/******************************************************************************
 *
 * 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 <linux/bitmap.h>

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

/*
 * Maximum number of unused free pages to keep in the internal buffer.
 * Setting this to a value too low will reduce memory used in each backend,
 * but can have a performance penalty.
 *
 * A sane value is xen_blkif_reqs * BLKIF_MAX_SEGMENTS_PER_REQUEST, but can
 * be set to a lower value that might degrade performance on some intensive
 * IO workloads.
 */

static int xen_blkif_max_buffer_pages = 1024;
module_param_named(max_buffer_pages, xen_blkif_max_buffer_pages, int, 0644);
MODULE_PARM_DESC(max_buffer_pages,
"Maximum number of free pages to keep in each block backend buffer");

/*
 * Maximum number of grants to map persistently in blkback. For maximum
 * performance this should be the total numbers of grants that can be used
 * to fill the ring, but since this might become too high, specially with
 * the use of indirect descriptors, we set it to a value that provides good
 * performance without using too much memory.
 *
 * When the list of persistent grants is full we clean it up using a LRU
 * algorithm.
 */

static int xen_blkif_max_pgrants = 1056;
module_param_named(max_persistent_grants, xen_blkif_max_pgrants, int, 0644);
MODULE_PARM_DESC(max_persistent_grants,
                 "Maximum number of grants to map persistently");

/*
 * The LRU mechanism to clean the lists of persistent grants needs to
 * be executed periodically. The time interval between consecutive executions
 * of the purge mechanism is set in ms.
 */
#define LRU_INTERVAL 100

/*
 * When the persistent grants list is full we will remove unused grants
 * from the list. The percent number of grants to be removed at each LRU
 * execution.
 */
#define LRU_PERCENT_CLEAN 5

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

#define BLKBACK_INVALID_HANDLE (~0)

/* Number of free pages to remove on each call to free_xenballooned_pages */
#define NUM_BATCH_FREE_PAGES 10

static inline int get_free_page(struct xen_blkif *blkif, struct page **page)
{
        unsigned long flags;

        spin_lock_irqsave(&blkif->free_pages_lock, flags);
        if (list_empty(&blkif->free_pages)) {
                BUG_ON(blkif->free_pages_num != 0);
                spin_unlock_irqrestore(&blkif->free_pages_lock, flags);
                return alloc_xenballooned_pages(1, page, false);
        }
        BUG_ON(blkif->free_pages_num == 0);
        page[0] = list_first_entry(&blkif->free_pages, struct page, lru);
        list_del(&page[0]->lru);
        blkif->free_pages_num--;
        spin_unlock_irqrestore(&blkif->free_pages_lock, flags);

        return 0;
}

static inline void put_free_pages(struct xen_blkif *blkif, struct page **page,
                                  int num)
{
        unsigned long flags;
        int i;

        spin_lock_irqsave(&blkif->free_pages_lock, flags);
        for (i = 0; i < num; i++)
                list_add(&page[i]->lru, &blkif->free_pages);
        blkif->free_pages_num += num;
        spin_unlock_irqrestore(&blkif->free_pages_lock, flags);
}

static inline void shrink_free_pagepool(struct xen_blkif *blkif, int num)
{
        /* Remove requested pages in batches of NUM_BATCH_FREE_PAGES */
        struct page *page[NUM_BATCH_FREE_PAGES];
        unsigned int num_pages = 0;
        unsigned long flags;

        spin_lock_irqsave(&blkif->free_pages_lock, flags);
        while (blkif->free_pages_num > num) {
                BUG_ON(list_empty(&blkif->free_pages));
                page[num_pages] = list_first_entry(&blkif->free_pages,
                                                   struct page, lru);
                list_del(&page[num_pages]->lru);
                blkif->free_pages_num--;
                if (++num_pages == NUM_BATCH_FREE_PAGES) {
                        spin_unlock_irqrestore(&blkif->free_pages_lock, flags);
                        free_xenballooned_pages(num_pages, page);
                        spin_lock_irqsave(&blkif->free_pages_lock, flags);
                        num_pages = 0;
                }
        }
        spin_unlock_irqrestore(&blkif->free_pages_lock, flags);
        if (num_pages != 0)
                free_xenballooned_pages(num_pages, page);
}

#define vaddr(page) ((unsigned long)pfn_to_kaddr(page_to_pfn(page)))

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

#define foreach_grant_safe(pos, n, rbtree, node) \
        for ((pos) = container_of(rb_first((rbtree)), typeof(*(pos)), node), \
             (n) = (&(pos)->node != NULL) ? rb_next(&(pos)->node) : NULL; \
             &(pos)->node != NULL; \
             (pos) = container_of(n, typeof(*(pos)), node), \
             (n) = (&(pos)->node != NULL) ? rb_next(&(pos)->node) : NULL)


/*
 * We don't need locking around the persistent grant helpers
 * because blkback uses a single-thread for each backed, so we
 * can be sure that this functions will never be called recursively.
 *
 * The only exception to that is put_persistent_grant, that can be called
 * from interrupt context (by xen_blkbk_unmap), so we have to use atomic
 * bit operations to modify the flags of a persistent grant and to count
 * the number of used grants.
 */
static int add_persistent_gnt(struct xen_blkif *blkif,
                               struct persistent_gnt *persistent_gnt)
{
        struct rb_node **new = NULL, *parent = NULL;
        struct persistent_gnt *this;

        if (blkif->persistent_gnt_c >= xen_blkif_max_pgrants) {
                if (!blkif->vbd.overflow_max_grants)
                        blkif->vbd.overflow_max_grants = 1;
                return -EBUSY;
        }
        /* Figure out where to put new node */
        new = &blkif->persistent_gnts.rb_node;
        while (*new) {
                this = container_of(*new, struct persistent_gnt, node);

                parent = *new;
                if (persistent_gnt->gnt < this->gnt)
                        new = &((*new)->rb_left);
                else if (persistent_gnt->gnt > this->gnt)
                        new = &((*new)->rb_right);
                else {
                        pr_alert_ratelimited(DRV_PFX " trying to add a gref that's already in the tree\n");
                        return -EINVAL;
                }
        }

        bitmap_zero(persistent_gnt->flags, PERSISTENT_GNT_FLAGS_SIZE);
        set_bit(PERSISTENT_GNT_ACTIVE, persistent_gnt->flags);
        /* Add new node and rebalance tree. */
        rb_link_node(&(persistent_gnt->node), parent, new);
        rb_insert_color(&(persistent_gnt->node), &blkif->persistent_gnts);
        blkif->persistent_gnt_c++;
        atomic_inc(&blkif->persistent_gnt_in_use);
        return 0;
}

static struct persistent_gnt *get_persistent_gnt(struct xen_blkif *blkif,
                                                 grant_ref_t gref)
{
        struct persistent_gnt *data;
        struct rb_node *node = NULL;

        node = blkif->persistent_gnts.rb_node;
        while (node) {
                data = container_of(node, struct persistent_gnt, node);

                if (gref < data->gnt)
                        node = node->rb_left;
                else if (gref > data->gnt)
                        node = node->rb_right;
                else {
                        if(test_bit(PERSISTENT_GNT_ACTIVE, data->flags)) {
                                pr_alert_ratelimited(DRV_PFX " requesting a grant already in use\n");
                                return NULL;
                        }
                        set_bit(PERSISTENT_GNT_ACTIVE, data->flags);
                        atomic_inc(&blkif->persistent_gnt_in_use);
                        return data;
                }
        }
        return NULL;
}

static void put_persistent_gnt(struct xen_blkif *blkif,
                               struct persistent_gnt *persistent_gnt)
{
        if(!test_bit(PERSISTENT_GNT_ACTIVE, persistent_gnt->flags))
                  pr_alert_ratelimited(DRV_PFX " freeing a grant already unused");
        set_bit(PERSISTENT_GNT_WAS_ACTIVE, persistent_gnt->flags);
        clear_bit(PERSISTENT_GNT_ACTIVE, persistent_gnt->flags);
        atomic_dec(&blkif->persistent_gnt_in_use);
}

static void free_persistent_gnts(struct xen_blkif *blkif, struct rb_root *root,
                                 unsigned int num)
{
        struct gnttab_unmap_grant_ref unmap[BLKIF_MAX_SEGMENTS_PER_REQUEST];
        struct page *pages[BLKIF_MAX_SEGMENTS_PER_REQUEST];
        struct persistent_gnt *persistent_gnt;
        struct rb_node *n;
        int ret = 0;
        int segs_to_unmap = 0;

        foreach_grant_safe(persistent_gnt, n, root, node) {
                BUG_ON(persistent_gnt->handle ==
                        BLKBACK_INVALID_HANDLE);
                gnttab_set_unmap_op(&unmap[segs_to_unmap],
                        (unsigned long) pfn_to_kaddr(page_to_pfn(
                                persistent_gnt->page)),
                        GNTMAP_host_map,
                        persistent_gnt->handle);

                pages[segs_to_unmap] = persistent_gnt->page;

                if (++segs_to_unmap == BLKIF_MAX_SEGMENTS_PER_REQUEST ||
                        !rb_next(&persistent_gnt->node)) {
                        ret = gnttab_unmap_refs(unmap, NULL, pages,
                                segs_to_unmap);
                        BUG_ON(ret);
                        put_free_pages(blkif, pages, segs_to_unmap);
                        segs_to_unmap = 0;
                }

                rb_erase(&persistent_gnt->node, root);
                kfree(persistent_gnt);
                num--;
        }
        BUG_ON(num != 0);
}

static void unmap_purged_grants(struct work_struct *work)
{
        struct gnttab_unmap_grant_ref unmap[BLKIF_MAX_SEGMENTS_PER_REQUEST];
        struct page *pages[BLKIF_MAX_SEGMENTS_PER_REQUEST];
        struct persistent_gnt *persistent_gnt;
        int ret, segs_to_unmap = 0;
        struct xen_blkif *blkif = container_of(work, typeof(*blkif), persistent_purge_work);

        while(!list_empty(&blkif->persistent_purge_list)) {
                persistent_gnt = list_first_entry(&blkif->persistent_purge_list,
                                                  struct persistent_gnt,
                                                  remove_node);
                list_del(&persistent_gnt->remove_node);

                gnttab_set_unmap_op(&unmap[segs_to_unmap],
                        vaddr(persistent_gnt->page),
                        GNTMAP_host_map,
                        persistent_gnt->handle);

                pages[segs_to_unmap] = persistent_gnt->page;

                if (++segs_to_unmap == BLKIF_MAX_SEGMENTS_PER_REQUEST) {
                        ret = gnttab_unmap_refs(unmap, NULL, pages,
                                segs_to_unmap);
                        BUG_ON(ret);
                        put_free_pages(blkif, pages, segs_to_unmap);
                        segs_to_unmap = 0;
                }
                kfree(persistent_gnt);
        }
        if (segs_to_unmap > 0) {
                ret = gnttab_unmap_refs(unmap, NULL, pages, segs_to_unmap);
                BUG_ON(ret);
                put_free_pages(blkif, pages, segs_to_unmap);
        }
}

static void purge_persistent_gnt(struct xen_blkif *blkif)
{
        struct persistent_gnt *persistent_gnt;
        struct rb_node *n;
        unsigned int num_clean, total;
        bool scan_used = false, clean_used = false;
        struct rb_root *root;

        if (blkif->persistent_gnt_c < xen_blkif_max_pgrants ||
            (blkif->persistent_gnt_c == xen_blkif_max_pgrants &&
            !blkif->vbd.overflow_max_grants)) {
                return;
        }

        if (work_pending(&blkif->persistent_purge_work)) {
                pr_alert_ratelimited(DRV_PFX "Scheduled work from previous purge is still pending, cannot purge list\n");
                return;
        }

        num_clean = (xen_blkif_max_pgrants / 100) * LRU_PERCENT_CLEAN;
        num_clean = blkif->persistent_gnt_c - xen_blkif_max_pgrants + num_clean;
        num_clean = min(blkif->persistent_gnt_c, num_clean);
        if ((num_clean == 0) ||
            (num_clean > (blkif->persistent_gnt_c - atomic_read(&blkif->persistent_gnt_in_use))))
                return;

        /*
         * At this point, we can assure that there will be no calls
         * to get_persistent_grant (because we are executing this code from
         * xen_blkif_schedule), there can only be calls to put_persistent_gnt,
         * which means that the number of currently used grants will go down,
         * but never up, so we will always be able to remove the requested
         * number of grants.
         */

        total = num_clean;

        pr_debug(DRV_PFX "Going to purge %u persistent grants\n", num_clean);

        INIT_LIST_HEAD(&blkif->persistent_purge_list);
        root = &blkif->persistent_gnts;
purge_list:
        foreach_grant_safe(persistent_gnt, n, root, node) {
                BUG_ON(persistent_gnt->handle ==
                        BLKBACK_INVALID_HANDLE);

                if (clean_used) {
                        clear_bit(PERSISTENT_GNT_WAS_ACTIVE, persistent_gnt->flags);
                        continue;
                }

                if (test_bit(PERSISTENT_GNT_ACTIVE, persistent_gnt->flags))
                        continue;
                if (!scan_used &&
                    (test_bit(PERSISTENT_GNT_WAS_ACTIVE, persistent_gnt->flags)))
                        continue;

                rb_erase(&persistent_gnt->node, root);
                list_add(&persistent_gnt->remove_node,
                         &blkif->persistent_purge_list);
                if (--num_clean == 0)
                        goto finished;
        }
        /*
         * If we get here it means we also need to start cleaning
         * grants that were used since last purge in order to cope
         * with the requested num
         */
        if (!scan_used && !clean_used) {
                pr_debug(DRV_PFX "Still missing %u purged frames\n", num_clean);
                scan_used = true;
                goto purge_list;
        }
finished:
        if (!clean_used) {
                pr_debug(DRV_PFX "Finished scanning for grants to clean, removing used flag\n");
                clean_used = true;
                goto purge_list;
        }

        blkif->persistent_gnt_c -= (total - num_clean);
        blkif->vbd.overflow_max_grants = 0;

        /* We can defer this work */
        INIT_WORK(&blkif->persistent_purge_work, unmap_purged_grants);
        schedule_work(&blkif->persistent_purge_work);
        pr_debug(DRV_PFX "Purged %u/%u\n", (total - num_clean), total);
        return;
}

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

        spin_lock_irqsave(&blkif->pending_free_lock, flags);
        if (!list_empty(&blkif->pending_free)) {
                req = list_entry(blkif->pending_free.next, struct pending_req,
                                 free_list);
                list_del(&req->free_list);
        }
        spin_unlock_irqrestore(&blkif->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 xen_blkif *blkif, struct pending_req *req)
{
        unsigned long flags;
        int was_empty;

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

/*
 * Routines for managing virtual block devices (vbds).
 */
static int xen_vbd_translate(struct phys_req *req, struct xen_blkif *blkif,
                             int operation)
{
        struct xen_vbd *vbd = &blkif->vbd;
        int rc = -EACCES;

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

        if (likely(req->nr_sects)) {
                blkif_sector_t end = req->sector_number + req->nr_sects;

                if (unlikely(end < req->sector_number))
                        goto out;
                if (unlikely(end > vbd_sz(vbd)))
                        goto out;
        }

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

 out:
        return rc;
}

static void xen_vbd_resize(struct xen_blkif *blkif)
{
        struct xen_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);

        pr_info(DRV_PFX "VBD Resize: Domid: %d, Device: (%d, %d)\n",
                blkif->domid, MAJOR(vbd->pdevice), MINOR(vbd->pdevice));
        pr_info(DRV_PFX "VBD Resize: new size %llu\n", new_size);
        vbd->size = new_size;
again:
        err = xenbus_transaction_start(&xbt);
        if (err) {
                pr_warn(DRV_PFX "Error starting transaction");
                return;
        }
        err = xenbus_printf(xbt, dev->nodename, "sectors", "%llu",
                            (unsigned long long)vbd_sz(vbd));
        if (err) {
                pr_warn(DRV_PFX "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) {
                pr_warn(DRV_PFX "Error writing the state");
                goto abort;
        }

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

/*
 * Notification from the guest OS.
 */
static void blkif_notify_work(struct xen_blkif *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 xen_blkif *blkif)
{
        pr_info("xen-blkback (%s): oo %3llu  |  rd %4llu  |  wr %4llu  |  f %4llu"
                 "  |  ds %4llu | pg: %4u/%4d\n",
                 current->comm, blkif->st_oo_req,
                 blkif->st_rd_req, blkif->st_wr_req,
                 blkif->st_f_req, blkif->st_ds_req,
                 blkif->persistent_gnt_c,
                 xen_blkif_max_pgrants);
        blkif->st_print = jiffies + msecs_to_jiffies(10 * 1000);
        blkif->st_rd_req = 0;
        blkif->st_wr_req = 0;
        blkif->st_oo_req = 0;
        blkif->st_ds_req = 0;
}

int xen_blkif_schedule(void *arg)
{
        struct xen_blkif *blkif = arg;
        struct xen_vbd *vbd = &blkif->vbd;
        unsigned long timeout;
        int ret;

        xen_blkif_get(blkif);

        while (!kthread_should_stop()) {
                if (try_to_freeze())
                        continue;
                if (unlikely(vbd->size != vbd_sz(vbd)))
                        xen_vbd_resize(blkif);

                timeout = msecs_to_jiffies(LRU_INTERVAL);

                timeout = wait_event_interruptible_timeout(
                        blkif->wq,
                        blkif->waiting_reqs || kthread_should_stop(),
                        timeout);
                if (timeout == 0)
                        goto purge_gnt_list;
                timeout = wait_event_interruptible_timeout(
                        blkif->pending_free_wq,
                        !list_empty(&blkif->pending_free) ||
                        kthread_should_stop(),
                        timeout);
                if (timeout == 0)
                        goto purge_gnt_list;

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

                ret = do_block_io_op(blkif);
                if (ret > 0)
                        blkif->waiting_reqs = 1;
                if (ret == -EACCES)
                        wait_event_interruptible(blkif->shutdown_wq,
                                                 kthread_should_stop());

purge_gnt_list:
                if (blkif->vbd.feature_gnt_persistent &&
                    time_after(jiffies, blkif->next_lru)) {
                        purge_persistent_gnt(blkif);
                        blkif->next_lru = jiffies + msecs_to_jiffies(LRU_INTERVAL);
                }

                /* Shrink if we have more than xen_blkif_max_buffer_pages */
                shrink_free_pagepool(blkif, xen_blkif_max_buffer_pages);

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

        /* Since we are shutting down remove all pages from the buffer */
        shrink_free_pagepool(blkif, 0 /* All */);

        /* Free all persistent grant pages */
        if (!RB_EMPTY_ROOT(&blkif->persistent_gnts))
                free_persistent_gnts(blkif, &blkif->persistent_gnts,
                        blkif->persistent_gnt_c);

        BUG_ON(!RB_EMPTY_ROOT(&blkif->persistent_gnts));
        blkif->persistent_gnt_c = 0;

        if (log_stats)
                print_stats(blkif);

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

        return 0;
}

/*
 * Unmap the grant references, and also remove the M2P over-rides
 * used in the 'pending_req'.
 */
static void xen_blkbk_unmap(struct xen_blkif *blkif,
                            struct grant_page *pages[],
                            int num)
{
        struct gnttab_unmap_grant_ref unmap[BLKIF_MAX_SEGMENTS_PER_REQUEST];
        struct page *unmap_pages[BLKIF_MAX_SEGMENTS_PER_REQUEST];
        unsigned int i, invcount = 0;
        int ret;

        for (i = 0; i < num; i++) {
                if (pages[i]->persistent_gnt != NULL) {
                        put_persistent_gnt(blkif, pages[i]->persistent_gnt);
                        continue;
                }
                if (pages[i]->handle == BLKBACK_INVALID_HANDLE)
                        continue;
                unmap_pages[invcount] = pages[i]->page;
                gnttab_set_unmap_op(&unmap[invcount], vaddr(pages[i]->page),
                                    GNTMAP_host_map, pages[i]->handle);
                pages[i]->handle = BLKBACK_INVALID_HANDLE;
                if (++invcount == BLKIF_MAX_SEGMENTS_PER_REQUEST) {
                        ret = gnttab_unmap_refs(unmap, NULL, unmap_pages,
                                                invcount);
                        BUG_ON(ret);
                        put_free_pages(blkif, unmap_pages, invcount);
                        invcount = 0;
                }
        }
        if (invcount) {
                ret = gnttab_unmap_refs(unmap, NULL, unmap_pages, invcount);
                BUG_ON(ret);
                put_free_pages(blkif, unmap_pages, invcount);
        }
}

static int xen_blkbk_map(struct xen_blkif *blkif,
                         struct grant_page *pages[],
                         int num, bool ro)
{
        struct gnttab_map_grant_ref map[BLKIF_MAX_SEGMENTS_PER_REQUEST];
        struct page *pages_to_gnt[BLKIF_MAX_SEGMENTS_PER_REQUEST];
        struct persistent_gnt *persistent_gnt = NULL;
        phys_addr_t addr = 0;
        int i, seg_idx, new_map_idx;
        int segs_to_map = 0;
        int ret = 0;
        int last_map = 0, map_until = 0;
        int use_persistent_gnts;

        use_persistent_gnts = (blkif->vbd.feature_gnt_persistent);

        /*
         * 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.
         */
again:
        for (i = map_until; i < num; i++) {
                uint32_t flags;

                if (use_persistent_gnts)
                        persistent_gnt = get_persistent_gnt(
                                blkif,
                                pages[i]->gref);

                if (persistent_gnt) {
                        /*
                         * We are using persistent grants and
                         * the grant is already mapped
                         */
                        pages[i]->page = persistent_gnt->page;
                        pages[i]->persistent_gnt = persistent_gnt;
                } else {
                        if (get_free_page(blkif, &pages[i]->page))
                                goto out_of_memory;
                        addr = vaddr(pages[i]->page);
                        pages_to_gnt[segs_to_map] = pages[i]->page;
                        pages[i]->persistent_gnt = NULL;
                        flags = GNTMAP_host_map;
                        if (!use_persistent_gnts && ro)
                                flags |= GNTMAP_readonly;
                        gnttab_set_map_op(&map[segs_to_map++], addr,
                                          flags, pages[i]->gref,
                                          blkif->domid);
                }
                map_until = i + 1;
                if (segs_to_map == BLKIF_MAX_SEGMENTS_PER_REQUEST)
                        break;
        }

        if (segs_to_map) {
                ret = gnttab_map_refs(map, NULL, pages_to_gnt, segs_to_map);
                BUG_ON(ret);
        }

        /*
         * Now swizzle 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 (seg_idx = last_map, new_map_idx = 0; seg_idx < map_until; seg_idx++) {
                if (!pages[seg_idx]->persistent_gnt) {
                        /* This is a newly mapped grant */
                        BUG_ON(new_map_idx >= segs_to_map);
                        if (unlikely(map[new_map_idx].status != 0)) {
                                pr_debug(DRV_PFX "invalid buffer -- could not remap it\n");
                                pages[seg_idx]->handle = BLKBACK_INVALID_HANDLE;
                                ret |= 1;
                                goto next;
                        }
                        pages[seg_idx]->handle = map[new_map_idx].handle;
                } else {
                        continue;
                }
                if (use_persistent_gnts &&
                    blkif->persistent_gnt_c < xen_blkif_max_pgrants) {
                        /*
                         * We are using persistent grants, the grant is
                         * not mapped but we might have room for it.
                         */
                        persistent_gnt = kmalloc(sizeof(struct persistent_gnt),
                                                 GFP_KERNEL);
                        if (!persistent_gnt) {
                                /*
                                 * If we don't have enough memory to
                                 * allocate the persistent_gnt struct
                                 * map this grant non-persistenly
                                 */
                                goto next;
                        }
                        persistent_gnt->gnt = map[new_map_idx].ref;
                        persistent_gnt->handle = map[new_map_idx].handle;
                        persistent_gnt->page = pages[seg_idx]->page;
                        if (add_persistent_gnt(blkif,
                                               persistent_gnt)) {
                                kfree(persistent_gnt);
                                persistent_gnt = NULL;
                                goto next;
                        }
                        pages[seg_idx]->persistent_gnt = persistent_gnt;
                        pr_debug(DRV_PFX " grant %u added to the tree of persistent grants, using %u/%u\n",
                                 persistent_gnt->gnt, blkif->persistent_gnt_c,
                                 xen_blkif_max_pgrants);
                        goto next;
                }
                if (use_persistent_gnts && !blkif->vbd.overflow_max_grants) {
                        blkif->vbd.overflow_max_grants = 1;
                        pr_debug(DRV_PFX " domain %u, device %#x is using maximum number of persistent grants\n",
                                 blkif->domid, blkif->vbd.handle);
                }
                /*
                 * We could not map this grant persistently, so use it as
                 * a non-persistent grant.
                 */
next:
                new_map_idx++;
        }
        segs_to_map = 0;
        last_map = map_until;
        if (map_until != num)
                goto again;

        return ret;

out_of_memory:
        pr_alert(DRV_PFX "%s: out of memory\n", __func__);
        put_free_pages(blkif, pages_to_gnt, segs_to_map);
        return -ENOMEM;
}

static int xen_blkbk_map_seg(struct pending_req *pending_req)
{
        int rc;

        rc = xen_blkbk_map(pending_req->blkif, pending_req->segments,
                           pending_req->nr_pages,
                           (pending_req->operation != BLKIF_OP_READ));

        return rc;
}

static int xen_blkbk_parse_indirect(struct blkif_request *req,
                                    struct pending_req *pending_req,
                                    struct seg_buf seg[],
                                    struct phys_req *preq)
{
        struct grant_page **pages = pending_req->indirect_pages;
        struct xen_blkif *blkif = pending_req->blkif;
        int indirect_grefs, rc, n, nseg, i;
        struct blkif_request_segment_aligned *segments = NULL;

        nseg = pending_req->nr_pages;
        indirect_grefs = INDIRECT_PAGES(nseg);
        BUG_ON(indirect_grefs > BLKIF_MAX_INDIRECT_PAGES_PER_REQUEST);

        for (i = 0; i < indirect_grefs; i++)
                pages[i]->gref = req->u.indirect.indirect_grefs[i];

        rc = xen_blkbk_map(blkif, pages, indirect_grefs, true);
        if (rc)
                goto unmap;

        for (n = 0, i = 0; n < nseg; n++) {
                if ((n % SEGS_PER_INDIRECT_FRAME) == 0) {
                        /* Map indirect segments */
                        if (segments)
                                kunmap_atomic(segments);
                        segments = kmap_atomic(pages[n/SEGS_PER_INDIRECT_FRAME]->page);
                }
                i = n % SEGS_PER_INDIRECT_FRAME;
                pending_req->segments[n]->gref = segments[i].gref;
                seg[n].nsec = segments[i].last_sect -
                        segments[i].first_sect + 1;
                seg[n].offset = (segments[i].first_sect << 9);
                if ((segments[i].last_sect >= (PAGE_SIZE >> 9)) ||
                    (segments[i].last_sect < segments[i].first_sect)) {
                        rc = -EINVAL;
                        goto unmap;
                }
                preq->nr_sects += seg[n].nsec;
        }

unmap:
        if (segments)
                kunmap_atomic(segments);
        xen_blkbk_unmap(blkif, pages, indirect_grefs);
        return rc;
}

static int dispatch_discard_io(struct xen_blkif *blkif,
                                struct blkif_request *req)
{
        int err = 0;
        int status = BLKIF_RSP_OKAY;
        struct block_device *bdev = blkif->vbd.bdev;
        unsigned long secure;
        struct phys_req preq;

        preq.sector_number = req->u.discard.sector_number;
        preq.nr_sects      = req->u.discard.nr_sectors;

        err = xen_vbd_translate(&preq, blkif, WRITE);
        if (err) {
                pr_warn(DRV_PFX "access denied: DISCARD [%llu->%llu] on dev=%04x\n",
                        preq.sector_number,
                        preq.sector_number + preq.nr_sects, blkif->vbd.pdevice);
                goto fail_response;
        }
        blkif->st_ds_req++;

        xen_blkif_get(blkif);
        secure = (blkif->vbd.discard_secure &&
                 (req->u.discard.flag & BLKIF_DISCARD_SECURE)) ?
                 BLKDEV_DISCARD_SECURE : 0;

        err = blkdev_issue_discard(bdev, req->u.discard.sector_number,
                                   req->u.discard.nr_sectors,
                                   GFP_KERNEL, secure);
fail_response:
        if (err == -EOPNOTSUPP) {
                pr_debug(DRV_PFX "discard op failed, not supported\n");
                status = BLKIF_RSP_EOPNOTSUPP;
        } else if (err)
                status = BLKIF_RSP_ERROR;

        make_response(blkif, req->u.discard.id, req->operation, status);
        xen_blkif_put(blkif);
        return err;
}

static int dispatch_other_io(struct xen_blkif *blkif,
                             struct blkif_request *req,
                             struct pending_req *pending_req)
{
        free_req(blkif, pending_req);
        make_response(blkif, req->u.other.id, req->operation,
                      BLKIF_RSP_EOPNOTSUPP);
        return -EIO;
}

static void xen_blk_drain_io(struct xen_blkif *blkif)
{
        atomic_set(&blkif->drain, 1);
        do {
                /* The initial value is one, and one refcnt taken at the
                 * start of the xen_blkif_schedule thread. */
                if (atomic_read(&blkif->refcnt) <= 2)
                        break;
                wait_for_completion_interruptible_timeout(
                                &blkif->drain_complete, HZ);

                if (!atomic_read(&blkif->drain))
                        break;
        } while (!kthread_should_stop());
        atomic_set(&blkif->drain, 0);
}

/*
 * 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_FLUSH_DISKCACHE) &&
            (error == -EOPNOTSUPP)) {
                pr_debug(DRV_PFX "flush diskcache op failed, not supported\n");
                xen_blkbk_flush_diskcache(XBT_NIL, pending_req->blkif->be, 0);
                pending_req->status = BLKIF_RSP_EOPNOTSUPP;
        } else if ((pending_req->operation == BLKIF_OP_WRITE_BARRIER) &&
                    (error == -EOPNOTSUPP)) {
                pr_debug(DRV_PFX "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) {
                pr_debug(DRV_PFX "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->blkif,
                                pending_req->segments,
                                pending_req->nr_pages);
                make_response(pending_req->blkif, pending_req->id,
                              pending_req->operation, pending_req->status);
                xen_blkif_put(pending_req->blkif);
                if (atomic_read(&pending_req->blkif->refcnt) <= 2) {
                        if (atomic_read(&pending_req->blkif->drain))
                                complete(&pending_req->blkif->drain_complete);
                }
                free_req(pending_req->blkif, 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 xen_blkif *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'. */

        if (RING_REQUEST_PROD_OVERFLOW(&blk_rings->common, rp)) {
                rc = blk_rings->common.rsp_prod_pvt;
                pr_warn(DRV_PFX "Frontend provided bogus ring requests (%d - %d = %d). Halting ring processing on dev=%04x\n",
                        rp, rc, rp - rc, blkif->vbd.pdevice);
                return -EACCES;
        }
        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(blkif);
                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:
                case BLKIF_OP_WRITE:
                case BLKIF_OP_WRITE_BARRIER:
                case BLKIF_OP_FLUSH_DISKCACHE:
                case BLKIF_OP_INDIRECT:
                        if (dispatch_rw_block_io(blkif, &req, pending_req))
                                goto done;
                        break;
                case BLKIF_OP_DISCARD:
                        free_req(blkif, pending_req);
                        if (dispatch_discard_io(blkif, &req))
                                goto done;
                        break;
                default:
                        if (dispatch_other_io(blkif, &req, pending_req))
                                goto done;
                        break;
                }

                /* Yield point for this unbounded loop. */
                cond_resched();
        }
done:
        return more_to_do;
}

static int
do_block_io_op(struct xen_blkif *blkif)
{
        union blkif_back_rings *blk_rings = &blkif->blk_rings;
        int more_to_do;

        do {
                more_to_do = __do_block_io_op(blkif);
                if (more_to_do)
                        break;

                RING_FINAL_CHECK_FOR_REQUESTS(&blk_rings->common, more_to_do);
        } while (more_to_do);

        return more_to_do;
}
/*
 * Transmutation of the 'struct blkif_request' to a proper 'struct bio'
 * and call the 'submit_bio' to pass it to the underlying storage.
 */
static int dispatch_rw_block_io(struct xen_blkif *blkif,
                                struct blkif_request *req,
                                struct pending_req *pending_req)
{
        struct phys_req preq;
        struct seg_buf *seg = pending_req->seg;
        unsigned int nseg;
        struct bio *bio = NULL;
        struct bio **biolist = pending_req->biolist;
        int i, nbio = 0;
        int operation;
        struct blk_plug plug;
        bool drain = false;
        struct grant_page **pages = pending_req->segments;
        unsigned short req_operation;

        req_operation = req->operation == BLKIF_OP_INDIRECT ?
                        req->u.indirect.indirect_op : req->operation;
        if ((req->operation == BLKIF_OP_INDIRECT) &&
            (req_operation != BLKIF_OP_READ) &&
            (req_operation != BLKIF_OP_WRITE)) {
                pr_debug(DRV_PFX "Invalid indirect operation (%u)\n",
                         req_operation);
                goto fail_response;
        }

        switch (req_operation) {
        case BLKIF_OP_READ:
                blkif->st_rd_req++;
                operation = READ;
                break;
        case BLKIF_OP_WRITE:
                blkif->st_wr_req++;
                operation = WRITE_ODIRECT;
                break;
        case BLKIF_OP_WRITE_BARRIER:
                drain = true;
        case BLKIF_OP_FLUSH_DISKCACHE:
                blkif->st_f_req++;
                operation = WRITE_FLUSH;
                break;
        default:
                operation = 0; /* make gcc happy */
                goto fail_response;
                break;
        }

        /* Check that the number of segments is sane. */
        nseg = req->operation == BLKIF_OP_INDIRECT ?
               req->u.indirect.nr_segments : req->u.rw.nr_segments;

        if (unlikely(nseg == 0 && operation != WRITE_FLUSH) ||
            unlikely((req->operation != BLKIF_OP_INDIRECT) &&
                     (nseg > BLKIF_MAX_SEGMENTS_PER_REQUEST)) ||
            unlikely((req->operation == BLKIF_OP_INDIRECT) &&
                     (nseg > MAX_INDIRECT_SEGMENTS))) {
                pr_debug(DRV_PFX "Bad number of segments in request (%d)\n",
                         nseg);
                /* Haven't submitted any bio's yet. */
                goto fail_response;
        }

        preq.nr_sects      = 0;

        pending_req->blkif     = blkif;
        pending_req->id        = req->u.rw.id;
        pending_req->operation = req_operation;
        pending_req->status    = BLKIF_RSP_OKAY;
        pending_req->nr_pages  = nseg;

        if (req->operation != BLKIF_OP_INDIRECT) {
                preq.dev               = req->u.rw.handle;
                preq.sector_number     = req->u.rw.sector_number;
                for (i = 0; i < nseg; i++) {
                        pages[i]->gref = req->u.rw.seg[i].gref;
                        seg[i].nsec = req->u.rw.seg[i].last_sect -
                                req->u.rw.seg[i].first_sect + 1;
                        seg[i].offset = (req->u.rw.seg[i].first_sect << 9);
                        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;
                }
        } else {
                preq.dev               = req->u.indirect.handle;
                preq.sector_number     = req->u.indirect.sector_number;
                if (xen_blkbk_parse_indirect(req, pending_req, seg, &preq))
                        goto fail_response;
        }

        if (xen_vbd_translate(&preq, blkif, operation) != 0) {
                pr_debug(DRV_PFX "access denied: %s of [%llu,%llu] on dev=%04x\n",
                         operation == READ ? "read" : "write",
                         preq.sector_number,
                         preq.sector_number + preq.nr_sects,
                         blkif->vbd.pdevice);
                goto fail_response;
        }

        /*
         * This check _MUST_ be done after xen_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)) {
                        pr_debug(DRV_PFX "Misaligned I/O request from domain %d",
                                 blkif->domid);
                        goto fail_response;
                }
        }

        /* Wait on all outstanding I/O's and once that has been completed
         * issue the WRITE_FLUSH.
         */
        if (drain)
                xen_blk_drain_io(pending_req->blkif);

        /*
         * 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_seg(pending_req))
                goto fail_flush;

        /*
         * This corresponding xen_blkif_put is done in __end_block_io_op, or
         * below (in "!bio") if we are handling a BLKIF_OP_DISCARD.
         */
        xen_blkif_get(blkif);

        for (i = 0; i < nseg; i++) {
                while ((bio == NULL) ||
                       (bio_add_page(bio,
                                     pages[i]->page,
                                     seg[i].nsec << 9,
                                     seg[i].offset) == 0)) {

                        int nr_iovecs = min_t(int, (nseg-i), BIO_MAX_PAGES);
                        bio = bio_alloc(GFP_KERNEL, nr_iovecs);
                        if (unlikely(bio == NULL))
                                goto fail_put_bio;

                        biolist[nbio++] = 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 flush or discard. */
        if (!bio) {
                BUG_ON(operation != WRITE_FLUSH);

                bio = bio_alloc(GFP_KERNEL, 0);
                if (unlikely(bio == NULL))
                        goto fail_put_bio;

                biolist[nbio++] = bio;
                bio->bi_bdev    = preq.bdev;
                bio->bi_private = pending_req;
                bio->bi_end_io  = end_block_io_op;
        }

        atomic_set(&pending_req->pendcnt, nbio);
        blk_start_plug(&plug);

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

        /* Let the I/Os go.. */
        blk_finish_plug(&plug);

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

        return 0;

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

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



/*
 * Put a response on the ring on how the operation fared.
 */
static void make_response(struct xen_blkif *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 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);
        spin_unlock_irqrestore(&blkif->blk_ring_lock, flags);
        if (notify)
                notify_remote_via_irq(blkif->irq);
}

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

        if (!xen_domain())
                return -ENODEV;

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

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

 failed_init:
        return rc;
}

module_init(xen_blkif_init);

MODULE_LICENSE("Dual BSD/GPL");
MODULE_ALIAS("xen-backend:vbd");