4 files changed, 474 insertions, 55 deletions

diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
index 280a13846e6..d7dd5cbdac5 100644
--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -39,6 +39,7 @@
 #include <linux/list.h>
 #include <linux/delay.h>
 #include <linux/freezer.h>
+#include <linux/bitmap.h>
 
 #include <xen/events.h>
 #include <xen/page.h>
@@ -79,6 +80,7 @@ struct pending_req {
 	unsigned short		operation;
 	int			status;
 	struct list_head	free_list;
+	DECLARE_BITMAP(unmap_seg, BLKIF_MAX_SEGMENTS_PER_REQUEST);
 };
 
 #define BLKBACK_INVALID_HANDLE (~0)
@@ -99,6 +101,36 @@ struct xen_blkbk {
 static struct xen_blkbk *blkbk;
 
 /*
+ * Maximum number of grant pages that can be mapped in blkback.
+ * BLKIF_MAX_SEGMENTS_PER_REQUEST * RING_SIZE is the maximum number of
+ * pages that blkback will persistently map.
+ * Currently, this is:
+ * RING_SIZE = 32 (for all known ring types)
+ * BLKIF_MAX_SEGMENTS_PER_REQUEST = 11
+ * sizeof(struct persistent_gnt) = 48
+ * So the maximum memory used to store the grants is:
+ * 32 * 11 * 48 = 16896 bytes
+ */
+static inline unsigned int max_mapped_grant_pages(enum blkif_protocol protocol)
+{
+	switch (protocol) {
+	case BLKIF_PROTOCOL_NATIVE:
+		return __CONST_RING_SIZE(blkif, PAGE_SIZE) *
+			   BLKIF_MAX_SEGMENTS_PER_REQUEST;
+	case BLKIF_PROTOCOL_X86_32:
+		return __CONST_RING_SIZE(blkif_x86_32, PAGE_SIZE) *
+			   BLKIF_MAX_SEGMENTS_PER_REQUEST;
+	case BLKIF_PROTOCOL_X86_64:
+		return __CONST_RING_SIZE(blkif_x86_64, PAGE_SIZE) *
+			   BLKIF_MAX_SEGMENTS_PER_REQUEST;
+	default:
+		BUG();
+	}
+	return 0;
+}
+
+
+/*
  * 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
@@ -129,6 +161,57 @@ static int dispatch_rw_block_io(struct xen_blkif *blkif,
 static void make_response(struct xen_blkif *blkif, u64 id,
 			  unsigned short op, int st);
 
+#define foreach_grant(pos, rbtree, node) \
+	for ((pos) = container_of(rb_first((rbtree)), typeof(*(pos)), node); \
+	     &(pos)->node != NULL; \
+	     (pos) = container_of(rb_next(&(pos)->node), typeof(*(pos)), node))
+
+
+static void add_persistent_gnt(struct rb_root *root,
+			       struct persistent_gnt *persistent_gnt)
+{
+	struct rb_node **new = &(root->rb_node), *parent = NULL;
+	struct persistent_gnt *this;
+
+	/* Figure out where to put new 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(DRV_PFX " trying to add a gref that's already in the tree\n");
+			BUG();
+		}
+	}
+
+	/* Add new node and rebalance tree. */
+	rb_link_node(&(persistent_gnt->node), parent, new);
+	rb_insert_color(&(persistent_gnt->node), root);
+}
+
+static struct persistent_gnt *get_persistent_gnt(struct rb_root *root,
+						 grant_ref_t gref)
+{
+	struct persistent_gnt *data;
+	struct rb_node *node = root->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
+			return data;
+	}
+	return NULL;
+}
+
 /*
  * Retrieve from the 'pending_reqs' a free pending_req structure to be used.
  */
@@ -275,6 +358,11 @@ int xen_blkif_schedule(void *arg)
 {
 	struct xen_blkif *blkif = arg;
 	struct xen_vbd *vbd = &blkif->vbd;
+	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 = 0;
+	int segs_to_unmap = 0;
 
 	xen_blkif_get(blkif);
 
@@ -302,6 +390,36 @@ int xen_blkif_schedule(void *arg)
 			print_stats(blkif);
 	}
 
+	/* Free all persistent grant pages */
+	if (!RB_EMPTY_ROOT(&blkif->persistent_gnts)) {
+		foreach_grant(persistent_gnt, &blkif->persistent_gnts, 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;
+			rb_erase(&persistent_gnt->node,
+				&blkif->persistent_gnts);
+			kfree(persistent_gnt);
+			blkif->persistent_gnt_c--;
+
+			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);
+				segs_to_unmap = 0;
+			}
+		}
+	}
+
+	BUG_ON(blkif->persistent_gnt_c != 0);
+	BUG_ON(!RB_EMPTY_ROOT(&blkif->persistent_gnts));
+
 	if (log_stats)
 		print_stats(blkif);
 
@@ -328,6 +446,8 @@ static void xen_blkbk_unmap(struct pending_req *req)
 	int ret;
 
 	for (i = 0; i < req->nr_pages; i++) {
+		if (!test_bit(i, req->unmap_seg))
+			continue;
 		handle = pending_handle(req, i);
 		if (handle == BLKBACK_INVALID_HANDLE)
 			continue;
@@ -344,12 +464,26 @@ static void xen_blkbk_unmap(struct pending_req *req)
 
 static int xen_blkbk_map(struct blkif_request *req,
 			 struct pending_req *pending_req,
-			 struct seg_buf seg[])
+			 struct seg_buf seg[],
+			 struct page *pages[])
 {
 	struct gnttab_map_grant_ref map[BLKIF_MAX_SEGMENTS_PER_REQUEST];
-	int i;
+	struct persistent_gnt *persistent_gnts[BLKIF_MAX_SEGMENTS_PER_REQUEST];
+	struct page *pages_to_gnt[BLKIF_MAX_SEGMENTS_PER_REQUEST];
+	struct persistent_gnt *persistent_gnt = NULL;
+	struct xen_blkif *blkif = pending_req->blkif;
+	phys_addr_t addr = 0;
+	int i, j;
+	bool new_map;
 	int nseg = req->u.rw.nr_segments;
+	int segs_to_map = 0;
 	int ret = 0;
+	int use_persistent_gnts;
+
+	use_persistent_gnts = (blkif->vbd.feature_gnt_persistent);
+
+	BUG_ON(blkif->persistent_gnt_c >
+		   max_mapped_grant_pages(pending_req->blkif->blk_protocol));
 
 	/*
 	 * Fill out preq.nr_sects with proper amount of sectors, and setup
@@ -359,36 +493,143 @@ static int xen_blkbk_map(struct blkif_request *req,
 	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);
+		if (use_persistent_gnts)
+			persistent_gnt = get_persistent_gnt(
+				&blkif->persistent_gnts,
+				req->u.rw.seg[i].gref);
+
+		if (persistent_gnt) {
+			/*
+			 * We are using persistent grants and
+			 * the grant is already mapped
+			 */
+			new_map = false;
+		} else if (use_persistent_gnts &&
+			   blkif->persistent_gnt_c <
+			   max_mapped_grant_pages(blkif->blk_protocol)) {
+			/*
+			 * We are using persistent grants, the grant is
+			 * not mapped but we have room for it
+			 */
+			new_map = true;
+			persistent_gnt = kzalloc(
+				sizeof(struct persistent_gnt),
+				GFP_KERNEL);
+			if (!persistent_gnt)
+				return -ENOMEM;
+			persistent_gnt->page = alloc_page(GFP_KERNEL);
+			if (!persistent_gnt->page) {
+				kfree(persistent_gnt);
+				return -ENOMEM;
+			}
+			persistent_gnt->gnt = req->u.rw.seg[i].gref;
+
+			pages_to_gnt[segs_to_map] =
+				persistent_gnt->page;
+			addr = (unsigned long) pfn_to_kaddr(
+				page_to_pfn(persistent_gnt->page));
+
+			add_persistent_gnt(&blkif->persistent_gnts,
+				persistent_gnt);
+			blkif->persistent_gnt_c++;
+			pr_debug(DRV_PFX " grant %u added to the tree of persistent grants, using %u/%u\n",
+				 persistent_gnt->gnt, blkif->persistent_gnt_c,
+				 max_mapped_grant_pages(blkif->blk_protocol));
+		} else {
+			/*
+			 * We are either using persistent grants and
+			 * hit the maximum limit of grants mapped,
+			 * or we are not using persistent grants.
+			 */
+			if (use_persistent_gnts &&
+				!blkif->vbd.overflow_max_grants) {
+				blkif->vbd.overflow_max_grants = 1;
+				pr_alert(DRV_PFX " domain %u, device %#x is using maximum number of persistent grants\n",
+					 blkif->domid, blkif->vbd.handle);
+			}
+			new_map = true;
+			pages[i] = blkbk->pending_page(pending_req, i);
+			addr = vaddr(pending_req, i);
+			pages_to_gnt[segs_to_map] =
+				blkbk->pending_page(pending_req, i);
+		}
+
+		if (persistent_gnt) {
+			pages[i] = persistent_gnt->page;
+			persistent_gnts[i] = persistent_gnt;
+		} else {
+			persistent_gnts[i] = NULL;
+		}
+
+		if (new_map) {
+			flags = GNTMAP_host_map;
+			if (!persistent_gnt &&
+			    (pending_req->operation != BLKIF_OP_READ))
+				flags |= GNTMAP_readonly;
+			gnttab_set_map_op(&map[segs_to_map++], addr,
+					  flags, req->u.rw.seg[i].gref,
+					  blkif->domid);
+		}
 	}
 
-	ret = gnttab_map_refs(map, NULL, &blkbk->pending_page(pending_req, 0), nseg);
-	BUG_ON(ret);
+	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 (i = 0; i < nseg; i++) {
-		if (unlikely(map[i].status != 0)) {
-			pr_debug(DRV_PFX "invalid buffer -- could not remap it\n");
-			map[i].handle = BLKBACK_INVALID_HANDLE;
-			ret |= 1;
+	bitmap_zero(pending_req->unmap_seg, BLKIF_MAX_SEGMENTS_PER_REQUEST);
+	for (i = 0, j = 0; i < nseg; i++) {
+		if (!persistent_gnts[i] || !persistent_gnts[i]->handle) {
+			/* This is a newly mapped grant */
+			BUG_ON(j >= segs_to_map);
+			if (unlikely(map[j].status != 0)) {
+				pr_debug(DRV_PFX "invalid buffer -- could not remap it\n");
+				map[j].handle = BLKBACK_INVALID_HANDLE;
+				ret |= 1;
+				if (persistent_gnts[i]) {
+					rb_erase(&persistent_gnts[i]->node,
+						 &blkif->persistent_gnts);
+					blkif->persistent_gnt_c--;
+					kfree(persistent_gnts[i]);
+					persistent_gnts[i] = NULL;
+				}
+			}
+		}
+		if (persistent_gnts[i]) {
+			if (!persistent_gnts[i]->handle) {
+				/*
+				 * If this is a new persistent grant
+				 * save the handler
+				 */
+				persistent_gnts[i]->handle = map[j].handle;
+				persistent_gnts[i]->dev_bus_addr =
+					map[j++].dev_bus_addr;
+			}
+			pending_handle(pending_req, i) =
+				persistent_gnts[i]->handle;
+
+			if (ret)
+				continue;
+
+			seg[i].buf = persistent_gnts[i]->dev_bus_addr |
+				(req->u.rw.seg[i].first_sect << 9);
+		} else {
+			pending_handle(pending_req, i) = map[j].handle;
+			bitmap_set(pending_req->unmap_seg, i, 1);
+
+			if (ret) {
+				j++;
+				continue;
+			}
+
+			seg[i].buf = map[j++].dev_bus_addr |
+				(req->u.rw.seg[i].first_sect << 9);
 		}
-
-		pending_handle(pending_req, i) = map[i].handle;
-
-		if (ret)
-			continue;
-
-		seg[i].buf  = map[i].dev_bus_addr |
-			(req->u.rw.seg[i].first_sect << 9);
 	}
 	return ret;
 }
@@ -591,6 +832,7 @@ static int dispatch_rw_block_io(struct xen_blkif *blkif,
 	int operation;
 	struct blk_plug plug;
 	bool drain = false;
+	struct page *pages[BLKIF_MAX_SEGMENTS_PER_REQUEST];
 
 	switch (req->operation) {
 	case BLKIF_OP_READ:
@@ -677,7 +919,7 @@ static int dispatch_rw_block_io(struct xen_blkif *blkif,
 	 * the hypercall to unmap the grants - that is all done in
 	 * xen_blkbk_unmap.
 	 */
-	if (xen_blkbk_map(req, pending_req, seg))
+	if (xen_blkbk_map(req, pending_req, seg, pages))
 		goto fail_flush;
 
 	/*
@@ -689,7 +931,7 @@ static int dispatch_rw_block_io(struct xen_blkif *blkif,
 	for (i = 0; i < nseg; i++) {
 		while ((bio == NULL) ||
 		       (bio_add_page(bio,
-				     blkbk->pending_page(pending_req, i),
+				     pages[i],
 				     seg[i].nsec << 9,
 				     seg[i].buf & ~PAGE_MASK) == 0)) {
 
diff --git a/drivers/block/xen-blkback/common.h b/drivers/block/xen-blkback/common.h
index 9ad3b5ec1dc..ae7951f0e26 100644
--- a/drivers/block/xen-blkback/common.h
+++ b/drivers/block/xen-blkback/common.h
@@ -34,6 +34,7 @@
 #include <linux/vmalloc.h>
 #include <linux/wait.h>
 #include <linux/io.h>
+#include <linux/rbtree.h>
 #include <asm/setup.h>
 #include <asm/pgalloc.h>
 #include <asm/hypervisor.h>
@@ -160,10 +161,22 @@ struct xen_vbd {
 	sector_t		size;
 	bool			flush_support;
 	bool			discard_secure;
+
+	unsigned int		feature_gnt_persistent:1;
+	unsigned int		overflow_max_grants:1;
 };
 
 struct backend_info;
 
+
+struct persistent_gnt {
+	struct page *page;
+	grant_ref_t gnt;
+	grant_handle_t handle;
+	uint64_t dev_bus_addr;
+	struct rb_node node;
+};
+
 struct xen_blkif {
 	/* Unique identifier for this interface. */
 	domid_t			domid;
@@ -190,6 +203,10 @@ struct xen_blkif {
 	struct task_struct	*xenblkd;
 	unsigned int		waiting_reqs;
 
+	/* tree to store persistent grants */
+	struct rb_root		persistent_gnts;
+	unsigned int		persistent_gnt_c;
+
 	/* statistics */
 	unsigned long		st_print;
 	int			st_rd_req;
diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
index 4f66171c668..b2250265308 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -118,6 +118,7 @@ static struct xen_blkif *xen_blkif_alloc(domid_t domid)
 	atomic_set(&blkif->drain, 0);
 	blkif->st_print = jiffies;
 	init_waitqueue_head(&blkif->waiting_to_free);
+	blkif->persistent_gnts.rb_node = NULL;
 
 	return blkif;
 }
@@ -673,6 +674,13 @@ again:
 
 	xen_blkbk_barrier(xbt, be, be->blkif->vbd.flush_support);
 
+	err = xenbus_printf(xbt, dev->nodename, "feature-persistent", "%u", 1);
+	if (err) {
+		xenbus_dev_fatal(dev, err, "writing %s/feature-persistent",
+				 dev->nodename);
+		goto abort;
+	}
+
 	err = xenbus_printf(xbt, dev->nodename, "sectors", "%llu",
 			    (unsigned long long)vbd_sz(&be->blkif->vbd));
 	if (err) {
@@ -721,6 +729,7 @@ static int connect_ring(struct backend_info *be)
 	struct xenbus_device *dev = be->dev;
 	unsigned long ring_ref;
 	unsigned int evtchn;
+	unsigned int pers_grants;
 	char protocol[64] = "";
 	int err;
 
@@ -750,8 +759,18 @@ static int connect_ring(struct backend_info *be)
 		xenbus_dev_fatal(dev, err, "unknown fe protocol %s", protocol);
 		return -1;
 	}
-	pr_info(DRV_PFX "ring-ref %ld, event-channel %d, protocol %d (%s)\n",
-		ring_ref, evtchn, be->blkif->blk_protocol, protocol);
+	err = xenbus_gather(XBT_NIL, dev->otherend,
+			    "feature-persistent-grants", "%u",
+			    &pers_grants, NULL);
+	if (err)
+		pers_grants = 0;
+
+	be->blkif->vbd.feature_gnt_persistent = pers_grants;
+	be->blkif->vbd.overflow_max_grants = 0;
+
+	pr_info(DRV_PFX "ring-ref %ld, event-channel %d, protocol %d (%s) %s\n",
+		ring_ref, evtchn, be->blkif->blk_protocol, protocol,
+		pers_grants ? "persistent grants" : "");
 
 	/* Map the shared frame, irq etc. */
 	err = xen_blkif_map(be->blkif, ring_ref, evtchn);
diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index 007db8986e8..911d733d21b 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -44,6 +44,7 @@
 #include <linux/mutex.h>
 #include <linux/scatterlist.h>
 #include <linux/bitmap.h>
+#include <linux/llist.h>
 
 #include <xen/xen.h>
 #include <xen/xenbus.h>
@@ -64,10 +65,17 @@ enum blkif_state {
 	BLKIF_STATE_SUSPENDED,
 };
 
+struct grant {
+	grant_ref_t gref;
+	unsigned long pfn;
+	struct llist_node node;
+};
+
 struct blk_shadow {
 	struct blkif_request req;
 	struct request *request;
 	unsigned long frame[BLKIF_MAX_SEGMENTS_PER_REQUEST];
+	struct grant *grants_used[BLKIF_MAX_SEGMENTS_PER_REQUEST];
 };
 
 static DEFINE_MUTEX(blkfront_mutex);
@@ -97,6 +105,8 @@ struct blkfront_info
 	struct work_struct work;
 	struct gnttab_free_callback callback;
 	struct blk_shadow shadow[BLK_RING_SIZE];
+	struct llist_head persistent_gnts;
+	unsigned int persistent_gnts_c;
 	unsigned long shadow_free;
 	unsigned int feature_flush;
 	unsigned int flush_op;
@@ -104,6 +114,7 @@ struct blkfront_info
 	unsigned int feature_secdiscard:1;
 	unsigned int discard_granularity;
 	unsigned int discard_alignment;
+	unsigned int feature_persistent:1;
 	int is_ready;
 };
 
@@ -287,21 +298,36 @@ static int blkif_queue_request(struct request *req)
 	unsigned long id;
 	unsigned int fsect, lsect;
 	int i, ref;
+
+	/*
+	 * Used to store if we are able to queue the request by just using
+	 * existing persistent grants, or if we have to get new grants,
+	 * as there are not sufficiently many free.
+	 */
+	bool new_persistent_gnts;
 	grant_ref_t gref_head;
+	struct page *granted_page;
+	struct grant *gnt_list_entry = NULL;
 	struct scatterlist *sg;
 
 	if (unlikely(info->connected != BLKIF_STATE_CONNECTED))
 		return 1;
 
-	if (gnttab_alloc_grant_references(
-		BLKIF_MAX_SEGMENTS_PER_REQUEST, &gref_head) < 0) {
-		gnttab_request_free_callback(
-			&info->callback,
-			blkif_restart_queue_callback,
-			info,
-			BLKIF_MAX_SEGMENTS_PER_REQUEST);
-		return 1;
-	}
+	/* Check if we have enought grants to allocate a requests */
+	if (info->persistent_gnts_c < BLKIF_MAX_SEGMENTS_PER_REQUEST) {
+		new_persistent_gnts = 1;
+		if (gnttab_alloc_grant_references(
+		    BLKIF_MAX_SEGMENTS_PER_REQUEST - info->persistent_gnts_c,
+		    &gref_head) < 0) {
+			gnttab_request_free_callback(
+				&info->callback,
+				blkif_restart_queue_callback,
+				info,
+				BLKIF_MAX_SEGMENTS_PER_REQUEST);
+			return 1;
+		}
+	} else
+		new_persistent_gnts = 0;
 
 	/* Fill out a communications ring structure. */
 	ring_req = RING_GET_REQUEST(&info->ring, info->ring.req_prod_pvt);
@@ -341,18 +367,73 @@ static int blkif_queue_request(struct request *req)
 		       BLKIF_MAX_SEGMENTS_PER_REQUEST);
 
 		for_each_sg(info->sg, sg, ring_req->u.rw.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,
+			if (info->persistent_gnts_c) {
+				BUG_ON(llist_empty(&info->persistent_gnts));
+				gnt_list_entry = llist_entry(
+					llist_del_first(&info->persistent_gnts),
+					struct grant, node);
+
+				ref = gnt_list_entry->gref;
+				buffer_mfn = pfn_to_mfn(gnt_list_entry->pfn);
+				info->persistent_gnts_c--;
+			} else {
+				ref = gnttab_claim_grant_reference(&gref_head);
+				BUG_ON(ref == -ENOSPC);
+
+				gnt_list_entry =
+					kmalloc(sizeof(struct grant),
+							 GFP_ATOMIC);
+				if (!gnt_list_entry)
+					return -ENOMEM;
+
+				granted_page = alloc_page(GFP_ATOMIC);
+				if (!granted_page) {
+					kfree(gnt_list_entry);
+					return -ENOMEM;
+				}
+
+				gnt_list_entry->pfn =
+					page_to_pfn(granted_page);
+				gnt_list_entry->gref = ref;
+
+				buffer_mfn = pfn_to_mfn(page_to_pfn(
+								granted_page));
+				gnttab_grant_foreign_access_ref(ref,
 					info->xbdev->otherend_id,
-					buffer_mfn,
-					rq_data_dir(req));
+					buffer_mfn, 0);
+			}
+
+			info->shadow[id].grants_used[i] = gnt_list_entry;
+
+			if (rq_data_dir(req)) {
+				char *bvec_data;
+				void *shared_data;
+
+				BUG_ON(sg->offset + sg->length > PAGE_SIZE);
+
+				shared_data = kmap_atomic(
+					pfn_to_page(gnt_list_entry->pfn));
+				bvec_data = kmap_atomic(sg_page(sg));
+
+				/*
+				 * this does not wipe data stored outside the
+				 * range sg->offset..sg->offset+sg->length.
+				 * Therefore, blkback *could* see data from
+				 * previous requests. This is OK as long as
+				 * persistent grants are shared with just one
+				 * domain. It may need refactoring if this
+				 * changes
+				 */
+				memcpy(shared_data + sg->offset,
+				       bvec_data   + sg->offset,
+				       sg->length);
+
+				kunmap_atomic(bvec_data);
+				kunmap_atomic(shared_data);
+			}
 
 			info->shadow[id].frame[i] = mfn_to_pfn(buffer_mfn);
 			ring_req->u.rw.seg[i] =
@@ -368,7 +449,8 @@ static int blkif_queue_request(struct request *req)
 	/* Keep a private copy so we can reissue requests when recovering. */
 	info->shadow[id].req = *ring_req;
 
-	gnttab_free_grant_references(gref_head);
+	if (new_persistent_gnts)
+		gnttab_free_grant_references(gref_head);
 
 	return 0;
 }
@@ -480,12 +562,13 @@ static int xlvbd_init_blk_queue(struct gendisk *gd, u16 sector_size)
 static void xlvbd_flush(struct blkfront_info *info)
 {
 	blk_queue_flush(info->rq, info->feature_flush);
-	printk(KERN_INFO "blkfront: %s: %s: %s\n",
+	printk(KERN_INFO "blkfront: %s: %s: %s %s\n",
 	       info->gd->disk_name,
 	       info->flush_op == BLKIF_OP_WRITE_BARRIER ?
 		"barrier" : (info->flush_op == BLKIF_OP_FLUSH_DISKCACHE ?
 		"flush diskcache" : "barrier or flush"),
-	       info->feature_flush ? "enabled" : "disabled");
+	       info->feature_flush ? "enabled" : "disabled",
+	       info->feature_persistent ? "using persistent grants" : "");
 }
 
 static int xen_translate_vdev(int vdevice, int *minor, unsigned int *offset)
@@ -707,6 +790,9 @@ static void blkif_restart_queue(struct work_struct *work)
 
 static void blkif_free(struct blkfront_info *info, int suspend)
 {
+	struct llist_node *all_gnts;
+	struct grant *persistent_gnt;
+
 	/* Prevent new requests being issued until we fix things up. */
 	spin_lock_irq(&info->io_lock);
 	info->connected = suspend ?
@@ -714,6 +800,17 @@ static void blkif_free(struct blkfront_info *info, int suspend)
 	/* No more blkif_request(). */
 	if (info->rq)
 		blk_stop_queue(info->rq);
+
+	/* Remove all persistent grants */
+	if (info->persistent_gnts_c) {
+		all_gnts = llist_del_all(&info->persistent_gnts);
+		llist_for_each_entry(persistent_gnt, all_gnts, node) {
+			gnttab_end_foreign_access(persistent_gnt->gref, 0, 0UL);
+			kfree(persistent_gnt);
+		}
+		info->persistent_gnts_c = 0;
+	}
+
 	/* No more gnttab callback work. */
 	gnttab_cancel_free_callback(&info->callback);
 	spin_unlock_irq(&info->io_lock);
@@ -734,13 +831,42 @@ static void blkif_free(struct blkfront_info *info, int suspend)
 
 }
 
-static void blkif_completion(struct blk_shadow *s)
+static void blkif_completion(struct blk_shadow *s, struct blkfront_info *info,
+			     struct blkif_response *bret)
 {
 	int i;
-	/* Do not let BLKIF_OP_DISCARD as nr_segment is in the same place
-	 * flag. */
-	for (i = 0; i < s->req.u.rw.nr_segments; i++)
-		gnttab_end_foreign_access(s->req.u.rw.seg[i].gref, 0, 0UL);
+	struct bio_vec *bvec;
+	struct req_iterator iter;
+	unsigned long flags;
+	char *bvec_data;
+	void *shared_data;
+	unsigned int offset = 0;
+
+	if (bret->operation == BLKIF_OP_READ) {
+		/*
+		 * Copy the data received from the backend into the bvec.
+		 * Since bv_offset can be different than 0, and bv_len different
+		 * than PAGE_SIZE, we have to keep track of the current offset,
+		 * to be sure we are copying the data from the right shared page.
+		 */
+		rq_for_each_segment(bvec, s->request, iter) {
+			BUG_ON((bvec->bv_offset + bvec->bv_len) > PAGE_SIZE);
+			i = offset >> PAGE_SHIFT;
+			shared_data = kmap_atomic(
+				pfn_to_page(s->grants_used[i]->pfn));
+			bvec_data = bvec_kmap_irq(bvec, &flags);
+			memcpy(bvec_data, shared_data + bvec->bv_offset,
+				bvec->bv_len);
+			bvec_kunmap_irq(bvec_data, &flags);
+			kunmap_atomic(shared_data);
+			offset += bvec->bv_len;
+		}
+	}
+	/* Add the persistent grant into the list of free grants */
+	for (i = 0; i < s->req.u.rw.nr_segments; i++) {
+		llist_add(&s->grants_used[i]->node, &info->persistent_gnts);
+		info->persistent_gnts_c++;
+	}
 }
 
 static irqreturn_t blkif_interrupt(int irq, void *dev_id)
@@ -783,7 +909,7 @@ static irqreturn_t blkif_interrupt(int irq, void *dev_id)
 		req  = info->shadow[id].request;
 
 		if (bret->operation != BLKIF_OP_DISCARD)
-			blkif_completion(&info->shadow[id]);
+			blkif_completion(&info->shadow[id], info, bret);
 
 		if (add_id_to_freelist(info, id)) {
 			WARN(1, "%s: response to %s (id %ld) couldn't be recycled!\n",
@@ -942,6 +1068,11 @@ again:
 		message = "writing protocol";
 		goto abort_transaction;
 	}
+	err = xenbus_printf(xbt, dev->nodename,
+			    "feature-persistent-grants", "%u", 1);
+	if (err)
+		dev_warn(&dev->dev,
+			 "writing persistent grants feature to xenbus");
 
 	err = xenbus_transaction_end(xbt, 0);
 	if (err) {
@@ -1029,6 +1160,8 @@ static int blkfront_probe(struct xenbus_device *dev,
 	spin_lock_init(&info->io_lock);
 	info->xbdev = dev;
 	info->vdevice = vdevice;
+	init_llist_head(&info->persistent_gnts);
+	info->persistent_gnts_c = 0;
 	info->connected = BLKIF_STATE_DISCONNECTED;
 	INIT_WORK(&info->work, blkif_restart_queue);
 
@@ -1093,7 +1226,7 @@ static int blkif_recover(struct blkfront_info *info)
 					req->u.rw.seg[j].gref,
 					info->xbdev->otherend_id,
 					pfn_to_mfn(info->shadow[req->u.rw.id].frame[j]),
-					rq_data_dir(info->shadow[req->u.rw.id].request));
+					0);
 		}
 		info->shadow[req->u.rw.id].req = *req;
 
@@ -1225,7 +1358,7 @@ static void blkfront_connect(struct blkfront_info *info)
 	unsigned long sector_size;
 	unsigned int binfo;
 	int err;
-	int barrier, flush, discard;
+	int barrier, flush, discard, persistent;
 
 	switch (info->connected) {
 	case BLKIF_STATE_CONNECTED:
@@ -1303,6 +1436,14 @@ static void blkfront_connect(struct blkfront_info *info)
 	if (!err && discard)
 		blkfront_setup_discard(info);
 
+	err = xenbus_gather(XBT_NIL, info->xbdev->otherend,
+			    "feature-persistent", "%u", &persistent,
+			    NULL);
+	if (err)
+		info->feature_persistent = 0;
+	else
+		info->feature_persistent = persistent;
+
 	err = xlvbd_alloc_gendisk(sectors, info, binfo, sector_size);
 	if (err) {
 		xenbus_dev_fatal(info->xbdev, err, "xlvbd_add at %s",