xen-scsifront.c

/*
 * Xen SCSI frontend driver
 *
 * Copyright (c) 2008, FUJITSU Limited
 *
 * 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/module.h>
#include <linux/kernel.h>
#include <linux/device.h>
#include <linux/wait.h>
#include <linux/interrupt.h>
#include <linux/mutex.h>
#include <linux/spinlock.h>
#include <linux/sched.h>
#include <linux/blkdev.h>
#include <linux/pfn.h>
#include <linux/slab.h>
#include <linux/bitops.h>

#include <scsi/scsi_cmnd.h>
#include <scsi/scsi_device.h>
#include <scsi/scsi.h>
#include <scsi/scsi_host.h>

#include <xen/xen.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/vscsiif.h>
#include <xen/interface/io/protocols.h>

#include <asm/xen/hypervisor.h>


#define GRANT_INVALID_REF	0

#define VSCSIFRONT_OP_ADD_LUN	1
#define VSCSIFRONT_OP_DEL_LUN	2

/* Tuning point. */
#define VSCSIIF_DEFAULT_CMD_PER_LUN 10
#define VSCSIIF_MAX_TARGET          64
#define VSCSIIF_MAX_LUN             255

#define VSCSIIF_RING_SIZE	__CONST_RING_SIZE(vscsiif, PAGE_SIZE)
#define VSCSIIF_MAX_REQS	VSCSIIF_RING_SIZE

#define vscsiif_grants_sg(_sg)	(PFN_UP((_sg) *		\
				sizeof(struct scsiif_request_segment)))

struct vscsifrnt_shadow {
	/* command between backend and frontend */
	unsigned char act;
	uint16_t rqid;

	unsigned int nr_grants;		/* number of grants in gref[] */
	struct scsiif_request_segment *sg;	/* scatter/gather elements */

	/* Do reset or abort function. */
	wait_queue_head_t wq_reset;	/* reset work queue           */
	int wait_reset;			/* reset work queue condition */
	int32_t rslt_reset;		/* reset response status:     */
					/* SUCCESS or FAILED or:      */
#define RSLT_RESET_WAITING	0
#define RSLT_RESET_ERR		-1

	/* Requested struct scsi_cmnd is stored from kernel. */
	struct scsi_cmnd *sc;
	int gref[vscsiif_grants_sg(SG_ALL) + SG_ALL];
};

struct vscsifrnt_info {
	struct xenbus_device *dev;

	struct Scsi_Host *host;
	int host_active;

	unsigned int evtchn;
	unsigned int irq;

	grant_ref_t ring_ref;
	struct vscsiif_front_ring ring;
	struct vscsiif_response	ring_rsp;

	spinlock_t shadow_lock;
	DECLARE_BITMAP(shadow_free_bitmap, VSCSIIF_MAX_REQS);
	struct vscsifrnt_shadow *shadow[VSCSIIF_MAX_REQS];

	wait_queue_head_t wq_sync;
	unsigned int wait_ring_available:1;

	char dev_state_path[64];
	struct task_struct *curr;
};

static DEFINE_MUTEX(scsifront_mutex);

static void scsifront_wake_up(struct vscsifrnt_info *info)
{
	info->wait_ring_available = 0;
	wake_up(&info->wq_sync);
}

static int scsifront_get_rqid(struct vscsifrnt_info *info)
{
	unsigned long flags;
	int free;

	spin_lock_irqsave(&info->shadow_lock, flags);

	free = find_first_bit(info->shadow_free_bitmap, VSCSIIF_MAX_REQS);
	__clear_bit(free, info->shadow_free_bitmap);

	spin_unlock_irqrestore(&info->shadow_lock, flags);

	return free;
}

static int _scsifront_put_rqid(struct vscsifrnt_info *info, uint32_t id)
{
	int empty = bitmap_empty(info->shadow_free_bitmap, VSCSIIF_MAX_REQS);

	__set_bit(id, info->shadow_free_bitmap);
	info->shadow[id] = NULL;

	return empty || info->wait_ring_available;
}

static void scsifront_put_rqid(struct vscsifrnt_info *info, uint32_t id)
{
	unsigned long flags;
	int kick;

	spin_lock_irqsave(&info->shadow_lock, flags);
	kick = _scsifront_put_rqid(info, id);
	spin_unlock_irqrestore(&info->shadow_lock, flags);

	if (kick)
		scsifront_wake_up(info);
}

static struct vscsiif_request *scsifront_pre_req(struct vscsifrnt_info *info)
{
	struct vscsiif_front_ring *ring = &(info->ring);
	struct vscsiif_request *ring_req;
	uint32_t id;

	id = scsifront_get_rqid(info);	/* use id in response */
	if (id >= VSCSIIF_MAX_REQS)
		return NULL;

	ring_req = RING_GET_REQUEST(&(info->ring), ring->req_prod_pvt);

	ring->req_prod_pvt++;

	ring_req->rqid = (uint16_t)id;

	return ring_req;
}

static void scsifront_do_request(struct vscsifrnt_info *info)
{
	struct vscsiif_front_ring *ring = &(info->ring);
	int notify;

	RING_PUSH_REQUESTS_AND_CHECK_NOTIFY(ring, notify);
	if (notify)
		notify_remote_via_irq(info->irq);
}

static void scsifront_gnttab_done(struct vscsifrnt_info *info, uint32_t id)
{
	struct vscsifrnt_shadow *s = info->shadow[id];
	int i;

	if (s->sc->sc_data_direction == DMA_NONE)
		return;

	for (i = 0; i < s->nr_grants; i++) {
		if (unlikely(gnttab_query_foreign_access(s->gref[i]) != 0)) {
			shost_printk(KERN_ALERT, info->host, KBUILD_MODNAME
				     "grant still in use by backend\n");
			BUG();
		}
		gnttab_end_foreign_access(s->gref[i], 0, 0UL);
	}

	kfree(s->sg);
}

static void scsifront_cdb_cmd_done(struct vscsifrnt_info *info,
				   struct vscsiif_response *ring_rsp)
{
	struct scsi_cmnd *sc;
	uint32_t id;
	uint8_t sense_len;

	id = ring_rsp->rqid;
	sc = info->shadow[id]->sc;

	BUG_ON(sc == NULL);

	scsifront_gnttab_done(info, id);
	scsifront_put_rqid(info, id);

	sc->result = ring_rsp->rslt;
	scsi_set_resid(sc, ring_rsp->residual_len);

	sense_len = min_t(uint8_t, VSCSIIF_SENSE_BUFFERSIZE,
			  ring_rsp->sense_len);

	if (sense_len)
		memcpy(sc->sense_buffer, ring_rsp->sense_buffer, sense_len);

	sc->scsi_done(sc);
}

static void scsifront_sync_cmd_done(struct vscsifrnt_info *info,
				    struct vscsiif_response *ring_rsp)
{
	uint16_t id = ring_rsp->rqid;
	unsigned long flags;
	struct vscsifrnt_shadow *shadow = info->shadow[id];
	int kick;

	spin_lock_irqsave(&info->shadow_lock, flags);
	shadow->wait_reset = 1;
	switch (shadow->rslt_reset) {
	case RSLT_RESET_WAITING:
		shadow->rslt_reset = ring_rsp->rslt;
		break;
	case RSLT_RESET_ERR:
		kick = _scsifront_put_rqid(info, id);
		spin_unlock_irqrestore(&info->shadow_lock, flags);
		kfree(shadow);
		if (kick)
			scsifront_wake_up(info);
		return;
	default:
		shost_printk(KERN_ERR, info->host, KBUILD_MODNAME
			     "bad reset state %d, possibly leaking %u\n",
			     shadow->rslt_reset, id);
		break;
	}
	spin_unlock_irqrestore(&info->shadow_lock, flags);

	wake_up(&shadow->wq_reset);
}

static int scsifront_cmd_done(struct vscsifrnt_info *info)
{
	struct vscsiif_response *ring_rsp;
	RING_IDX i, rp;
	int more_to_do = 0;
	unsigned long flags;

	spin_lock_irqsave(info->host->host_lock, flags);

	rp = info->ring.sring->rsp_prod;
	rmb();	/* ordering required respective to dom0 */
	for (i = info->ring.rsp_cons; i != rp; i++) {

		ring_rsp = RING_GET_RESPONSE(&info->ring, i);

		if (WARN(ring_rsp->rqid >= VSCSIIF_MAX_REQS ||
			 test_bit(ring_rsp->rqid, info->shadow_free_bitmap),
			 "illegal rqid %u returned by backend!\n",
			 ring_rsp->rqid))
			continue;

		if (info->shadow[ring_rsp->rqid]->act == VSCSIIF_ACT_SCSI_CDB)
			scsifront_cdb_cmd_done(info, ring_rsp);
		else
			scsifront_sync_cmd_done(info, ring_rsp);
	}

	info->ring.rsp_cons = i;

	if (i != info->ring.req_prod_pvt)
		RING_FINAL_CHECK_FOR_RESPONSES(&info->ring, more_to_do);
	else
		info->ring.sring->rsp_event = i + 1;

	info->wait_ring_available = 0;

	spin_unlock_irqrestore(info->host->host_lock, flags);

	wake_up(&info->wq_sync);

	return more_to_do;
}

static irqreturn_t scsifront_irq_fn(int irq, void *dev_id)
{
	struct vscsifrnt_info *info = dev_id;

	while (scsifront_cmd_done(info))
		/* Yield point for this unbounded loop. */
		cond_resched();

	return IRQ_HANDLED;
}

static int map_data_for_request(struct vscsifrnt_info *info,
				struct scsi_cmnd *sc,
				struct vscsiif_request *ring_req,
				struct vscsifrnt_shadow *shadow)
{
	grant_ref_t gref_head;
	struct page *page;
	int err, ref, ref_cnt = 0;
	int grant_ro = (sc->sc_data_direction == DMA_TO_DEVICE);
	unsigned int i, off, len, bytes;
	unsigned int data_len = scsi_bufflen(sc);
	unsigned int data_grants = 0, seg_grants = 0;
	struct scatterlist *sg;
	unsigned long mfn;
	struct scsiif_request_segment *seg;

	ring_req->nr_segments = 0;
	if (sc->sc_data_direction == DMA_NONE || !data_len)
		return 0;

	scsi_for_each_sg(sc, sg, scsi_sg_count(sc), i)
		data_grants += PFN_UP(sg->offset + sg->length);

	if (data_grants > VSCSIIF_SG_TABLESIZE) {
		if (data_grants > info->host->sg_tablesize) {
			shost_printk(KERN_ERR, info->host, KBUILD_MODNAME
			     "Unable to map request_buffer for command!\n");
			return -E2BIG;
		}
		seg_grants = vscsiif_grants_sg(data_grants);
		shadow->sg = kcalloc(data_grants,
			sizeof(struct scsiif_request_segment), GFP_ATOMIC);
		if (!shadow->sg)
			return -ENOMEM;
	}
	seg = shadow->sg ? : ring_req->seg;

	err = gnttab_alloc_grant_references(seg_grants + data_grants,
					    &gref_head);
	if (err) {
		kfree(shadow->sg);
		shost_printk(KERN_ERR, info->host, KBUILD_MODNAME
			     "gnttab_alloc_grant_references() error\n");
		return -ENOMEM;
	}

	if (seg_grants) {
		page = virt_to_page(seg);
		off = (unsigned long)seg & ~PAGE_MASK;
		len = sizeof(struct scsiif_request_segment) * data_grants;
		while (len > 0) {
			bytes = min_t(unsigned int, len, PAGE_SIZE - off);

			ref = gnttab_claim_grant_reference(&gref_head);
			BUG_ON(ref == -ENOSPC);

			mfn = pfn_to_mfn(page_to_pfn(page));
			gnttab_grant_foreign_access_ref(ref,
				info->dev->otherend_id, mfn, 1);
			shadow->gref[ref_cnt] = ref;
			ring_req->seg[ref_cnt].gref   = ref;
			ring_req->seg[ref_cnt].offset = (uint16_t)off;
			ring_req->seg[ref_cnt].length = (uint16_t)bytes;

			page++;
			len -= bytes;
			off = 0;
			ref_cnt++;
		}
		BUG_ON(seg_grants < ref_cnt);
		seg_grants = ref_cnt;
	}

	scsi_for_each_sg(sc, sg, scsi_sg_count(sc), i) {
		page = sg_page(sg);
		off = sg->offset;
		len = sg->length;

		while (len > 0 && data_len > 0) {
			/*
			 * sg sends a scatterlist that is larger than
			 * the data_len it wants transferred for certain
			 * IO sizes.
			 */
			bytes = min_t(unsigned int, len, PAGE_SIZE - off);
			bytes = min(bytes, data_len);

			ref = gnttab_claim_grant_reference(&gref_head);
			BUG_ON(ref == -ENOSPC);

			mfn = pfn_to_mfn(page_to_pfn(page));
			gnttab_grant_foreign_access_ref(ref,
				info->dev->otherend_id, mfn, grant_ro);

			shadow->gref[ref_cnt] = ref;
			seg->gref   = ref;
			seg->offset = (uint16_t)off;
			seg->length = (uint16_t)bytes;

			page++;
			seg++;
			len -= bytes;
			data_len -= bytes;
			off = 0;
			ref_cnt++;
		}
	}

	if (seg_grants)
		ring_req->nr_segments = VSCSIIF_SG_GRANT | seg_grants;
	else
		ring_req->nr_segments = (uint8_t)ref_cnt;
	shadow->nr_grants = ref_cnt;

	return 0;
}

static struct vscsiif_request *scsifront_command2ring(
		struct vscsifrnt_info *info, struct scsi_cmnd *sc,
		struct vscsifrnt_shadow *shadow)
{
	struct vscsiif_request *ring_req;

	memset(shadow, 0, sizeof(*shadow));

	ring_req = scsifront_pre_req(info);
	if (!ring_req)
		return NULL;

	info->shadow[ring_req->rqid] = shadow;
	shadow->rqid = ring_req->rqid;

	ring_req->id      = sc->device->id;
	ring_req->lun     = sc->device->lun;
	ring_req->channel = sc->device->channel;
	ring_req->cmd_len = sc->cmd_len;

	BUG_ON(sc->cmd_len > VSCSIIF_MAX_COMMAND_SIZE);

	memcpy(ring_req->cmnd, sc->cmnd, sc->cmd_len);

	ring_req->sc_data_direction   = (uint8_t)sc->sc_data_direction;
	ring_req->timeout_per_command = sc->request->timeout / HZ;

	return ring_req;
}

static int scsifront_queuecommand(struct Scsi_Host *shost,
				  struct scsi_cmnd *sc)
{
	struct vscsifrnt_info *info = shost_priv(shost);
	struct vscsiif_request *ring_req;
	struct vscsifrnt_shadow *shadow = scsi_cmd_priv(sc);
	unsigned long flags;
	int err;
	uint16_t rqid;

	spin_lock_irqsave(shost->host_lock, flags);
	if (RING_FULL(&info->ring))
		goto busy;

	ring_req = scsifront_command2ring(info, sc, shadow);
	if (!ring_req)
		goto busy;

	sc->result = 0;

	rqid = ring_req->rqid;
	ring_req->act = VSCSIIF_ACT_SCSI_CDB;

	shadow->sc  = sc;
	shadow->act = VSCSIIF_ACT_SCSI_CDB;

	err = map_data_for_request(info, sc, ring_req, shadow);
	if (err < 0) {
		pr_debug("%s: err %d\n", __func__, err);
		scsifront_put_rqid(info, rqid);
		spin_unlock_irqrestore(shost->host_lock, flags);
		if (err == -ENOMEM)
			return SCSI_MLQUEUE_HOST_BUSY;
		sc->result = DID_ERROR << 16;
		sc->scsi_done(sc);
		return 0;
	}

	scsifront_do_request(info);
	spin_unlock_irqrestore(shost->host_lock, flags);

	return 0;

busy:
	spin_unlock_irqrestore(shost->host_lock, flags);
	pr_debug("%s: busy\n", __func__);
	return SCSI_MLQUEUE_HOST_BUSY;
}

/*
 * Any exception handling (reset or abort) must be forwarded to the backend.
 * We have to wait until an answer is returned. This answer contains the
 * result to be returned to the requestor.
 */
static int scsifront_action_handler(struct scsi_cmnd *sc, uint8_t act)
{
	struct Scsi_Host *host = sc->device->host;
	struct vscsifrnt_info *info = shost_priv(host);
	struct vscsifrnt_shadow *shadow, *s = scsi_cmd_priv(sc);
	struct vscsiif_request *ring_req;
	int err = 0;

	shadow = kmalloc(sizeof(*shadow), GFP_NOIO);
	if (!shadow)
		return FAILED;

	spin_lock_irq(host->host_lock);

	for (;;) {
		if (!RING_FULL(&info->ring)) {
			ring_req = scsifront_command2ring(info, sc, shadow);
			if (ring_req)
				break;
		}
		if (err) {
			spin_unlock_irq(host->host_lock);
			kfree(shadow);
			return FAILED;
		}
		info->wait_ring_available = 1;
		spin_unlock_irq(host->host_lock);
		err = wait_event_interruptible(info->wq_sync,
					       !info->wait_ring_available);
		spin_lock_irq(host->host_lock);
	}

	ring_req->act = act;
	ring_req->ref_rqid = s->rqid;

	shadow->act = act;
	shadow->rslt_reset = RSLT_RESET_WAITING;
	init_waitqueue_head(&shadow->wq_reset);

	ring_req->nr_segments = 0;

	scsifront_do_request(info);

	spin_unlock_irq(host->host_lock);
	err = wait_event_interruptible(shadow->wq_reset, shadow->wait_reset);
	spin_lock_irq(host->host_lock);

	if (!err) {
		err = shadow->rslt_reset;
		scsifront_put_rqid(info, shadow->rqid);
		kfree(shadow);
	} else {
		spin_lock(&info->shadow_lock);
		shadow->rslt_reset = RSLT_RESET_ERR;
		spin_unlock(&info->shadow_lock);
		err = FAILED;
	}

	spin_unlock_irq(host->host_lock);
	return err;
}

static int scsifront_eh_abort_handler(struct scsi_cmnd *sc)
{
	pr_debug("%s\n", __func__);
	return scsifront_action_handler(sc, VSCSIIF_ACT_SCSI_ABORT);
}

static int scsifront_dev_reset_handler(struct scsi_cmnd *sc)
{
	pr_debug("%s\n", __func__);
	return scsifront_action_handler(sc, VSCSIIF_ACT_SCSI_RESET);
}

static int scsifront_sdev_configure(struct scsi_device *sdev)
{
	struct vscsifrnt_info *info = shost_priv(sdev->host);

	if (info && current == info->curr)
		xenbus_printf(XBT_NIL, info->dev->nodename,
			      info->dev_state_path, "%d", XenbusStateConnected);

	return 0;
}

static void scsifront_sdev_destroy(struct scsi_device *sdev)
{
	struct vscsifrnt_info *info = shost_priv(sdev->host);

	if (info && current == info->curr)
		xenbus_printf(XBT_NIL, info->dev->nodename,
			      info->dev_state_path, "%d", XenbusStateClosed);
}

static struct scsi_host_template scsifront_sht = {
	.module			= THIS_MODULE,
	.name			= "Xen SCSI frontend driver",
	.queuecommand		= scsifront_queuecommand,
	.eh_abort_handler	= scsifront_eh_abort_handler,
	.eh_device_reset_handler = scsifront_dev_reset_handler,
	.slave_configure	= scsifront_sdev_configure,
	.slave_destroy		= scsifront_sdev_destroy,
	.cmd_per_lun		= VSCSIIF_DEFAULT_CMD_PER_LUN,
	.can_queue		= VSCSIIF_MAX_REQS,
	.this_id		= -1,
	.cmd_size		= sizeof(struct vscsifrnt_shadow),
	.sg_tablesize		= VSCSIIF_SG_TABLESIZE,
	.use_clustering		= DISABLE_CLUSTERING,
	.proc_name		= "scsifront",
};

static int scsifront_alloc_ring(struct vscsifrnt_info *info)
{
	struct xenbus_device *dev = info->dev;
	struct vscsiif_sring *sring;
	int err = -ENOMEM;

	/***** Frontend to Backend ring start *****/
	sring = (struct vscsiif_sring *)__get_free_page(GFP_KERNEL);
	if (!sring) {
		xenbus_dev_fatal(dev, err,
			"fail to allocate shared ring (Front to Back)");
		return err;
	}
	SHARED_RING_INIT(sring);
	FRONT_RING_INIT(&info->ring, sring, PAGE_SIZE);

	err = xenbus_grant_ring(dev, virt_to_mfn(sring));
	if (err < 0) {
		free_page((unsigned long)sring);
		xenbus_dev_fatal(dev, err,
			"fail to grant shared ring (Front to Back)");
		return err;
	}
	info->ring_ref = err;

	err = xenbus_alloc_evtchn(dev, &info->evtchn);
	if (err) {
		xenbus_dev_fatal(dev, err, "xenbus_alloc_evtchn");
		goto free_gnttab;
	}

	err = bind_evtchn_to_irq(info->evtchn);
	if (err <= 0) {
		xenbus_dev_fatal(dev, err, "bind_evtchn_to_irq");
		goto free_gnttab;
	}

	info->irq = err;

	err = request_threaded_irq(info->irq, NULL, scsifront_irq_fn,
				   IRQF_ONESHOT, "scsifront", info);
	if (err) {
		xenbus_dev_fatal(dev, err, "request_threaded_irq");
		goto free_irq;
	}

	return 0;

/* free resource */
free_irq:
	unbind_from_irqhandler(info->irq, info);
free_gnttab:
	gnttab_end_foreign_access(info->ring_ref, 0,
				  (unsigned long)info->ring.sring);

	return err;
}

static int scsifront_init_ring(struct vscsifrnt_info *info)
{
	struct xenbus_device *dev = info->dev;
	struct xenbus_transaction xbt;
	int err;

	pr_debug("%s\n", __func__);

	err = scsifront_alloc_ring(info);
	if (err)
		return err;
	pr_debug("%s: %u %u\n", __func__, info->ring_ref, info->evtchn);

again:
	err = xenbus_transaction_start(&xbt);
	if (err)
		xenbus_dev_fatal(dev, err, "starting transaction");

	err = xenbus_printf(xbt, dev->nodename, "ring-ref", "%u",
			    info->ring_ref);
	if (err) {
		xenbus_dev_fatal(dev, err, "%s", "writing ring-ref");
		goto fail;
	}

	err = xenbus_printf(xbt, dev->nodename, "event-channel", "%u",
			    info->evtchn);

	if (err) {
		xenbus_dev_fatal(dev, err, "%s", "writing event-channel");
		goto fail;
	}

	err = xenbus_transaction_end(xbt, 0);
	if (err) {
		if (err == -EAGAIN)
			goto again;
		xenbus_dev_fatal(dev, err, "completing transaction");
		goto free_sring;
	}

	return 0;

fail:
	xenbus_transaction_end(xbt, 1);
free_sring:
	unbind_from_irqhandler(info->irq, info);
	gnttab_end_foreign_access(info->ring_ref, 0,
				  (unsigned long)info->ring.sring);

	return err;
}


static int scsifront_probe(struct xenbus_device *dev,
			   const struct xenbus_device_id *id)
{
	struct vscsifrnt_info *info;
	struct Scsi_Host *host;
	int err = -ENOMEM;
	char name[TASK_COMM_LEN];

	host = scsi_host_alloc(&scsifront_sht, sizeof(*info));
	if (!host) {
		xenbus_dev_fatal(dev, err, "fail to allocate scsi host");
		return err;
	}
	info = (struct vscsifrnt_info *)host->hostdata;

	dev_set_drvdata(&dev->dev, info);
	info->dev = dev;

	bitmap_fill(info->shadow_free_bitmap, VSCSIIF_MAX_REQS);

	err = scsifront_init_ring(info);
	if (err) {
		scsi_host_put(host);
		return err;
	}

	init_waitqueue_head(&info->wq_sync);
	spin_lock_init(&info->shadow_lock);

	snprintf(name, TASK_COMM_LEN, "vscsiif.%d", host->host_no);

	host->max_id      = VSCSIIF_MAX_TARGET;
	host->max_channel = 0;
	host->max_lun     = VSCSIIF_MAX_LUN;
	host->max_sectors = (host->sg_tablesize - 1) * PAGE_SIZE / 512;
	host->max_cmd_len = VSCSIIF_MAX_COMMAND_SIZE;

	err = scsi_add_host(host, &dev->dev);
	if (err) {
		dev_err(&dev->dev, "fail to add scsi host %d\n", err);
		goto free_sring;
	}
	info->host = host;
	info->host_active = 1;

	xenbus_switch_state(dev, XenbusStateInitialised);

	return 0;

free_sring:
	unbind_from_irqhandler(info->irq, info);
	gnttab_end_foreign_access(info->ring_ref, 0,
				  (unsigned long)info->ring.sring);
	scsi_host_put(host);
	return err;
}

static int scsifront_remove(struct xenbus_device *dev)
{
	struct vscsifrnt_info *info = dev_get_drvdata(&dev->dev);

	pr_debug("%s: %s removed\n", __func__, dev->nodename);

	mutex_lock(&scsifront_mutex);
	if (info->host_active) {
		/* Scsi_host not yet removed */
		scsi_remove_host(info->host);
		info->host_active = 0;
	}
	mutex_unlock(&scsifront_mutex);

	gnttab_end_foreign_access(info->ring_ref, 0,
				  (unsigned long)info->ring.sring);
	unbind_from_irqhandler(info->irq, info);

	scsi_host_put(info->host);

	return 0;
}

static void scsifront_disconnect(struct vscsifrnt_info *info)
{
	struct xenbus_device *dev = info->dev;
	struct Scsi_Host *host = info->host;

	pr_debug("%s: %s disconnect\n", __func__, dev->nodename);

	/*
	 * When this function is executed, all devices of
	 * Frontend have been deleted.
	 * Therefore, it need not block I/O before remove_host.
	 */

	mutex_lock(&scsifront_mutex);
	if (info->host_active) {
		scsi_remove_host(host);
		info->host_active = 0;
	}
	mutex_unlock(&scsifront_mutex);

	xenbus_frontend_closed(dev);
}

static void scsifront_do_lun_hotplug(struct vscsifrnt_info *info, int op)
{
	struct xenbus_device *dev = info->dev;
	int i, err = 0;
	char str[64];
	char **dir;
	unsigned int dir_n = 0;
	unsigned int device_state;
	unsigned int hst, chn, tgt, lun;
	struct scsi_device *sdev;

	dir = xenbus_directory(XBT_NIL, dev->otherend, "vscsi-devs", &dir_n);
	if (IS_ERR(dir))
		return;

	/* mark current task as the one allowed to modify device states */
	BUG_ON(info->curr);
	info->curr = current;

	for (i = 0; i < dir_n; i++) {
		/* read status */
		snprintf(str, sizeof(str), "vscsi-devs/%s/state", dir[i]);
		err = xenbus_scanf(XBT_NIL, dev->otherend, str, "%u",
				   &device_state);
		if (XENBUS_EXIST_ERR(err))
			continue;

		/* virtual SCSI device */
		snprintf(str, sizeof(str), "vscsi-devs/%s/v-dev", dir[i]);
		err = xenbus_scanf(XBT_NIL, dev->otherend, str,
				   "%u:%u:%u:%u", &hst, &chn, &tgt, &lun);
		if (XENBUS_EXIST_ERR(err))
			continue;

		/*
		 * Front device state path, used in slave_configure called
		 * on successfull scsi_add_device, and in slave_destroy called
		 * on remove of a device.
		 */
		snprintf(info->dev_state_path, sizeof(info->dev_state_path),
			 "vscsi-devs/%s/state", dir[i]);

		switch (op) {
		case VSCSIFRONT_OP_ADD_LUN:
			if (device_state != XenbusStateInitialised)
				break;

			if (scsi_add_device(info->host, chn, tgt, lun)) {
				dev_err(&dev->dev, "scsi_add_device\n");
				xenbus_printf(XBT_NIL, dev->nodename,
					      info->dev_state_path,
					      "%d", XenbusStateClosed);
			}
			break;
		case VSCSIFRONT_OP_DEL_LUN:
			if (device_state != XenbusStateClosing)
				break;

			sdev = scsi_device_lookup(info->host, chn, tgt, lun);
			if (sdev) {
				scsi_remove_device(sdev);
				scsi_device_put(sdev);
			}
			break;
		default:
			break;
		}
	}

	info->curr = NULL;

	kfree(dir);
}

static void scsifront_read_backend_params(struct xenbus_device *dev,
					  struct vscsifrnt_info *info)
{
	unsigned int sg_grant;
	int ret;
	struct Scsi_Host *host = info->host;

	ret = xenbus_scanf(XBT_NIL, dev->otherend, "feature-sg-grant", "%u",
			   &sg_grant);
	if (ret == 1 && sg_grant) {
		sg_grant = min_t(unsigned int, sg_grant, SG_ALL);
		sg_grant = max_t(unsigned int, sg_grant, VSCSIIF_SG_TABLESIZE);
		host->sg_tablesize = min_t(unsigned int, sg_grant,
			VSCSIIF_SG_TABLESIZE * PAGE_SIZE /
			sizeof(struct scsiif_request_segment));
		host->max_sectors = (host->sg_tablesize - 1) * PAGE_SIZE / 512;
	}
	dev_info(&dev->dev, "using up to %d SG entries\n", host->sg_tablesize);
}

static void scsifront_backend_changed(struct xenbus_device *dev,
				      enum xenbus_state backend_state)
{
	struct vscsifrnt_info *info = dev_get_drvdata(&dev->dev);

	pr_debug("%s: %p %u %u\n", __func__, dev, dev->state, backend_state);

	switch (backend_state) {
	case XenbusStateUnknown:
	case XenbusStateInitialising:
	case XenbusStateInitWait:
	case XenbusStateInitialised:
		break;

	case XenbusStateConnected:
		scsifront_read_backend_params(dev, info);
		if (xenbus_read_driver_state(dev->nodename) ==
		    XenbusStateInitialised)
			scsifront_do_lun_hotplug(info, VSCSIFRONT_OP_ADD_LUN);

		if (dev->state != XenbusStateConnected)
			xenbus_switch_state(dev, XenbusStateConnected);
		break;

	case XenbusStateClosed:
		if (dev->state == XenbusStateClosed)
			break;
		/* Missed the backend's Closing state -- fallthrough */
	case XenbusStateClosing:
		scsifront_disconnect(info);
		break;

	case XenbusStateReconfiguring:
		scsifront_do_lun_hotplug(info, VSCSIFRONT_OP_DEL_LUN);
		xenbus_switch_state(dev, XenbusStateReconfiguring);
		break;

	case XenbusStateReconfigured:
		scsifront_do_lun_hotplug(info, VSCSIFRONT_OP_ADD_LUN);
		xenbus_switch_state(dev, XenbusStateConnected);
		break;
	}
}

static const struct xenbus_device_id scsifront_ids[] = {
	{ "vscsi" },
	{ "" }
};

static struct xenbus_driver scsifront_driver = {
	.ids			= scsifront_ids,
	.probe			= scsifront_probe,
	.remove			= scsifront_remove,
	.otherend_changed	= scsifront_backend_changed,
};

static int __init scsifront_init(void)
{
	if (!xen_domain())
		return -ENODEV;

	return xenbus_register_frontend(&scsifront_driver);
}
module_init(scsifront_init);

static void __exit scsifront_exit(void)
{
	xenbus_unregister_driver(&scsifront_driver);
}
module_exit(scsifront_exit);

MODULE_DESCRIPTION("Xen SCSI frontend driver");
MODULE_LICENSE("GPL");
MODULE_ALIAS("xen:vscsi");
MODULE_AUTHOR("Juergen Gross <jgross@suse.com>");