drivers/block/xen-blkback/blkback.c

/******************************************************************************
 *
 * Back-end of the driver for virtual block devices. This portion of the
 * driver exports a 'unified' block-device interface that can be accessed
 * by any operating system that implements a compatible front end. A
 * reference front-end implementation can be found in:
 *  drivers/block/xen-blkfront.c
 *
 * Copyright (c) 2003-2004, Keir Fraser & Steve Hand
 * Copyright (c) 2005, Christopher Clark
 *
 * 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/spinlock.h>
#include <linux/kthread.h>
#include <linux/list.h>
#include <linux/delay.h>
#include <linux/freezer.h>

#include <xen/events.h>
#include <xen/page.h>
#include <asm/xen/hypervisor.h>
#include <asm/xen/hypercall.h>
#include "common.h"

#define WRITE_BARRIER	(REQ_WRITE | REQ_FLUSH | REQ_FUA)

/*
 * These are rather arbitrary. They are fairly large because adjacent requests
 * pulled from a communication ring are quite likely to end up being part of
 * the same scatter/gather request at the disc.
 *
 * ** TRY INCREASING 'xen_blkif_reqs' IF WRITE SPEEDS SEEM TOO LOW **
 *
 * This will increase the chances of being able to write whole tracks.
 * 64 should be enough to keep us competitive with Linux.
 */
static int xen_blkif_reqs = 64;
module_param_named(reqs, xen_blkif_reqs, int, 0);
MODULE_PARM_DESC(reqs, "Number of blkback requests to allocate");

/* Run-time switchable: /sys/module/blkback/parameters/ */
static unsigned int log_stats;
static unsigned int debug_lvl;
module_param(log_stats, int, 0644);
module_param(debug_lvl, int, 0644);

/*
 * Each outstanding request that we've passed to the lower device layers has a
 * 'pending_req' allocated to it. Each buffer_head that completes decrements
 * the pendcnt towards zero. When it hits zero, the specified domain has a
 * response queued for it, with the saved 'id' passed back.
 */
struct pending_req {
	struct blkif_st       *blkif;
	u64            id;
	int            nr_pages;
	atomic_t       pendcnt;
	unsigned short operation;
	int            status;
	struct list_head free_list;
};

#define BLKBACK_INVALID_HANDLE (~0)

struct xen_blkbk {
	struct pending_req	*pending_reqs;
	/* List of all 'pending_req' available */
	struct list_head	pending_free;
	/* And its spinlock. */
	spinlock_t		pending_free_lock;
	wait_queue_head_t	pending_free_wq;
	/* The list of all pages that are available. */
	struct page		**pending_pages;
	/* And the grant handles that are available. */
	grant_handle_t		*pending_grant_handles;
};

static struct xen_blkbk *blkbk;

/*
 * Little helpful macro to figure out the index and virtual address of the
 * pending_pages[..]. For each 'pending_req' we have have up to
 * BLKIF_MAX_SEGMENTS_PER_REQUEST (11) pages. The seg would be from 0 through
 * 10 and would index in the pending_pages[..]. */
static inline int vaddr_pagenr(struct pending_req *req, int seg)
{
	return (req - blkbk->pending_reqs) *
		BLKIF_MAX_SEGMENTS_PER_REQUEST + seg;
}

#define pending_page(req, seg) pending_pages[vaddr_pagenr(req, seg)]

static inline unsigned long vaddr(struct pending_req *req, int seg)
{
	unsigned long pfn = page_to_pfn(blkbk->pending_page(req, seg));
	return (unsigned long)pfn_to_kaddr(pfn);
}

#define pending_handle(_req, _seg) \
	(blkbk->pending_grant_handles[vaddr_pagenr(_req, _seg)])


static int do_block_io_op(struct blkif_st *blkif);
static void dispatch_rw_block_io(struct blkif_st *blkif,
				 struct blkif_request *req,
				 struct pending_req *pending_req);
static void make_response(struct blkif_st *blkif, u64 id,
			  unsigned short op, int st);

/*
 * Retrieve from the 'pending_reqs' a free pending_req structure to be used.
 */
static struct pending_req *alloc_req(void)
{
	struct pending_req *req = NULL;
	unsigned long flags;

	spin_lock_irqsave(&blkbk->pending_free_lock, flags);
	if (!list_empty(&blkbk->pending_free)) {
		req = list_entry(blkbk->pending_free.next, struct pending_req,
				 free_list);
		list_del(&req->free_list);
	}
	spin_unlock_irqrestore(&blkbk->pending_free_lock, flags);
	return req;
}

/*
 * Return the 'pending_req' structure back to the freepool. We also
 * wake up the thread if it was waiting for a free page.
 */
static void free_req(struct pending_req *req)
{
	unsigned long flags;
	int was_empty;

	spin_lock_irqsave(&blkbk->pending_free_lock, flags);
	was_empty = list_empty(&blkbk->pending_free);
	list_add(&req->free_list, &blkbk->pending_free);
	spin_unlock_irqrestore(&blkbk->pending_free_lock, flags);
	if (was_empty)
		wake_up(&blkbk->pending_free_wq);
}

/*
 * Routines for managing virtual block devices (vbds).
 */


static int vbd_translate(struct phys_req *req, struct blkif_st *blkif,
			 int operation)
{
	struct vbd *vbd = &blkif->vbd;
	int rc = -EACCES;

	if ((operation != READ) && vbd->readonly)
		goto out;

	if (unlikely((req->sector_number + req->nr_sects) > vbd_sz(vbd)))
		goto out;

	req->dev  = vbd->pdevice;
	req->bdev = vbd->bdev;
	rc = 0;

 out:
	return rc;
}

static void vbd_resize(struct blkif_st *blkif)
{
	struct vbd *vbd = &blkif->vbd;
	struct xenbus_transaction xbt;
	int err;
	struct xenbus_device *dev = xen_blkbk_xenbus(blkif->be);
	unsigned long long new_size = vbd_sz(vbd);

	printk(KERN_INFO "VBD Resize: Domid: %d, Device: (%d, %d)\n",
		blkif->domid, MAJOR(vbd->pdevice), MINOR(vbd->pdevice));
	printk(KERN_INFO "VBD Resize: new size %llu\n", new_size);
	vbd->size = new_size;
again:
	err = xenbus_transaction_start(&xbt);
	if (err) {
		printk(KERN_WARNING "Error starting transaction");
		return;
	}
	err = xenbus_printf(xbt, dev->nodename, "sectors", "%llu",
			    (unsigned long long)vbd_sz(vbd));
	if (err) {
		printk(KERN_WARNING "Error writing new size");
		goto abort;
	}
	/*
	 * Write the current state; we will use this to synchronize
	 * the front-end. If the current state is "connected" the
	 * front-end will get the new size information online.
	 */
	err = xenbus_printf(xbt, dev->nodename, "state", "%d", dev->state);
	if (err) {
		printk(KERN_WARNING "Error writing the state");
		goto abort;
	}

	err = xenbus_transaction_end(xbt, 0);
	if (err == -EAGAIN)
		goto again;
	if (err)
		printk(KERN_WARNING "Error ending transaction");
abort:
	xenbus_transaction_end(xbt, 1);
}

/*
 * Notification from the guest OS.
 */
static void blkif_notify_work(struct blkif_st *blkif)
{
	blkif->waiting_reqs = 1;
	wake_up(&blkif->wq);
}

irqreturn_t xen_blkif_be_int(int irq, void *dev_id)
{
	blkif_notify_work(dev_id);
	return IRQ_HANDLED;
}

/*
 * SCHEDULER FUNCTIONS
 */

static void print_stats(struct blkif_st *blkif)
{
	printk(KERN_DEBUG "%s: oo %3d  |  rd %4d  |  wr %4d  |  br %4d\n",
	       current->comm, blkif->st_oo_req,
	       blkif->st_rd_req, blkif->st_wr_req, blkif->st_br_req);
	blkif->st_print = jiffies + msecs_to_jiffies(10 * 1000);
	blkif->st_rd_req = 0;
	blkif->st_wr_req = 0;
	blkif->st_oo_req = 0;
}

int xen_blkif_schedule(void *arg)
{
	struct blkif_st *blkif = arg;
	struct vbd *vbd = &blkif->vbd;

	xen_blkif_get(blkif);

	if (debug_lvl)
		printk(KERN_DEBUG "%s: started\n", current->comm);

	while (!kthread_should_stop()) {
		struct blk_plug plug;

		if (try_to_freeze())
			continue;
		if (unlikely(vbd->size != vbd_sz(vbd)))
			vbd_resize(blkif);

		wait_event_interruptible(
			blkif->wq,
			blkif->waiting_reqs || kthread_should_stop());
		wait_event_interruptible(
			blkbk->pending_free_wq,
			!list_empty(&blkbk->pending_free) ||
			kthread_should_stop());

		blkif->waiting_reqs = 0;
		smp_mb(); /* clear flag *before* checking for work */

		blk_start_plug(&plug);

		if (do_block_io_op(blkif))
			blkif->waiting_reqs = 1;

		blk_finish_plug(&plug);

		if (log_stats && time_after(jiffies, blkif->st_print))
			print_stats(blkif);
	}

	if (log_stats)
		print_stats(blkif);
	if (debug_lvl)
		printk(KERN_DEBUG "%s: exiting\n", current->comm);

	blkif->xenblkd = NULL;
	xen_blkif_put(blkif);

	return 0;
}

struct seg_buf {
	unsigned long buf;
	unsigned int nsec;
};
/*
 * Unmap the grant references, and also remove the M2P over-rides
 * used in the 'pending_req'.
*/
static void xen_blkbk_unmap(struct pending_req *req)
{
	struct gnttab_unmap_grant_ref unmap[BLKIF_MAX_SEGMENTS_PER_REQUEST];
	unsigned int i, invcount = 0;
	grant_handle_t handle;
	int ret;

	for (i = 0; i < req->nr_pages; i++) {
		handle = pending_handle(req, i);
		if (handle == BLKBACK_INVALID_HANDLE)
			continue;
		gnttab_set_unmap_op(&unmap[invcount], vaddr(req, i),
				    GNTMAP_host_map, handle);
		pending_handle(req, i) = BLKBACK_INVALID_HANDLE;
		invcount++;
	}

	ret = HYPERVISOR_grant_table_op(
		GNTTABOP_unmap_grant_ref, unmap, invcount);
	BUG_ON(ret);
	/* Note, we use invcount, so nr->pages, so we can't index
	 * using vaddr(req, i).
	 */
	for (i = 0; i < invcount; i++) {
		ret = m2p_remove_override(
			virt_to_page(unmap[i].host_addr), false);
		if (ret) {
			printk(KERN_ALERT "Failed to remove M2P override for " \
				"%lx\n", (unsigned long)unmap[i].host_addr);
			continue;
		}
	}
}
static int xen_blkbk_map(struct blkif_request *req, struct pending_req *pending_req,
			 struct seg_buf seg[])
{
	struct gnttab_map_grant_ref map[BLKIF_MAX_SEGMENTS_PER_REQUEST];
	int i;
	int nseg = req->nr_segments;
	int ret = 0;
	/* Fill out preq.nr_sects with proper amount of sectors, and setup
	 * assign map[..] with the PFN of the page in our domain with the
	 * corresponding grant reference for each page.
	 */
	for (i = 0; i < nseg; i++) {
		uint32_t flags;

		flags = GNTMAP_host_map;
		if (pending_req->operation != BLKIF_OP_READ)
			flags |= GNTMAP_readonly;
		gnttab_set_map_op(&map[i], vaddr(pending_req, i), flags,
				  req->u.rw.seg[i].gref, pending_req->blkif->domid);
	}

	ret = HYPERVISOR_grant_table_op(GNTTABOP_map_grant_ref, map, nseg);
	BUG_ON(ret);

	/* Now swizzel the MFN in our domain with the MFN from the other domain
	 * so that when we access vaddr(pending_req,i) it has the contents of
	 * the page from the other domain.
	 */
	for (i = 0; i < nseg; i++) {
		if (unlikely(map[i].status != 0)) {
			DPRINTK("invalid buffer -- could not remap it\n");
			map[i].handle = BLKBACK_INVALID_HANDLE;
			ret |= 1;
		}

		pending_handle(pending_req, i) = map[i].handle;

		if (ret)
			continue;

		ret = m2p_add_override(PFN_DOWN(map[i].dev_bus_addr),
			blkbk->pending_page(pending_req, i), false);
		if (ret) {
			printk(KERN_ALERT "Failed to install M2P override for"\
				" %lx (ret: %d)\n", (unsigned long)
				map[i].dev_bus_addr, ret);
			/* We could switch over to GNTTABOP_copy */
			continue;
		}

		seg[i].buf  = map[i].dev_bus_addr |
			(req->u.rw.seg[i].first_sect << 9);
	}
	return ret;
}

/*
 * Completion callback on the bio's. Called as bh->b_end_io()
 */

static void __end_block_io_op(struct pending_req *pending_req, int error)
{
	/* An error fails the entire request. */
	if ((pending_req->operation == BLKIF_OP_WRITE_BARRIER) &&
	    (error == -EOPNOTSUPP)) {
		DPRINTK("blkback: write barrier op failed, not supported\n");
		xen_blkbk_barrier(XBT_NIL, pending_req->blkif->be, 0);
		pending_req->status = BLKIF_RSP_EOPNOTSUPP;
	} else if (error) {
		DPRINTK("Buffer not up-to-date at end of operation, "
			"error=%d\n", error);
		pending_req->status = BLKIF_RSP_ERROR;
	}

	/* If all of the bio's have completed it is time to unmap
	 * the grant references associated with 'request' and provide
	 * the proper response on the ring.
	 */
	if (atomic_dec_and_test(&pending_req->pendcnt)) {
		xen_blkbk_unmap(pending_req);
		make_response(pending_req->blkif, pending_req->id,
			      pending_req->operation, pending_req->status);
		xen_blkif_put(pending_req->blkif);
		free_req(pending_req);
	}
}

/*
 * bio callback.
 */
static void end_block_io_op(struct bio *bio, int error)
{
	__end_block_io_op(bio->bi_private, error);
	bio_put(bio);
}



/*
 * Function to copy the from the ring buffer the 'struct blkif_request'
 * (which has the sectors we want, number of them, grant references, etc),
 * and transmute  it to the block API to hand it over to the proper block disk.
 */
static int do_block_io_op(struct blkif_st *blkif)
{
	union blkif_back_rings *blk_rings = &blkif->blk_rings;
	struct blkif_request req;
	struct pending_req *pending_req;
	RING_IDX rc, rp;
	int more_to_do = 0;

	rc = blk_rings->common.req_cons;
	rp = blk_rings->common.sring->req_prod;
	rmb(); /* Ensure we see queued requests up to 'rp'. */

	while (rc != rp) {

		if (RING_REQUEST_CONS_OVERFLOW(&blk_rings->common, rc))
			break;

		if (kthread_should_stop()) {
			more_to_do = 1;
			break;
		}

		pending_req = alloc_req();
		if (NULL == pending_req) {
			blkif->st_oo_req++;
			more_to_do = 1;
			break;
		}

		switch (blkif->blk_protocol) {
		case BLKIF_PROTOCOL_NATIVE:
			memcpy(&req, RING_GET_REQUEST(&blk_rings->native, rc), sizeof(req));
			break;
		case BLKIF_PROTOCOL_X86_32:
			blkif_get_x86_32_req(&req, RING_GET_REQUEST(&blk_rings->x86_32, rc));
			break;
		case BLKIF_PROTOCOL_X86_64:
			blkif_get_x86_64_req(&req, RING_GET_REQUEST(&blk_rings->x86_64, rc));
			break;
		default:
			BUG();
		}
		blk_rings->common.req_cons = ++rc; /* before make_response() */

		/* Apply all sanity checks to /private copy/ of request. */
		barrier();

		switch (req.operation) {
		case BLKIF_OP_READ:
			blkif->st_rd_req++;
			dispatch_rw_block_io(blkif, &req, pending_req);
			break;
		case BLKIF_OP_WRITE_BARRIER:
			blkif->st_br_req++;
			/* fall through */
		case BLKIF_OP_WRITE:
			blkif->st_wr_req++;
			dispatch_rw_block_io(blkif, &req, pending_req);
			break;
		default:
			/* A good sign something is wrong: sleep for a while to
			 * avoid excessive CPU consumption by a bad guest. */
			msleep(1);
			DPRINTK("error: unknown block io operation [%d]\n",
				req.operation);
			make_response(blkif, req.id, req.operation,
				      BLKIF_RSP_ERROR);
			free_req(pending_req);
			break;
		}

		/* Yield point for this unbounded loop. */
		cond_resched();
	}

	return more_to_do;
}

/*
 * Transumation of the 'struct blkif_request' to a proper 'struct bio'
 * and call the 'submit_bio' to pass it to the underlaying storage.
 */
static void dispatch_rw_block_io(struct blkif_st *blkif,
				 struct blkif_request *req,
				 struct pending_req *pending_req)
{
	struct phys_req preq;
	struct seg_buf seg[BLKIF_MAX_SEGMENTS_PER_REQUEST];
	unsigned int nseg;
	struct bio *bio = NULL;
	struct bio *biolist[BLKIF_MAX_SEGMENTS_PER_REQUEST];
	int i, nbio = 0;
	int operation;

	switch (req->operation) {
	case BLKIF_OP_READ:
		operation = READ;
		break;
	case BLKIF_OP_WRITE:
		operation = WRITE;
		break;
	case BLKIF_OP_WRITE_BARRIER:
		operation = WRITE_BARRIER;
		break;
	default:
		operation = 0; /* make gcc happy */
		BUG();
	}

	/* Check that the number of segments is sane. */
	nseg = req->nr_segments;
	if (unlikely(nseg == 0 && operation != WRITE_BARRIER) ||
	    unlikely(nseg > BLKIF_MAX_SEGMENTS_PER_REQUEST)) {
		DPRINTK("Bad number of segments in request (%d)\n", nseg);
		/* Haven't submitted any bio's yet. */
		goto fail_response;
	}

	preq.dev           = req->handle;
	preq.sector_number = req->u.rw.sector_number;
	preq.nr_sects      = 0;

	pending_req->blkif     = blkif;
	pending_req->id        = req->id;
	pending_req->operation = req->operation;
	pending_req->status    = BLKIF_RSP_OKAY;
	pending_req->nr_pages  = nseg;

	for (i = 0; i < nseg; i++) {
		seg[i].nsec = req->u.rw.seg[i].last_sect -
			req->u.rw.seg[i].first_sect + 1;
		if ((req->u.rw.seg[i].last_sect >= (PAGE_SIZE >> 9)) ||
		    (req->u.rw.seg[i].last_sect < req->u.rw.seg[i].first_sect))
			goto fail_response;
		preq.nr_sects += seg[i].nsec;

	}

	if (vbd_translate(&preq, blkif, operation) != 0) {
		DPRINTK("access denied: %s of [%llu,%llu] on dev=%04x\n",
			operation == READ ? "read" : "write",
			preq.sector_number,
			preq.sector_number + preq.nr_sects, preq.dev);
		goto fail_response;
	}
	/* This check _MUST_ be done after vbd_translate as the preq.bdev
	 * is set there. */
	for (i = 0; i < nseg; i++) {
		if (((int)preq.sector_number|(int)seg[i].nsec) &
		    ((bdev_logical_block_size(preq.bdev) >> 9) - 1)) {
			DPRINTK("Misaligned I/O request from domain %d",
				blkif->domid);
			goto fail_response;
		}
	}
	/* If we have failed at this point, we need to undo the M2P override,
	 * set gnttab_set_unmap_op on all of the grant references and perform
	 * the hypercall to unmap the grants - that is all done in
	 * xen_blkbk_unmap.
	 */
	if (xen_blkbk_map(req, pending_req, seg))
		goto fail_flush;

	/* This corresponding blkif_put is done in __end_block_io_op */
	xen_blkif_get(blkif);

	for (i = 0; i < nseg; i++) {
		while ((bio == NULL) ||
		       (bio_add_page(bio,
				     blkbk->pending_page(pending_req, i),
				     seg[i].nsec << 9,
				     seg[i].buf & ~PAGE_MASK) == 0)) {

			bio = biolist[nbio++] = bio_alloc(GFP_KERNEL, nseg-i);
			if (unlikely(bio == NULL))
				goto fail_put_bio;

			bio->bi_bdev    = preq.bdev;
			bio->bi_private = pending_req;
			bio->bi_end_io  = end_block_io_op;
			bio->bi_sector  = preq.sector_number;
		}

		preq.sector_number += seg[i].nsec;
	}

	/* This will be hit if the operation was a barrier. */
	if (!bio) {
		BUG_ON(operation != WRITE_BARRIER);
		bio = biolist[nbio++] = bio_alloc(GFP_KERNEL, 0);
		if (unlikely(bio == NULL))
			goto fail_put_bio;

		bio->bi_bdev    = preq.bdev;
		bio->bi_private = pending_req;
		bio->bi_end_io  = end_block_io_op;
		bio->bi_sector  = -1;
	}


	/* We set it one so that the last submit_bio does not have to call
	 * atomic_inc.
	 */
	atomic_set(&pending_req->pendcnt, nbio);

	for (i = 0; i < nbio; i++)
		submit_bio(operation, biolist[i]);

	if (operation == READ)
		blkif->st_rd_sect += preq.nr_sects;
	else if (operation == WRITE || operation == WRITE_BARRIER)
		blkif->st_wr_sect += preq.nr_sects;

	return;

 fail_flush:
	xen_blkbk_unmap(pending_req);
 fail_response:
	/* Haven't submitted any bio's yet. */
	make_response(blkif, req->id, req->operation, BLKIF_RSP_ERROR);
	free_req(pending_req);
	msleep(1); /* back off a bit */
	return;

 fail_put_bio:
	for (i = 0; i < (nbio-1); i++)
		bio_put(biolist[i]);
	__end_block_io_op(pending_req, -EINVAL);
	msleep(1); /* back off a bit */
	return;
}



/*
 * Put a response on the ring on how the operation fared.
 */
static void make_response(struct blkif_st *blkif, u64 id,
			  unsigned short op, int st)
{
	struct blkif_response  resp;
	unsigned long     flags;
	union blkif_back_rings *blk_rings = &blkif->blk_rings;
	int more_to_do = 0;
	int notify;

	resp.id        = id;
	resp.operation = op;
	resp.status    = st;

	spin_lock_irqsave(&blkif->blk_ring_lock, flags);
	/* Place on the response ring for the relevant domain. */
	switch (blkif->blk_protocol) {
	case BLKIF_PROTOCOL_NATIVE:
		memcpy(RING_GET_RESPONSE(&blk_rings->native, blk_rings->native.rsp_prod_pvt),
		       &resp, sizeof(resp));
		break;
	case BLKIF_PROTOCOL_X86_32:
		memcpy(RING_GET_RESPONSE(&blk_rings->x86_32, blk_rings->x86_32.rsp_prod_pvt),
		       &resp, sizeof(resp));
		break;
	case BLKIF_PROTOCOL_X86_64:
		memcpy(RING_GET_RESPONSE(&blk_rings->x86_64, blk_rings->x86_64.rsp_prod_pvt),
		       &resp, sizeof(resp));
		break;
	default:
		BUG();
	}
	blk_rings->common.rsp_prod_pvt++;
	RING_PUSH_RESPONSES_AND_CHECK_NOTIFY(&blk_rings->common, notify);
	if (blk_rings->common.rsp_prod_pvt == blk_rings->common.req_cons) {
		/*
		 * Tail check for pending requests. Allows frontend to avoid
		 * notifications if requests are already in flight (lower
		 * overheads and promotes batching).
		 */
		RING_FINAL_CHECK_FOR_REQUESTS(&blk_rings->common, more_to_do);

	} else if (RING_HAS_UNCONSUMED_REQUESTS(&blk_rings->common)) {
		more_to_do = 1;
	}

	spin_unlock_irqrestore(&blkif->blk_ring_lock, flags);

	if (more_to_do)
		blkif_notify_work(blkif);
	if (notify)
		notify_remote_via_irq(blkif->irq);
}

static int __init xen_blkif_init(void)
{
	int i, mmap_pages;
	int rc = 0;

	if (!xen_pv_domain())
		return -ENODEV;

	blkbk = kzalloc(sizeof(struct xen_blkbk), GFP_KERNEL);
	if (!blkbk) {
		printk(KERN_ALERT "%s: out of memory!\n", __func__);
		return -ENOMEM;
	}

	mmap_pages = xen_blkif_reqs * BLKIF_MAX_SEGMENTS_PER_REQUEST;

	blkbk->pending_reqs          = kmalloc(sizeof(blkbk->pending_reqs[0]) *
					xen_blkif_reqs, GFP_KERNEL);
	blkbk->pending_grant_handles = kzalloc(sizeof(blkbk->pending_grant_handles[0]) *
					mmap_pages, GFP_KERNEL);
	blkbk->pending_pages         = kzalloc(sizeof(blkbk->pending_pages[0]) *
					mmap_pages, GFP_KERNEL);

	if (!blkbk->pending_reqs || !blkbk->pending_grant_handles ||
	    !blkbk->pending_pages) {
		rc = -ENOMEM;
		goto out_of_memory;
	}

	for (i = 0; i < mmap_pages; i++) {
		blkbk->pending_grant_handles[i] = BLKBACK_INVALID_HANDLE;
		blkbk->pending_pages[i] = alloc_page(GFP_KERNEL);
		if (blkbk->pending_pages[i] == NULL) {
			rc = -ENOMEM;
			goto out_of_memory;
		}
	}
	rc = xen_blkif_interface_init();
	if (rc)
		goto failed_init;

	memset(blkbk->pending_reqs, 0, sizeof(blkbk->pending_reqs));

	INIT_LIST_HEAD(&blkbk->pending_free);
	spin_lock_init(&blkbk->pending_free_lock);
	init_waitqueue_head(&blkbk->pending_free_wq);

	for (i = 0; i < xen_blkif_reqs; i++)
		list_add_tail(&blkbk->pending_reqs[i].free_list,
			      &blkbk->pending_free);

	rc = xen_blkif_xenbus_init();
	if (rc)
		goto failed_init;

	return 0;

 out_of_memory:
	printk(KERN_ERR "%s: out of memory\n", __func__);
 failed_init:
	kfree(blkbk->pending_reqs);
	kfree(blkbk->pending_grant_handles);
	for (i = 0; i < mmap_pages; i++) {
		if (blkbk->pending_pages[i])
			__free_page(blkbk->pending_pages[i]);
	}
	kfree(blkbk->pending_pages);
	kfree(blkbk);
	blkbk = NULL;
	return rc;
}

module_init(xen_blkif_init);

MODULE_LICENSE("Dual BSD/GPL");