5 files changed, 403 insertions, 66 deletions

diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
index 2330a9ad5e95..79efec24569b 100644
--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -39,6 +39,9 @@
 #include <linux/list.h>
 #include <linux/delay.h>
 #include <linux/freezer.h>
+#include <linux/loop.h>
+#include <linux/falloc.h>
+#include <linux/fs.h>
 
 #include <xen/events.h>
 #include <xen/page.h>
@@ -258,13 +261,16 @@ irqreturn_t xen_blkif_be_int(int irq, void *dev_id)
 
 static void print_stats(struct xen_blkif *blkif)
 {
-        pr_info("xen-blkback (%s): oo %3d  |  rd %4d  |  wr %4d  |  f %4d\n",
+        pr_info("xen-blkback (%s): oo %3d  |  rd %4d  |  wr %4d  |  f %4d"
+                 "  |  ds %4d\n",
                  current->comm, blkif->st_oo_req,
-                 blkif->st_rd_req, blkif->st_wr_req, blkif->st_f_req);
+                 blkif->st_rd_req, blkif->st_wr_req,
+                 blkif->st_f_req, blkif->st_ds_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;
+        blkif->st_ds_req = 0;
 }
 
 int xen_blkif_schedule(void *arg)
@@ -410,6 +416,59 @@ static int xen_blkbk_map(struct blkif_request *req,
         return ret;
 }
 
+static void xen_blk_discard(struct xen_blkif *blkif, struct blkif_request *req)
+{
+        int err = 0;
+        int status = BLKIF_RSP_OKAY;
+        struct block_device *bdev = blkif->vbd.bdev;
+
+        if (blkif->blk_backend_type == BLKIF_BACKEND_PHY)
+                /* just forward the discard request */
+                err = blkdev_issue_discard(bdev,
+                                req->u.discard.sector_number,
+                                req->u.discard.nr_sectors,
+                                GFP_KERNEL, 0);
+        else if (blkif->blk_backend_type == BLKIF_BACKEND_FILE) {
+                /* punch a hole in the backing file */
+                struct loop_device *lo = bdev->bd_disk->private_data;
+                struct file *file = lo->lo_backing_file;
+
+                if (file->f_op->fallocate)
+                        err = file->f_op->fallocate(file,
+                                FALLOC_FL_KEEP_SIZE | FALLOC_FL_PUNCH_HOLE,
+                                req->u.discard.sector_number << 9,
+                                req->u.discard.nr_sectors << 9);
+                else
+                        err = -EOPNOTSUPP;
+        } else
+                err = -EOPNOTSUPP;
+
+        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->id, req->operation, status);
+}
+
+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()
  */
@@ -422,6 +481,11 @@ static void __end_block_io_op(struct pending_req *pending_req, int error)
                 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);
@@ -438,6 +502,10 @@ static void __end_block_io_op(struct pending_req *pending_req, int error)
                 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);
         }
 }
@@ -532,7 +600,6 @@ do_block_io_op(struct xen_blkif *blkif)
 
         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.
@@ -549,6 +616,7 @@ static int dispatch_rw_block_io(struct xen_blkif *blkif,
         int i, nbio = 0;
         int operation;
         struct blk_plug plug;
+        bool drain = false;
 
         switch (req->operation) {
         case BLKIF_OP_READ:
@@ -559,11 +627,16 @@ static int dispatch_rw_block_io(struct xen_blkif *blkif,
                 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;
-        case BLKIF_OP_WRITE_BARRIER:
+        case BLKIF_OP_DISCARD:
+                blkif->st_ds_req++;
+                operation = REQ_DISCARD;
+                break;
         default:
                 operation = 0; /* make gcc happy */
                 goto fail_response;
@@ -572,7 +645,8 @@ static int dispatch_rw_block_io(struct xen_blkif *blkif,
 
         /* Check that the number of segments is sane. */
         nseg = req->nr_segments;
-        if (unlikely(nseg == 0 && operation != WRITE_FLUSH) ||
+        if (unlikely(nseg == 0 && operation != WRITE_FLUSH &&
+                                operation != REQ_DISCARD) ||
             unlikely(nseg > BLKIF_MAX_SEGMENTS_PER_REQUEST)) {
                 pr_debug(DRV_PFX "Bad number of segments in request (%d)\n",
                          nseg);
@@ -621,16 +695,25 @@ static int dispatch_rw_block_io(struct xen_blkif *blkif,
                 }
         }
 
+        /* 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(req, pending_req, seg))
+        if (operation != REQ_DISCARD && xen_blkbk_map(req, pending_req, seg))
                 goto fail_flush;
 
-        /* This corresponding xen_blkif_put is done in __end_block_io_op */
+        /*
+         * 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++) {
@@ -654,18 +737,25 @@ static int dispatch_rw_block_io(struct xen_blkif *blkif,
                 preq.sector_number += seg[i].nsec;
         }
 
-        /* This will be hit if the operation was a flush. */
+        /* This will be hit if the operation was a flush or discard. */
         if (!bio) {
-                BUG_ON(operation != WRITE_FLUSH);
+                BUG_ON(operation != WRITE_FLUSH && operation != REQ_DISCARD);
 
-                bio = bio_alloc(GFP_KERNEL, 0);
-                if (unlikely(bio == NULL))
-                        goto fail_put_bio;
+                if (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;
+                        biolist[nbio++] = bio;
+                        bio->bi_bdev    = preq.bdev;
+                        bio->bi_private = pending_req;
+                        bio->bi_end_io  = end_block_io_op;
+                } else if (operation == REQ_DISCARD) {
+                        xen_blk_discard(blkif, req);
+                        xen_blkif_put(blkif);
+                        free_req(pending_req);
+                        return 0;
+                }
         }
 
         /*
@@ -685,7 +775,7 @@ static int dispatch_rw_block_io(struct xen_blkif *blkif,
 
         if (operation == READ)
                 blkif->st_rd_sect += preq.nr_sects;
-        else if (operation == WRITE || operation == WRITE_FLUSH)
+        else if (operation & WRITE)
                 blkif->st_wr_sect += preq.nr_sects;
 
         return 0;
@@ -765,9 +855,9 @@ static int __init xen_blkif_init(void)
 
         mmap_pages = xen_blkif_reqs * BLKIF_MAX_SEGMENTS_PER_REQUEST;
 
-        blkbk->pending_reqs          = kmalloc(sizeof(blkbk->pending_reqs[0]) *
+        blkbk->pending_reqs          = kzalloc(sizeof(blkbk->pending_reqs[0]) *
                                         xen_blkif_reqs, GFP_KERNEL);
-        blkbk->pending_grant_handles = kzalloc(sizeof(blkbk->pending_grant_handles[0]) *
+        blkbk->pending_grant_handles = kmalloc(sizeof(blkbk->pending_grant_handles[0]) *
                                         mmap_pages, GFP_KERNEL);
         blkbk->pending_pages         = kzalloc(sizeof(blkbk->pending_pages[0]) *
                                         mmap_pages, GFP_KERNEL);
@@ -790,8 +880,6 @@ static int __init xen_blkif_init(void)
         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);
diff --git a/drivers/block/xen-blkback/common.h b/drivers/block/xen-blkback/common.h
index 9e40b283a468..e638457d9de4 100644
--- a/drivers/block/xen-blkback/common.h
+++ b/drivers/block/xen-blkback/common.h
@@ -46,7 +46,7 @@
 
 #define DRV_PFX "xen-blkback:"
 #define DPRINTK(fmt, args...)                           \
-        pr_debug(DRV_PFX "(%s:%d) " fmt ".\n",  \
+        pr_debug(DRV_PFX "(%s:%d) " fmt ".\n",          \
                  __func__, __LINE__, ##args)
 
 
@@ -63,13 +63,26 @@ struct blkif_common_response {
 
 /* i386 protocol version */
 #pragma pack(push, 4)
+
+struct blkif_x86_32_request_rw {
+        blkif_sector_t sector_number;/* start sector idx on disk (r/w only)  */
+        struct blkif_request_segment seg[BLKIF_MAX_SEGMENTS_PER_REQUEST];
+};
+
+struct blkif_x86_32_request_discard {
+        blkif_sector_t sector_number;/* start sector idx on disk (r/w only)  */
+        uint64_t nr_sectors;
+};
+
 struct blkif_x86_32_request {
         uint8_t        operation;    /* BLKIF_OP_???                         */
         uint8_t        nr_segments;  /* number of segments                   */
         blkif_vdev_t   handle;       /* only for read/write requests         */
         uint64_t       id;           /* private guest value, echoed in resp  */
-        blkif_sector_t sector_number;/* start sector idx on disk (r/w only)  */
-        struct blkif_request_segment seg[BLKIF_MAX_SEGMENTS_PER_REQUEST];
+        union {
+                struct blkif_x86_32_request_rw rw;
+                struct blkif_x86_32_request_discard discard;
+        } u;
 };
 struct blkif_x86_32_response {
         uint64_t        id;              /* copied from request */
@@ -79,13 +92,26 @@ struct blkif_x86_32_response {
 #pragma pack(pop)
 
 /* x86_64 protocol version */
+
+struct blkif_x86_64_request_rw {
+        blkif_sector_t sector_number;/* start sector idx on disk (r/w only)  */
+        struct blkif_request_segment seg[BLKIF_MAX_SEGMENTS_PER_REQUEST];
+};
+
+struct blkif_x86_64_request_discard {
+        blkif_sector_t sector_number;/* start sector idx on disk (r/w only)  */
+        uint64_t nr_sectors;
+};
+
 struct blkif_x86_64_request {
         uint8_t        operation;    /* BLKIF_OP_???                         */
         uint8_t        nr_segments;  /* number of segments                   */
         blkif_vdev_t   handle;       /* only for read/write requests         */
         uint64_t       __attribute__((__aligned__(8))) id;
-        blkif_sector_t sector_number;/* start sector idx on disk (r/w only)  */
-        struct blkif_request_segment seg[BLKIF_MAX_SEGMENTS_PER_REQUEST];
+        union {
+                struct blkif_x86_64_request_rw rw;
+                struct blkif_x86_64_request_discard discard;
+        } u;
 };
 struct blkif_x86_64_response {
         uint64_t       __attribute__((__aligned__(8))) id;
@@ -113,6 +139,11 @@ enum blkif_protocol {
         BLKIF_PROTOCOL_X86_64 = 3,
 };
 
+enum blkif_backend_type {
+        BLKIF_BACKEND_PHY  = 1,
+        BLKIF_BACKEND_FILE = 2,
+};
+
 struct xen_vbd {
         /* What the domain refers to this vbd as. */
         blkif_vdev_t            handle;
@@ -138,6 +169,7 @@ struct xen_blkif {
         unsigned int            irq;
         /* Comms information. */
         enum blkif_protocol     blk_protocol;
+        enum blkif_backend_type blk_backend_type;
         union blkif_back_rings  blk_rings;
         struct vm_struct        *blk_ring_area;
         /* The VBD attached to this interface. */
@@ -149,6 +181,9 @@ struct xen_blkif {
         atomic_t                refcnt;
 
         wait_queue_head_t       wq;
+        /* for barrier (drain) requests */
+        struct completion       drain_complete;
+        atomic_t                drain;
         /* One thread per one blkif. */
         struct task_struct      *xenblkd;
         unsigned int            waiting_reqs;
@@ -159,6 +194,7 @@ struct xen_blkif {
         int                     st_wr_req;
         int                     st_oo_req;
         int                     st_f_req;
+        int                     st_ds_req;
         int                     st_rd_sect;
         int                     st_wr_sect;
 
@@ -182,7 +218,7 @@ struct xen_blkif {
 
 struct phys_req {
         unsigned short          dev;
-        unsigned short          nr_sects;
+        blkif_sector_t          nr_sects;
         struct block_device     *bdev;
         blkif_sector_t          sector_number;
 };
@@ -196,6 +232,8 @@ int xen_blkif_schedule(void *arg);
 int xen_blkbk_flush_diskcache(struct xenbus_transaction xbt,
                               struct backend_info *be, int state);
 
+int xen_blkbk_barrier(struct xenbus_transaction xbt,
+                      struct backend_info *be, int state);
 struct xenbus_device *xen_blkbk_xenbus(struct backend_info *be);
 
 static inline void blkif_get_x86_32_req(struct blkif_request *dst,
@@ -206,12 +244,25 @@ static inline void blkif_get_x86_32_req(struct blkif_request *dst,
         dst->nr_segments = src->nr_segments;
         dst->handle = src->handle;
         dst->id = src->id;
-        dst->u.rw.sector_number = src->sector_number;
-        barrier();
-        if (n > dst->nr_segments)
-                n = dst->nr_segments;
-        for (i = 0; i < n; i++)
-                dst->u.rw.seg[i] = src->seg[i];
+        switch (src->operation) {
+        case BLKIF_OP_READ:
+        case BLKIF_OP_WRITE:
+        case BLKIF_OP_WRITE_BARRIER:
+        case BLKIF_OP_FLUSH_DISKCACHE:
+                dst->u.rw.sector_number = src->u.rw.sector_number;
+                barrier();
+                if (n > dst->nr_segments)
+                        n = dst->nr_segments;
+                for (i = 0; i < n; i++)
+                        dst->u.rw.seg[i] = src->u.rw.seg[i];
+                break;
+        case BLKIF_OP_DISCARD:
+                dst->u.discard.sector_number = src->u.discard.sector_number;
+                dst->u.discard.nr_sectors = src->u.discard.nr_sectors;
+                break;
+        default:
+                break;
+        }
 }
 
 static inline void blkif_get_x86_64_req(struct blkif_request *dst,
@@ -222,12 +273,25 @@ static inline void blkif_get_x86_64_req(struct blkif_request *dst,
         dst->nr_segments = src->nr_segments;
         dst->handle = src->handle;
         dst->id = src->id;
-        dst->u.rw.sector_number = src->sector_number;
-        barrier();
-        if (n > dst->nr_segments)
-                n = dst->nr_segments;
-        for (i = 0; i < n; i++)
-                dst->u.rw.seg[i] = src->seg[i];
+        switch (src->operation) {
+        case BLKIF_OP_READ:
+        case BLKIF_OP_WRITE:
+        case BLKIF_OP_WRITE_BARRIER:
+        case BLKIF_OP_FLUSH_DISKCACHE:
+                dst->u.rw.sector_number = src->u.rw.sector_number;
+                barrier();
+                if (n > dst->nr_segments)
+                        n = dst->nr_segments;
+                for (i = 0; i < n; i++)
+                        dst->u.rw.seg[i] = src->u.rw.seg[i];
+                break;
+        case BLKIF_OP_DISCARD:
+                dst->u.discard.sector_number = src->u.discard.sector_number;
+                dst->u.discard.nr_sectors = src->u.discard.nr_sectors;
+                break;
+        default:
+                break;
+        }
 }
 
 #endif /* __XEN_BLKIF__BACKEND__COMMON_H__ */
diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
index 3f129b45451a..a6d43030b107 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -114,6 +114,8 @@ static struct xen_blkif *xen_blkif_alloc(domid_t domid)
         spin_lock_init(&blkif->blk_ring_lock);
         atomic_set(&blkif->refcnt, 1);
         init_waitqueue_head(&blkif->wq);
+        init_completion(&blkif->drain_complete);
+        atomic_set(&blkif->drain, 0);
         blkif->st_print = jiffies;
         init_waitqueue_head(&blkif->waiting_to_free);
 
@@ -272,6 +274,7 @@ VBD_SHOW(oo_req,  "%d\n", be->blkif->st_oo_req);
 VBD_SHOW(rd_req,  "%d\n", be->blkif->st_rd_req);
 VBD_SHOW(wr_req,  "%d\n", be->blkif->st_wr_req);
 VBD_SHOW(f_req,  "%d\n", be->blkif->st_f_req);
+VBD_SHOW(ds_req,  "%d\n", be->blkif->st_ds_req);
 VBD_SHOW(rd_sect, "%d\n", be->blkif->st_rd_sect);
 VBD_SHOW(wr_sect, "%d\n", be->blkif->st_wr_sect);
 
@@ -280,6 +283,7 @@ static struct attribute *xen_vbdstat_attrs[] = {
         &dev_attr_rd_req.attr,
         &dev_attr_wr_req.attr,
         &dev_attr_f_req.attr,
+        &dev_attr_ds_req.attr,
         &dev_attr_rd_sect.attr,
         &dev_attr_wr_sect.attr,
         NULL
@@ -419,6 +423,73 @@ int xen_blkbk_flush_diskcache(struct xenbus_transaction xbt,
         return err;
 }
 
+int xen_blkbk_discard(struct xenbus_transaction xbt, struct backend_info *be)
+{
+        struct xenbus_device *dev = be->dev;
+        struct xen_blkif *blkif = be->blkif;
+        char *type;
+        int err;
+        int state = 0;
+
+        type = xenbus_read(XBT_NIL, dev->nodename, "type", NULL);
+        if (!IS_ERR(type)) {
+                if (strncmp(type, "file", 4) == 0) {
+                        state = 1;
+                        blkif->blk_backend_type = BLKIF_BACKEND_FILE;
+                }
+                if (strncmp(type, "phy", 3) == 0) {
+                        struct block_device *bdev = be->blkif->vbd.bdev;
+                        struct request_queue *q = bdev_get_queue(bdev);
+                        if (blk_queue_discard(q)) {
+                                err = xenbus_printf(xbt, dev->nodename,
+                                        "discard-granularity", "%u",
+                                        q->limits.discard_granularity);
+                                if (err) {
+                                        xenbus_dev_fatal(dev, err,
+                                                "writing discard-granularity");
+                                        goto kfree;
+                                }
+                                err = xenbus_printf(xbt, dev->nodename,
+                                        "discard-alignment", "%u",
+                                        q->limits.discard_alignment);
+                                if (err) {
+                                        xenbus_dev_fatal(dev, err,
+                                                "writing discard-alignment");
+                                        goto kfree;
+                                }
+                                state = 1;
+                                blkif->blk_backend_type = BLKIF_BACKEND_PHY;
+                        }
+                }
+        } else {
+                err = PTR_ERR(type);
+                xenbus_dev_fatal(dev, err, "reading type");
+                goto out;
+        }
+
+        err = xenbus_printf(xbt, dev->nodename, "feature-discard",
+                            "%d", state);
+        if (err)
+                xenbus_dev_fatal(dev, err, "writing feature-discard");
+kfree:
+        kfree(type);
+out:
+        return err;
+}
+int xen_blkbk_barrier(struct xenbus_transaction xbt,
+                      struct backend_info *be, int state)
+{
+        struct xenbus_device *dev = be->dev;
+        int err;
+
+        err = xenbus_printf(xbt, dev->nodename, "feature-barrier",
+                            "%d", state);
+        if (err)
+                xenbus_dev_fatal(dev, err, "writing feature-barrier");
+
+        return err;
+}
+
 /*
  * Entry point to this code when a new device is created.  Allocate the basic
  * structures, and watch the store waiting for the hotplug scripts to tell us
@@ -590,7 +661,7 @@ static void frontend_changed(struct xenbus_device *dev,
 
                 /*
                  * Enforce precondition before potential leak point.
-                 * blkif_disconnect() is idempotent.
+                 * xen_blkif_disconnect() is idempotent.
                  */
                 xen_blkif_disconnect(be->blkif);
 
@@ -611,7 +682,7 @@ static void frontend_changed(struct xenbus_device *dev,
                         break;
                 /* fall through if not online */
         case XenbusStateUnknown:
-                /* implies blkif_disconnect() via blkback_remove() */
+                /* implies xen_blkif_disconnect() via xen_blkbk_remove() */
                 device_unregister(&dev->dev);
                 break;
 
@@ -650,6 +721,11 @@ again:
         if (err)
                 goto abort;
 
+        err = xen_blkbk_discard(xbt, be);
+
+        /* If we can't advertise it is OK. */
+        err = xen_blkbk_barrier(xbt, be, be->blkif->vbd.flush_support);
+
         err = xenbus_printf(xbt, dev->nodename, "sectors", "%llu",
                             (unsigned long long)vbd_sz(&be->blkif->vbd));
         if (err) {
diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index b536a9cef917..773da7d6491e 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -98,6 +98,9 @@ struct blkfront_info
         unsigned long shadow_free;
         unsigned int feature_flush;
         unsigned int flush_op;
+        unsigned int feature_discard;
+        unsigned int discard_granularity;
+        unsigned int discard_alignment;
         int is_ready;
 };
 
@@ -302,29 +305,36 @@ static int blkif_queue_request(struct request *req)
                 ring_req->operation = info->flush_op;
         }
 
-        ring_req->nr_segments = blk_rq_map_sg(req->q, req, info->sg);
-        BUG_ON(ring_req->nr_segments > BLKIF_MAX_SEGMENTS_PER_REQUEST);
+        if (unlikely(req->cmd_flags & REQ_DISCARD)) {
+                /* id, sector_number and handle are set above. */
+                ring_req->operation = BLKIF_OP_DISCARD;
+                ring_req->nr_segments = 0;
+                ring_req->u.discard.nr_sectors = blk_rq_sectors(req);
+        } else {
+                ring_req->nr_segments = blk_rq_map_sg(req->q, req, info->sg);
+                BUG_ON(ring_req->nr_segments > BLKIF_MAX_SEGMENTS_PER_REQUEST);
 
-        for_each_sg(info->sg, sg, ring_req->nr_segments, i) {
-                buffer_mfn = pfn_to_mfn(page_to_pfn(sg_page(sg)));
-                fsect = sg->offset >> 9;
-                lsect = fsect + (sg->length >> 9) - 1;
-                /* install a grant reference. */
-                ref = gnttab_claim_grant_reference(&gref_head);
-                BUG_ON(ref == -ENOSPC);
+                for_each_sg(info->sg, sg, ring_req->nr_segments, i) {
+                        buffer_mfn = pfn_to_mfn(page_to_pfn(sg_page(sg)));
+                        fsect = sg->offset >> 9;
+                        lsect = fsect + (sg->length >> 9) - 1;
+                        /* install a grant reference. */
+                        ref = gnttab_claim_grant_reference(&gref_head);
+                        BUG_ON(ref == -ENOSPC);
 
-                gnttab_grant_foreign_access_ref(
-                                ref,
-                                info->xbdev->otherend_id,
-                                buffer_mfn,
-                                rq_data_dir(req) );
-
-                info->shadow[id].frame[i] = mfn_to_pfn(buffer_mfn);
-                ring_req->u.rw.seg[i] =
-                                (struct blkif_request_segment) {
-                                        .gref       = ref,
-                                        .first_sect = fsect,
-                                        .last_sect  = lsect };
+                        gnttab_grant_foreign_access_ref(
+                                        ref,
+                                        info->xbdev->otherend_id,
+                                        buffer_mfn,
+                                        rq_data_dir(req));
+
+                        info->shadow[id].frame[i] = mfn_to_pfn(buffer_mfn);
+                        ring_req->u.rw.seg[i] =
+                                        (struct blkif_request_segment) {
+                                                .gref       = ref,
+                                                .first_sect = fsect,
+                                                .last_sect  = lsect };
+                }
         }
 
         info->ring.req_prod_pvt++;
@@ -370,7 +380,9 @@ static void do_blkif_request(struct request_queue *rq)
 
                 blk_start_request(req);
 
-                if (req->cmd_type != REQ_TYPE_FS) {
+                if ((req->cmd_type != REQ_TYPE_FS) ||
+                    ((req->cmd_flags & (REQ_FLUSH | REQ_FUA)) &&
+                    !info->flush_op)) {
                         __blk_end_request_all(req, -EIO);
                         continue;
                 }
@@ -399,6 +411,7 @@ wait:
 static int xlvbd_init_blk_queue(struct gendisk *gd, u16 sector_size)
 {
         struct request_queue *rq;
+        struct blkfront_info *info = gd->private_data;
 
         rq = blk_init_queue(do_blkif_request, &blkif_io_lock);
         if (rq == NULL)
@@ -406,6 +419,13 @@ static int xlvbd_init_blk_queue(struct gendisk *gd, u16 sector_size)
 
         queue_flag_set_unlocked(QUEUE_FLAG_VIRT, rq);
 
+        if (info->feature_discard) {
+                queue_flag_set_unlocked(QUEUE_FLAG_DISCARD, rq);
+                blk_queue_max_discard_sectors(rq, get_capacity(gd));
+                rq->limits.discard_granularity = info->discard_granularity;
+                rq->limits.discard_alignment = info->discard_alignment;
+        }
+
         /* Hard sector size and max sectors impersonate the equiv. hardware. */
         blk_queue_logical_block_size(rq, sector_size);
         blk_queue_max_hw_sectors(rq, 512);
@@ -722,6 +742,17 @@ static irqreturn_t blkif_interrupt(int irq, void *dev_id)
 
                 error = (bret->status == BLKIF_RSP_OKAY) ? 0 : -EIO;
                 switch (bret->operation) {
+                case BLKIF_OP_DISCARD:
+                        if (unlikely(bret->status == BLKIF_RSP_EOPNOTSUPP)) {
+                                struct request_queue *rq = info->rq;
+                                printk(KERN_WARNING "blkfront: %s: discard op failed\n",
+                                           info->gd->disk_name);
+                                error = -EOPNOTSUPP;
+                                info->feature_discard = 0;
+                                queue_flag_clear(QUEUE_FLAG_DISCARD, rq);
+                        }
+                        __blk_end_request_all(req, error);
+                        break;
                 case BLKIF_OP_FLUSH_DISKCACHE:
                 case BLKIF_OP_WRITE_BARRIER:
                         if (unlikely(bret->status == BLKIF_RSP_EOPNOTSUPP)) {
@@ -1098,6 +1129,33 @@ blkfront_closing(struct blkfront_info *info)
         bdput(bdev);
 }
 
+static void blkfront_setup_discard(struct blkfront_info *info)
+{
+        int err;
+        char *type;
+        unsigned int discard_granularity;
+        unsigned int discard_alignment;
+
+        type = xenbus_read(XBT_NIL, info->xbdev->otherend, "type", NULL);
+        if (IS_ERR(type))
+                return;
+
+        if (strncmp(type, "phy", 3) == 0) {
+                err = xenbus_gather(XBT_NIL, info->xbdev->otherend,
+                        "discard-granularity", "%u", &discard_granularity,
+                        "discard-alignment", "%u", &discard_alignment,
+                        NULL);
+                if (!err) {
+                        info->feature_discard = 1;
+                        info->discard_granularity = discard_granularity;
+                        info->discard_alignment = discard_alignment;
+                }
+        } else if (strncmp(type, "file", 4) == 0)
+                info->feature_discard = 1;
+
+        kfree(type);
+}
+
 /*
  * Invoked when the backend is finally 'ready' (and has told produced
  * the details about the physical device - #sectors, size, etc).
@@ -1108,7 +1166,7 @@ static void blkfront_connect(struct blkfront_info *info)
         unsigned long sector_size;
         unsigned int binfo;
         int err;
-        int barrier, flush;
+        int barrier, flush, discard;
 
         switch (info->connected) {
         case BLKIF_STATE_CONNECTED:
@@ -1178,7 +1236,14 @@ static void blkfront_connect(struct blkfront_info *info)
                 info->feature_flush = REQ_FLUSH;
                 info->flush_op = BLKIF_OP_FLUSH_DISKCACHE;
         }
-                
+
+        err = xenbus_gather(XBT_NIL, info->xbdev->otherend,
+                            "feature-discard", "%d", &discard,
+                            NULL);
+
+        if (!err && discard)
+                blkfront_setup_discard(info);
+
         err = xlvbd_alloc_gendisk(sectors, info, binfo, sector_size);
         if (err) {
                 xenbus_dev_fatal(info->xbdev, err, "xlvbd_add at %s",
@@ -1385,6 +1450,8 @@ static struct xenbus_driver blkfront = {
 
 static int __init xlblk_init(void)
 {
+        int ret;
+
         if (!xen_domain())
                 return -ENODEV;
 
@@ -1394,7 +1461,13 @@ static int __init xlblk_init(void)
                 return -ENODEV;
         }
 
-        return xenbus_register_frontend(&blkfront);
+        ret = xenbus_register_frontend(&blkfront);
+        if (ret) {
+                unregister_blkdev(XENVBD_MAJOR, DEV_NAME);
+                return ret;
+        }
+
+        return 0;
 }
 module_init(xlblk_init);
 
diff --git a/include/xen/interface/io/blkif.h b/include/xen/interface/io/blkif.h
index 3d5d6db864fe..9324488f23f0 100644
--- a/include/xen/interface/io/blkif.h
+++ b/include/xen/interface/io/blkif.h
@@ -57,6 +57,36 @@ typedef uint64_t blkif_sector_t;
  * "feature-flush-cache" node!
  */
 #define BLKIF_OP_FLUSH_DISKCACHE   3
+
+/*
+ * Recognised only if "feature-discard" is present in backend xenbus info.
+ * The "feature-discard" node contains a boolean indicating whether trim
+ * (ATA) or unmap (SCSI) - conviently called discard requests are likely
+ * to succeed or fail. Either way, a discard request
+ * may fail at any time with BLKIF_RSP_EOPNOTSUPP if it is unsupported by
+ * the underlying block-device hardware. The boolean simply indicates whether
+ * or not it is worthwhile for the frontend to attempt discard requests.
+ * If a backend does not recognise BLKIF_OP_DISCARD, it should *not*
+ * create the "feature-discard" node!
+ *
+ * Discard operation is a request for the underlying block device to mark
+ * extents to be erased. However, discard does not guarantee that the blocks
+ * will be erased from the device - it is just a hint to the device
+ * controller that these blocks are no longer in use. What the device
+ * controller does with that information is left to the controller.
+ * Discard operations are passed with sector_number as the
+ * sector index to begin discard operations at and nr_sectors as the number of
+ * sectors to be discarded. The specified sectors should be discarded if the
+ * underlying block device supports trim (ATA) or unmap (SCSI) operations,
+ * or a BLKIF_RSP_EOPNOTSUPP  should be returned.
+ * More information about trim/unmap operations at:
+ * http://t13.org/Documents/UploadedDocuments/docs2008/
+ *     e07154r6-Data_Set_Management_Proposal_for_ATA-ACS2.doc
+ * http://www.seagate.com/staticfiles/support/disc/manuals/
+ *     Interface%20manuals/100293068c.pdf
+ */
+#define BLKIF_OP_DISCARD           5
+
 /*
  * Maximum scatter/gather segments per request.
  * This is carefully chosen so that sizeof(struct blkif_ring) <= PAGE_SIZE.
@@ -74,6 +104,11 @@ struct blkif_request_rw {
         } seg[BLKIF_MAX_SEGMENTS_PER_REQUEST];
 };
 
+struct blkif_request_discard {
+        blkif_sector_t sector_number;
+        uint64_t nr_sectors;
+};
+
 struct blkif_request {
         uint8_t        operation;    /* BLKIF_OP_???                         */
         uint8_t        nr_segments;  /* number of segments                   */
@@ -81,6 +116,7 @@ struct blkif_request {
         uint64_t       id;           /* private guest value, echoed in resp  */
         union {
                 struct blkif_request_rw rw;
+                struct blkif_request_discard discard;
         } u;
 };