq, void *dev_id)
+{
+        struct request *req;
+        struct blkif_response *bret;
+        RING_IDX i, rp;
+        unsigned long flags;
+        struct blkfront_info *info = (struct blkfront_info *)dev_id;
+        int uptodate;
+
+        spin_lock_irqsave(&blkif_io_lock, flags);
+
+        if (unlikely(info->connected != BLKIF_STATE_CONNECTED)) {
+                spin_unlock_irqrestore(&blkif_io_lock, flags);
+                return IRQ_HANDLED;
+        }
+
+ again:
+        rp = info->ring.sring->rsp_prod;
+        rmb(); /* Ensure we see queued responses up to 'rp'. */
+
+        for (i = info->ring.rsp_cons; i != rp; i++) {
+                unsigned long id;
+                int ret;
+
+                bret = RING_GET_RESPONSE(&info->ring, i);
+                id   = bret->id;
+                req  = (struct request *)info->shadow[id].request;
+
+                blkif_completion(&info->shadow[id]);
+
+                add_id_to_freelist(info, id);
+
+                uptodate = (bret->status == BLKIF_RSP_OKAY);
+                switch (bret->operation) {
+                case BLKIF_OP_WRITE_BARRIER:
+                        if (unlikely(bret->status == BLKIF_RSP_EOPNOTSUPP)) {
+                                printk(KERN_WARNING "blkfront: %s: write barrier op failed\n",
+                                       info->gd->disk_name);
+                                uptodate = -EOPNOTSUPP;
+                                info->feature_barrier = 0;
+                                xlvbd_barrier(info);
+                        }
+                        /* fall through */
+                case BLKIF_OP_READ:
+                case BLKIF_OP_WRITE:
+                        if (unlikely(bret->status != BLKIF_RSP_OKAY))
+                                dev_dbg(&info->xbdev->dev, "Bad return from blkdev data "
+                                        "request: %x\n", bret->status);
+
+                        ret = end_that_request_first(req, uptodate,
+                                req->hard_nr_sectors);
+                        BUG_ON(ret);
+                        end_that_request_last(req, uptodate);
+                        break;
+                default:
+                        BUG();
+                }
+        }
+
+        info->ring.rsp_cons = i;
+
+        if (i != info->ring.req_prod_pvt) {
+                int more_to_do;
+                RING_FINAL_CHECK_FOR_RESPONSES(&info->ring, more_to_do);
+                if (more_to_do)
+                        goto again;
+        } else
+                info->ring.sring->rsp_event = i + 1;
+
+        kick_pending_request_queues(info);
+
+        spin_unlock_irqrestore(&blkif_io_lock, flags);
+
+        return IRQ_HANDLED;
+}
+
+
+static int setup_blkring(struct xenbus_device *dev,
+                         struct blkfront_info *info)
+{
+        struct blkif_sring *sring;
+        int err;
+
+        info->ring_ref = GRANT_INVALID_REF;
+
+        sring = (struct blkif_sring *)__get_free_page(GFP_KERNEL);
+        if (!sring) {
+                xenbus_dev_fatal(dev, -ENOMEM, "allocating shared ring");
+                return -ENOMEM;
+        }
+        SHARED_RING_INIT(sring);
+        FRONT_RING_INIT(&info->ring, sring, PAGE_SIZE);
+
+        err = xenbus_grant_ring(dev, virt_to_mfn(info->ring.sring));
+        if (err < 0) {
+                free_page((unsigned long)sring);
+                info->ring.sring = NULL;
+                goto fail;
+        }
+        info->ring_ref = err;
+
+        err = xenbus_alloc_evtchn(dev, &info->evtchn);
+        if (err)
+                goto fail;
+
+        err = bind_evtchn_to_irqhandler(info->evtchn,
+                                        blkif_interrupt,
+                                        IRQF_SAMPLE_RANDOM, "blkif", info);
+        if (err <= 0) {
+                xenbus_dev_fatal(dev, err,
+                                 "bind_evtchn_to_irqhandler failed");
+                goto fail;
+        }
+        info->irq = err;
+
+        return 0;
+fail:
+        blkif_free(info, 0);
+        return err;
+}
+
+
+/* Common code used when first setting up, and when resuming. */
+static int talk_to_backend(struct xenbus_device *dev,
+                           struct blkfront_info *info)
+{
+        const char *message = NULL;
+        struct xenbus_transaction xbt;
+        int err;
+
+        /* Create shared ring, alloc event channel. */
+        err = setup_blkring(dev, info);
+        if (err)
+                goto out;
+
+again:
+        err = xenbus_transaction_start(&xbt);
+        if (err) {
+                xenbus_dev_fatal(dev, err, "starting transaction");
+                goto destroy_blkring;
+        }
+
+        err = xenbus_printf(xbt, dev->nodename,
+                            "ring-ref", "%u", info->ring_ref);
+        if (err) {
+                message = "writing ring-ref";
+                goto abort_transaction;
+        }
+        err = xenbus_printf(xbt, dev->nodename,
+                            "event-channel", "%u", info->evtchn);
+        if (err) {
+                message = "writing event-channel";
+                goto abort_transaction;
+        }
+
+        err = xenbus_transaction_end(xbt, 0);
+        if (err) {
+                if (err == -EAGAIN)
+                        goto again;
+                xenbus_dev_fatal(dev, err, "completing transaction");
+                goto destroy_blkring;
+        }
+
+        xenbus_switch_state(dev, XenbusStateInitialised);
+
+        return 0;
+
+ abort_transaction:
+        xenbus_transaction_end(xbt, 1);
+        if (message)
+                xenbus_dev_fatal(dev, err, "%s", message);
+ destroy_blkring:
+        blkif_free(info, 0);
+ out:
+        return err;
+}
+
+
+/**
+ * Entry point to this code when a new device is created.  Allocate the basic
+ * structures and the ring buffer for communication with the backend, and
+ * inform the backend of the appropriate details for those.  Switch to
+ * Initialised state.
+ */
+static int blkfront_probe(struct xenbus_device *dev,
+                          const struct xenbus_device_id *id)
+{
+        int err, vdevice, i;
+        struct blkfront_info *info;
+
+        /* FIXME: Use dynamic device id if this is not set. */
+        err = xenbus_scanf(XBT_NIL, dev->nodename,
+                           "virtual-device", "%i", &vdevice);
+        if (err != 1) {
+                xenbus_dev_fatal(dev, err, "reading virtual-device");
+                return err;
+        }
+
+        info = kzalloc(sizeof(*info), GFP_KERNEL);
+        if (!info) {
+                xenbus_dev_fatal(dev, -ENOMEM, "allocating info structure");
+                return -ENOMEM;
+        }
+
+        info->xbdev = dev;
+        info->vdevice = vdevice;
+        info->connected = BLKIF_STATE_DISCONNECTED;
+        INIT_WORK(&info->work, blkif_restart_queue);
+
+        for (i = 0; i < BLK_RING_SIZE; i++)
+                info->shadow[i].req.id = i+1;
+        info->shadow[BLK_RING_SIZE-1].req.id = 0x0fffffff;
+
+        /* Front end dir is a number, which is used as the id. */
+        info->handle = simple_strtoul(strrchr(dev->nodename, '/')+1, NULL, 0);
+        dev->dev.driver_data = info;
+
+        err = talk_to_backend(dev, info);
+        if (err) {
+                kfree(info);
+                dev->dev.driver_data = NULL;
+                return err;
+        }
+
+        return 0;
+}
+
+
+static int blkif_recover(struct blkfront_info *info)
+{
+        int i;
+        struct blkif_request *req;
+        struct blk_shadow *copy;
+        int j;
+
+        /* Stage 1: Make a safe copy of the shadow state. */
+        copy = kmalloc(sizeof(info->shadow), GFP_KERNEL);
+        if (!copy)
+                return -ENOMEM;
+        memcpy(copy, info->shadow, sizeof(info->shadow));
+
+        /* Stage 2: Set up free list. */
+        memset(&info->shadow, 0, sizeof(info->shadow));
+        for (i = 0; i < BLK_RING_SIZE; i++)
+                info->shadow[i].req.id = i+1;
+        info->shadow_free = info->ring.req_prod_pvt;
+        info->shadow[BLK_RING_SIZE-1].req.id = 0x0fffffff;
+
+        /* Stage 3: Find pending requests and requeue them. */
+        for (i = 0; i < BLK_RING_SIZE; i++) {
+                /* Not in use? */
+                if (copy[i].request == 0)
+                        continue;
+
+                /* Grab a request slot and copy shadow state into it. */
+                req = RING_GET_REQUEST(&info->ring, info->ring.req_prod_pvt);
+                *req = copy[i].req;
+
+                /* We get a new request id, and must reset the shadow state. */
+                req->id = get_id_from_freelist(info);
+                memcpy(&info->shadow[req->id], &copy[i], sizeof(copy[i]));
+
+                /* Rewrite any grant references invalidated by susp/resume. */
+                for (j = 0; j < req->nr_segments; j++)
+                        gnttab_grant_foreign_access_ref(
+                                req->seg[j].gref,
+                                info->xbdev->otherend_id,
+                                pfn_to_mfn(info->shadow[req->id].frame[j]),
+                                rq_data_dir(
+                                        (struct request *)
+                                        info->shadow[req->id].request));
+                info->shadow[req->id].req = *req;
+
+                info->ring.req_prod_pvt++;
+        }
+
+        kfree(copy);
+
+        xenbus_switch_state(info->xbdev, XenbusStateConnected);
+
+        spin_lock_irq(&blkif_io_lock);
+
+        /* Now safe for us to use the shared ring */
+        info->connected = BLKIF_STATE_CONNECTED;
+
+        /* Send off requeued requests */
+        flush_requests(info);
+
+        /* Kick any other new requests queued since we resumed */
+        kick_pending_request_queues(info);
+
+        spin_unlock_irq(&blkif_io_lock);
+
+        return 0;
+}
+
+/**
+ * We are reconnecting to the backend, due to a suspend/resume, or a backend
+ * driver restart.  We tear down our blkif structure and recreate it, but
+ * leave the device-layer structures intact so that this is transparent to the
+ * rest of the kernel.
+ */
+static int blkfront_resume(struct xenbus_device *dev)
+{
+        struct blkfront_info *info = dev->dev.driver_data;
+        int err;
+
+        dev_dbg(&dev->dev, "blkfront_resume: %s\n", dev->nodename);
+
+        blkif_free(info, info->connected == BLKIF_STATE_CONNECTED);
+
+        err = talk_to_backend(dev, info);
+        if (info->connected == BLKIF_STATE_SUSPENDED && !err)
+                err = blkif_recover(info);
+
+        return err;
+}
+
+
+/*
+ * Invoked when the backend is finally 'ready' (and has told produced
+ * the details about the physical device - #sectors, size, etc).
+ */
+static void blkfront_connect(struct blkfront_info *info)
+{
+        unsigned long long sectors;
+        unsigned long sector_size;
+        unsigned int binfo;
+        int err;
+
+        if ((info->connected == BLKIF_STATE_CONNECTED) ||
+            (info->connected == BLKIF_STATE_SUSPENDED) )
+                return;
+
+        dev_dbg(&info->xbdev->dev, "%s:%s.\n",
+                __func__, info->xbdev->otherend);
+
+        err = xenbus_gather(XBT_NIL, info->xbdev->otherend,
+                            "sectors", "%llu", &sectors,
+                            "info", "%u", &binfo,
+                            "sector-size", "%lu", &sector_size,
+                            NULL);
+        if (err) {
+                xenbus_dev_fatal(info->xbdev, err,
+                                 "reading backend fields at %s",
+                                 info->xbdev->otherend);
+                return;
+        }
+
+        err = xenbus_gather(XBT_NIL, info->xbdev->otherend,
+                            "feature-barrier", "%lu", &info->feature_barrier,
+                            NULL);
+        if (err)
+                info->feature_barrier = 0;
+
+        err = xlvbd_alloc_gendisk(BLKIF_MINOR(info->vdevice),
+                                  sectors, info->vdevice,
+                                  binfo, sector_size, info);
+        if (err) {
+                xenbus_dev_fatal(info->xbdev, err, "xlvbd_add at %s",
+                                 info->xbdev->otherend);
+                return;
+        }
+
+        xenbus_switch_state(info->xbdev, XenbusStateConnected);
+
+        /* Kick pending requests. */
+        spin_lock_irq(&blkif_io_lock);
+        info->connected = BLKIF_STATE_CONNECTED;
+        kick_pending_request_queues(info);
+        spin_unlock_irq(&blkif_io_lock);
+
+        add_disk(info->gd);
+}
+
+/**
+ * Handle the change of state of the backend to Closing.  We must delete our
+ * device-layer structures now, to ensure that writes are flushed through to
+ * the backend.  Once is this done, we can switch to Closed in
+ * acknowledgement.
+ */
+static void blkfront_closing(struct xenbus_device *dev)
+{
+        struct blkfront_info *info = dev->dev.driver_data;
+        unsigned long flags;
+
+        dev_dbg(&dev->dev, "blkfront_closing: %s removed\n", dev->nodename);
+
+        if (info->rq == NULL)
+                goto out;
+
+        spin_lock_irqsave(&blkif_io_lock, flags);
+
+        del_gendisk(info->gd);
+
+        /* No more blkif_request(). */
+        blk_stop_queue(info->rq);
+
+        /* No more gnttab callback work. */
+        gnttab_cancel_free_callback(&info->callback);
+        spin_unlock_irqrestore(&blkif_io_lock, flags);
+
+        /* Flush gnttab callback work. Must be done with no locks held. */
+        flush_scheduled_work();
+
+        blk_cleanup_queue(info->rq);
+        info->rq = NULL;
+
+ out:
+        xenbus_frontend_closed(dev);
+}
+
+/**
+ * Callback received when the backend's state changes.
+ */
+static void backend_changed(struct xenbus_device *dev,
+                            enum xenbus_state backend_state)
+{
+        struct blkfront_info *info = dev->dev.driver_data;
+        struct block_device *bd;
+
+        dev_dbg(&dev->dev, "blkfront:backend_changed.\n");
+
+        switch (backend_state) {
+        case XenbusStateInitialising:
+        case XenbusStateInitWait:
+        case XenbusStateInitialised:
+        case XenbusStateUnknown:
+        case XenbusStateClosed:
+                break;
+
+        case XenbusStateConnected:
+                blkfront_connect(info);
+                break;
+
+        case XenbusStateClosing:
+                bd = bdget(info->dev);
+                if (bd == NULL)
+                        xenbus_dev_fatal(dev, -ENODEV, "bdget failed");
+
+                mutex_lock(&bd->bd_mutex);
+                if (info->users > 0)
+                        xenbus_dev_error(dev, -EBUSY,
+                                         "Device in use; refusing to close");
+                else
+                        blkfront_closing(dev);
+                mutex_unlock(&bd->bd_mutex);
+                bdput(bd);
+                break;
+        }
+}
+
+static int blkfront_remove(struct xenbus_device *dev)
+{
+        struct blkfront_info *info = dev->dev.driver_data;
+
+        dev_dbg(&dev->dev, "blkfront_remove: %s removed\n", dev->nodename);
+
+        blkif_free(info, 0);
+
+        kfree(info);
+
+        return 0;
+}
+
+static int blkif_open(struct inode *inode, struct file *filep)
+{
+        struct blkfront_info *info = inode->i_bdev->bd_disk->private_data;
+        info->users++;
+        return 0;
+}
+
+static int blkif_release(struct inode *inode, struct file *filep)
+{
+        struct blkfront_info *info = inode->i_bdev->bd_disk->private_data;
+        info->users--;
+        if (info->users == 0) {
+                /* Check whether we have been instructed to close.  We will
+                   have ignored this request initially, as the device was
+                   still mounted. */
+                struct xenbus_device *dev = info->xbdev;
+                enum xenbus_state state = xenbus_read_driver_state(dev->otherend);
+
+                if (state == XenbusStateClosing)
+                        blkfront_closing(dev);
+        }
+        return 0;
+}
+
+static struct block_device_operations xlvbd_block_fops =
+{
+        .owner = THIS_MODULE,
+        .open = blkif_open,
+        .release = blkif_release,
+};
+
+
+static struct xenbus_device_id blkfront_ids[] = {
+        { "vbd" },
+        { "" }
+};
+
+static struct xenbus_driver blkfront = {
+        .name = "vbd",
+        .owner = THIS_MODULE,
+        .ids = blkfront_ids,
+        .probe = blkfront_probe,
+        .remove = blkfront_remove,
+        .resume = blkfront_resume,
+        .otherend_changed = backend_changed,
+};
+
+static int __init xlblk_init(void)
+{
+        if (!is_running_on_xen())
+                return -ENODEV;
+
+        if (register_blkdev(XENVBD_MAJOR, DEV_NAME)) {
+                printk(KERN_WARNING "xen_blk: can't get major %d with name %s\n",
+                       XENVBD_MAJOR, DEV_NAME);
+                return -ENODEV;
+        }
+
+        return xenbus_register_frontend(&blkfront);
+}
+module_init(xlblk_init);
+
+
+static void xlblk_exit(void)
+{
+        return xenbus_unregister_driver(&blkfront);
+}
+module_exit(xlblk_exit);
+
+MODULE_DESCRIPTION("Xen virtual block device frontend");
+MODULE_LICENSE("GPL");
+MODULE_ALIAS_BLOCKDEV_MAJOR(XENVBD_MAJOR);
diff --git a/include/linux/major.h b/include/linux/major.h
index 7e7c9093919a..0cb98053537a 100644
--- a/include/linux/major.h
+++ b/include/linux/major.h
@@ -158,6 +158,8 @@
 #define VXSPEC_MAJOR            200     /* VERITAS volume config driver */
 #define VXDMP_MAJOR             201     /* VERITAS volume multipath driver */
 
+#define XENVBD_MAJOR            202     /* Xen virtual block device */
+
 #define MSR_MAJOR               202
 #define CPUID_MAJOR             203