Xen devel - Feb 2013 - [PATCH RFC 12/12] xen-block: implement indirect descriptors

Indirect descriptors introduce a new block operation
(BLKIF_OP_INDIRECT) that passes grant references instead of segments
in the request. This grant references are filled with arrays of
blkif_request_segment_aligned, this way we can send more segments in a
request.

The proposed implementation sets the maximum number of indirect grefs
(frames filled with blkif_request_segment_aligned) to 256 in the
backend and 64 in the frontend. The value in the frontend has been
chosen experimentally, and the backend value has been set to a sane
value that allows expanding the maximum number of indirect descriptors
in the frontend if needed.

The migration code has changed from the previous implementation, in
which we simply remapped the segments on the shared ring. Now the
maximum number of segments allowed in a request can change depending
on the backend, so we have to requeue all the requests in the ring and
in the queue and split the bios in them if they are bigger than the
new maximum number of segments.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: xen-devel@lists.xen.org
---
 drivers/block/xen-blkback/blkback.c |  129 +++++++---
 drivers/block/xen-blkback/common.h  |   80 ++++++-
 drivers/block/xen-blkback/xenbus.c  |    8 +
 drivers/block/xen-blkfront.c        |  498 +++++++++++++++++++++++++++++------
 include/xen/interface/io/blkif.h    |   25 ++
 5 files changed, 622 insertions(+), 118 deletions(-)

diff --git a/drivers/block/xen-blkback/blkback.c
b/drivers/block/xen-blkback/blkback.c
index 0fa30db..98eb16b 100644
--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -70,7 +70,7 @@ MODULE_PARM_DESC(reqs, "Number of blkback requests to
allocate per backend");
  * algorithm.
  */
 
-static int xen_blkif_max_pgrants = 352;
+static int xen_blkif_max_pgrants = 1024;
 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");
@@ -578,10 +578,6 @@ purge_gnt_list:
 	return 0;
 }
 
-struct seg_buf {
-	unsigned long buf;
-	unsigned int nsec;
-};
 /*
  * Unmap the grant references, and also remove the M2P over-rides
  * used in the 'pending_req'.
@@ -761,32 +757,79 @@ out_of_memory:
 	return -ENOMEM;
 }
 
-static int xen_blkbk_map_seg(struct blkif_request *req,
-			     struct pending_req *pending_req,
+static int xen_blkbk_map_seg(struct pending_req *pending_req,
 			     struct seg_buf seg[],
 			     struct page *pages[])
 {
 	int i, rc;
-	grant_ref_t grefs[BLKIF_MAX_SEGMENTS_PER_REQUEST];
 
-	for (i = 0; i < req->u.rw.nr_segments; i++)
-		grefs[i] = req->u.rw.seg[i].gref;
-
-	rc = xen_blkbk_map(pending_req->blkif, grefs,
+	rc = xen_blkbk_map(pending_req->blkif, pending_req->grefs,
 	                   pending_req->persistent_gnts,
 	                   pending_req->grant_handles, pending_req->pages,
-	                   req->u.rw.nr_segments,
+	                   pending_req->nr_pages,
 	                   (pending_req->operation != BLKIF_OP_READ));
 	if (rc)
 		return rc;
 
-	for (i = 0; i < req->u.rw.nr_segments; i++)
-		seg[i].buf = pfn_to_mfn(page_to_pfn(pending_req->pages[i]))
-		             << PAGE_SHIFT | (req->u.rw.seg[i].first_sect <<
9);
+	for (i = 0; i < pending_req->nr_pages; i++)
+		seg[i].buf |= pfn_to_mfn(page_to_pfn(pending_req->pages[i]))
+		             << PAGE_SHIFT;
 
 	return 0;
 }
 
+static int xen_blkbk_parse_indirect(struct blkif_request *req,
+                                    struct pending_req *pending_req,
+                                    struct seg_buf seg[],
+                                    struct phys_req *preq)
+{
+	struct persistent_gnt **persistent +	
pending_req->indirect_persistent_gnts;
+	struct 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 = (nseg + SEGS_PER_INDIRECT_FRAME - 1) /
+		         SEGS_PER_INDIRECT_FRAME;
+
+	rc = xen_blkbk_map(blkif, req->u.indirect.indirect_grefs,
+	                   persistent, pending_req->indirect_handles,
+	                   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]);
+		}
+		i = n % SEGS_PER_INDIRECT_FRAME;
+		pending_req->grefs[n] = segments[i].gref;
+		seg[n].nsec = segments[i].last_sect -
+			segments[i].first_sect + 1;
+		seg[n].buf = 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, pending_req->indirect_handles,
+                        pages, persistent, indirect_grefs);
+	return rc;
+}
+
 static int dispatch_discard_io(struct xen_blkif *blkif,
 				struct blkif_request *req)
 {
@@ -980,17 +1023,21 @@ static int dispatch_rw_block_io(struct xen_blkif *blkif,
 				struct pending_req *pending_req)
 {
 	struct phys_req preq;
-	struct seg_buf seg[BLKIF_MAX_SEGMENTS_PER_REQUEST];
+	struct seg_buf *seg = pending_req->seg;
 	unsigned int nseg;
 	struct bio *bio = NULL;
-	struct bio *biolist[BLKIF_MAX_SEGMENTS_PER_REQUEST];
+	struct bio **biolist = pending_req->biolist;
 	int i, nbio = 0;
 	int operation;
 	struct blk_plug plug;
 	bool drain = false;
 	struct page **pages = pending_req->pages;
+	unsigned short req_operation;
+
+	req_operation = req->operation == BLKIF_OP_INDIRECT ?
+	                req->u.indirect.indirect_op : req->operation;
 
-	switch (req->operation) {
+	switch (req_operation) {
 	case BLKIF_OP_READ:
 		blkif->st_rd_req++;
 		operation = READ;
@@ -1012,33 +1059,49 @@ static int dispatch_rw_block_io(struct xen_blkif *blkif,
 	}
 
 	/* Check that the number of segments is sane. */
-	nseg = req->u.rw.nr_segments;
+	nseg = req->operation == BLKIF_OP_INDIRECT ?
+	       req->u.indirect.nr_segments : req->u.rw.nr_segments;
 
 	if (unlikely(nseg == 0 && operation != WRITE_FLUSH) ||
-	    unlikely(nseg > BLKIF_MAX_SEGMENTS_PER_REQUEST)) {
+	    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.sector_number = req->u.rw.sector_number;
 	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;
 
-	for (i = 0; i < nseg; i++) {
-		seg[i].nsec = req->u.rw.seg[i].last_sect -
-			req->u.rw.seg[i].first_sect + 1;
-		if ((req->u.rw.seg[i].last_sect >= (PAGE_SIZE >> 9)) ||
-		    (req->u.rw.seg[i].last_sect < req->u.rw.seg[i].first_sect))
+	if (req->operation != BLKIF_OP_INDIRECT) {
+		preq.dev               = req->u.rw.handle;
+		preq.sector_number     = req->u.rw.sector_number;
+		pending_req->id        = req->u.rw.id;
+		pending_req->operation = req->operation;
+		for (i = 0; i < nseg; i++) {
+			pending_req->grefs[i] = 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].buf = 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;
+		pending_req->id        = req->u.indirect.id;
+		pending_req->operation = req->u.indirect.indirect_op;
+		if (xen_blkbk_parse_indirect(req, pending_req, seg, &preq))
 			goto fail_response;
-		preq.nr_sects += seg[i].nsec;
-
 	}
 
 	if (xen_vbd_translate(&preq, blkif, operation) != 0) {
@@ -1074,7 +1137,7 @@ 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_seg(req, pending_req, seg, pages))
+	if (xen_blkbk_map_seg(pending_req, seg, pages))
 		goto fail_flush;
 
 	/*
@@ -1146,7 +1209,7 @@ static int dispatch_rw_block_io(struct xen_blkif *blkif,
 	                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);
+	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;
diff --git a/drivers/block/xen-blkback/common.h
b/drivers/block/xen-blkback/common.h
index 0b0ad3f..d3656d2 100644
--- a/drivers/block/xen-blkback/common.h
+++ b/drivers/block/xen-blkback/common.h
@@ -50,6 +50,17 @@
 		 __func__, __LINE__, ##args)
 
 
+/*
+ * This is the maximum number of segments that would be allowed in indirect
+ * requests. This value will also be passed to the frontend.
+ */
+#define MAX_INDIRECT_SEGMENTS 256
+
+#define SEGS_PER_INDIRECT_FRAME \
+(PAGE_SIZE/sizeof(struct blkif_request_segment_aligned))
+#define MAX_INDIRECT_GREFS \
+((MAX_INDIRECT_SEGMENTS + SEGS_PER_INDIRECT_FRAME - 1)/SEGS_PER_INDIRECT_FRAME)
+
 /* Not a real protocol.  Used to generate ring structs which contain
  * the elements common to all protocols only.  This way we get a
  * compiler-checkable way to use common struct elements, so we can
@@ -77,11 +88,21 @@ struct blkif_x86_32_request_discard {
 	uint64_t       nr_sectors;
 } __attribute__((__packed__));
 
+struct blkif_x86_32_request_indirect {
+	uint8_t        indirect_op;
+	uint16_t       nr_segments;
+	uint64_t       id;
+	blkif_vdev_t   handle;
+	blkif_sector_t sector_number;
+	grant_ref_t    indirect_grefs[BLKIF_MAX_INDIRECT_GREFS_PER_REQUEST];
+} __attribute__((__packed__));
+
 struct blkif_x86_32_request {
 	uint8_t        operation;    /* BLKIF_OP_???                         */
 	union {
 		struct blkif_x86_32_request_rw rw;
 		struct blkif_x86_32_request_discard discard;
+		struct blkif_x86_32_request_indirect indirect;
 	} u;
 } __attribute__((__packed__));
 
@@ -113,11 +134,22 @@ struct blkif_x86_64_request_discard {
 	uint64_t       nr_sectors;
 } __attribute__((__packed__));
 
+struct blkif_x86_64_request_indirect {
+	uint8_t        indirect_op;
+	uint16_t       nr_segments;
+	uint32_t       _pad1;        /* offsetof(blkif_..,u.indirect.id)==8   */
+	uint64_t       id;
+	blkif_vdev_t   handle;
+	blkif_sector_t sector_number;
+	grant_ref_t    indirect_grefs[BLKIF_MAX_INDIRECT_GREFS_PER_REQUEST];
+} __attribute__((__packed__));
+
 struct blkif_x86_64_request {
 	uint8_t        operation;    /* BLKIF_OP_???                         */
 	union {
 		struct blkif_x86_64_request_rw rw;
 		struct blkif_x86_64_request_discard discard;
+		struct blkif_x86_64_request_indirect indirect;
 	} u;
 } __attribute__((__packed__));
 
@@ -235,6 +267,11 @@ struct xen_blkif {
 	wait_queue_head_t	waiting_to_free;
 };
 
+struct seg_buf {
+	unsigned long buf;
+	unsigned int nsec;
+};
+
 /*
  * Each outstanding request that we've passed to the lower device layers
has a
  * 'pending_req' allocated to it. Each buffer_head that completes
decrements
@@ -249,9 +286,16 @@ struct pending_req {
 	unsigned short		operation;
 	int			status;
 	struct list_head	free_list;
-	struct persistent_gnt	*persistent_gnts[BLKIF_MAX_SEGMENTS_PER_REQUEST];
-	struct page		*pages[BLKIF_MAX_SEGMENTS_PER_REQUEST];
-	grant_handle_t		grant_handles[BLKIF_MAX_SEGMENTS_PER_REQUEST];
+	struct persistent_gnt	*persistent_gnts[MAX_INDIRECT_SEGMENTS];
+	struct page		*pages[MAX_INDIRECT_SEGMENTS];
+	grant_handle_t		grant_handles[MAX_INDIRECT_SEGMENTS];
+	grant_ref_t		grefs[MAX_INDIRECT_SEGMENTS];
+	/* Indirect descriptors */
+	struct persistent_gnt	*indirect_persistent_gnts[MAX_INDIRECT_GREFS];
+	struct page		*indirect_pages[MAX_INDIRECT_GREFS];
+	grant_handle_t		indirect_handles[MAX_INDIRECT_GREFS];
+	struct seg_buf		seg[MAX_INDIRECT_SEGMENTS];
+	struct bio		*biolist[MAX_INDIRECT_SEGMENTS];
 };
 
 
@@ -289,7 +333,7 @@ struct xenbus_device *xen_blkbk_xenbus(struct backend_info
*be);
 static inline void blkif_get_x86_32_req(struct blkif_request *dst,
 					struct blkif_x86_32_request *src)
 {
-	int i, n = BLKIF_MAX_SEGMENTS_PER_REQUEST;
+	int i, n = BLKIF_MAX_SEGMENTS_PER_REQUEST, j = MAX_INDIRECT_GREFS;
 	dst->operation = src->operation;
 	switch (src->operation) {
 	case BLKIF_OP_READ:
@@ -312,6 +356,19 @@ static inline void blkif_get_x86_32_req(struct
blkif_request *dst,
 		dst->u.discard.sector_number = src->u.discard.sector_number;
 		dst->u.discard.nr_sectors = src->u.discard.nr_sectors;
 		break;
+	case BLKIF_OP_INDIRECT:
+		dst->u.indirect.indirect_op = src->u.indirect.indirect_op;
+		dst->u.indirect.nr_segments = src->u.indirect.nr_segments;
+		dst->u.indirect.handle = src->u.indirect.handle;
+		dst->u.indirect.id = src->u.indirect.id;
+		dst->u.indirect.sector_number = src->u.indirect.sector_number;
+		barrier();
+		if (j > dst->u.indirect.nr_segments)
+			j = dst->u.indirect.nr_segments;
+		for (i = 0; i < j; i++)
+			dst->u.indirect.indirect_grefs[i] +			
src->u.indirect.indirect_grefs[i];
+		break;
 	default:
 		break;
 	}
@@ -320,7 +377,7 @@ static inline void blkif_get_x86_32_req(struct blkif_request
*dst,
 static inline void blkif_get_x86_64_req(struct blkif_request *dst,
 					struct blkif_x86_64_request *src)
 {
-	int i, n = BLKIF_MAX_SEGMENTS_PER_REQUEST;
+	int i, n = BLKIF_MAX_SEGMENTS_PER_REQUEST, j = MAX_INDIRECT_GREFS;
 	dst->operation = src->operation;
 	switch (src->operation) {
 	case BLKIF_OP_READ:
@@ -343,6 +400,19 @@ static inline void blkif_get_x86_64_req(struct
blkif_request *dst,
 		dst->u.discard.sector_number = src->u.discard.sector_number;
 		dst->u.discard.nr_sectors = src->u.discard.nr_sectors;
 		break;
+	case BLKIF_OP_INDIRECT:
+		dst->u.indirect.indirect_op = src->u.indirect.indirect_op;
+		dst->u.indirect.nr_segments = src->u.indirect.nr_segments;
+		dst->u.indirect.handle = src->u.indirect.handle;
+		dst->u.indirect.id = src->u.indirect.id;
+		dst->u.indirect.sector_number = src->u.indirect.sector_number;
+		barrier();
+		if (j > dst->u.indirect.nr_segments)
+			j = dst->u.indirect.nr_segments;
+		for (i = 0; i < j; i++)
+			dst->u.indirect.indirect_grefs[i] +			
src->u.indirect.indirect_grefs[i];
+		break;
 	default:
 		break;
 	}
diff --git a/drivers/block/xen-blkback/xenbus.c
b/drivers/block/xen-blkback/xenbus.c
index 8f929cb..9e16abb 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -700,6 +700,14 @@ again:
 		goto abort;
 	}
 
+	err = xenbus_printf(xbt, dev->nodename, "max-indirect-segments",
"%u",
+	                    MAX_INDIRECT_SEGMENTS);
+	if (err) {
+		xenbus_dev_fatal(dev, err, "writing %s/max-indirect-segments",
+				 dev->nodename);
+		goto abort;
+	}
+
 	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 4d81fcc..074d302 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -74,12 +74,30 @@ struct grant {
 struct blk_shadow {
 	struct blkif_request req;
 	struct request *request;
-	struct grant *grants_used[BLKIF_MAX_SEGMENTS_PER_REQUEST];
+	struct grant **grants_used;
+	struct grant **indirect_grants;
+};
+
+struct split_bio {
+	struct bio *bio;
+	atomic_t pending;
+	int err;
 };
 
 static DEFINE_MUTEX(blkfront_mutex);
 static const struct block_device_operations xlvbd_block_fops;
 
+/*
+ * Maximum number of segments in indirect requests, the actual value used by
+ * the frontend driver is the minimum of this value and the value provided
+ * by the backend driver.
+ */
+
+static int xen_blkif_max_segments = 64;
+module_param_named(max_segments, xen_blkif_max_segments, int, 0);
+MODULE_PARM_DESC(max_segments,
+"Maximum number of segments in indirect requests");
+
 #define BLK_RING_SIZE __CONST_RING_SIZE(blkif, PAGE_SIZE)
 
 /*
@@ -98,7 +116,7 @@ struct blkfront_info
 	enum blkif_state connected;
 	int ring_ref;
 	struct blkif_front_ring ring;
-	struct scatterlist sg[BLKIF_MAX_SEGMENTS_PER_REQUEST];
+	struct scatterlist *sg;
 	unsigned int evtchn, irq;
 	struct request_queue *rq;
 	struct work_struct work;
@@ -114,6 +132,8 @@ struct blkfront_info
 	unsigned int discard_granularity;
 	unsigned int discard_alignment;
 	unsigned int feature_persistent:1;
+	unsigned int max_indirect_segments;
+	unsigned int sector_size;
 	int is_ready;
 };
 
@@ -142,6 +162,14 @@ static DEFINE_SPINLOCK(minor_lock);
 
 #define DEV_NAME	"xvd"	/* name in /dev */
 
+#define SEGS_PER_INDIRECT_FRAME \
+	(PAGE_SIZE/sizeof(struct blkif_request_segment_aligned))
+#define INDIRECT_GREFS(_segs) \
+	((_segs + SEGS_PER_INDIRECT_FRAME - 1)/SEGS_PER_INDIRECT_FRAME)
+#define MIN(_a, _b) ((_a) < (_b) ? (_a) : (_b))
+
+static int blkfront_setup_indirect(struct blkfront_info *info);
+
 static int get_id_from_freelist(struct blkfront_info *info)
 {
 	unsigned long free = info->shadow_free;
@@ -358,7 +386,8 @@ static int blkif_queue_request(struct request *req)
 	struct blkif_request *ring_req;
 	unsigned long id;
 	unsigned int fsect, lsect;
-	int i, ref;
+	int i, ref, n;
+	struct blkif_request_segment_aligned *segments = NULL;
 
 	/*
 	 * Used to store if we are able to queue the request by just using
@@ -369,21 +398,27 @@ static int blkif_queue_request(struct request *req)
 	grant_ref_t gref_head;
 	struct grant *gnt_list_entry = NULL;
 	struct scatterlist *sg;
+	int nseg, max_grefs;
 
 	if (unlikely(info->connected != BLKIF_STATE_CONNECTED))
 		return 1;
 
-	/* Check if we have enought grants to allocate a requests */
-	if (info->persistent_gnts_c < BLKIF_MAX_SEGMENTS_PER_REQUEST) {
+	max_grefs = info->max_indirect_segments ?
+	            info->max_indirect_segments +
+	            INDIRECT_GREFS(info->max_indirect_segments) :
+	            BLKIF_MAX_SEGMENTS_PER_REQUEST;
+
+	/* Check if we have enough grants to allocate a requests */
+	if (info->persistent_gnts_c < max_grefs) {
 		new_persistent_gnts = 1;
 		if (gnttab_alloc_grant_references(
-		    BLKIF_MAX_SEGMENTS_PER_REQUEST - info->persistent_gnts_c,
+		    max_grefs - info->persistent_gnts_c,
 		    &gref_head) < 0) {
 			gnttab_request_free_callback(
 				&info->callback,
 				blkif_restart_queue_callback,
 				info,
-				BLKIF_MAX_SEGMENTS_PER_REQUEST);
+				max_grefs);
 			return 1;
 		}
 	} else
@@ -394,42 +429,82 @@ static int blkif_queue_request(struct request *req)
 	id = get_id_from_freelist(info);
 	info->shadow[id].request = req;
 
-	ring_req->u.rw.id = id;
-	ring_req->u.rw.sector_number = (blkif_sector_t)blk_rq_pos(req);
-	ring_req->u.rw.handle = info->handle;
-
-	ring_req->operation = rq_data_dir(req) ?
-		BLKIF_OP_WRITE : BLKIF_OP_READ;
-
-	if (req->cmd_flags & (REQ_FLUSH | REQ_FUA)) {
-		/*
-		 * Ideally we can do an unordered flush-to-disk. In case the
-		 * backend onlysupports barriers, use that. A barrier request
-		 * a superset of FUA, so we can implement it the same
-		 * way.  (It's also a FLUSH+FUA, since it is
-		 * guaranteed ordered WRT previous writes.)
-		 */
-		ring_req->operation = info->flush_op;
-	}
-
 	if (unlikely(req->cmd_flags & (REQ_DISCARD | REQ_SECURE))) {
 		/* id, sector_number and handle are set above. */
 		ring_req->operation = BLKIF_OP_DISCARD;
 		ring_req->u.discard.nr_sectors = blk_rq_sectors(req);
+		ring_req->u.discard.id = id;
+		ring_req->u.discard.sector_number +			(blkif_sector_t)blk_rq_pos(req);
 		if ((req->cmd_flags & REQ_SECURE) &&
info->feature_secdiscard)
 			ring_req->u.discard.flag = BLKIF_DISCARD_SECURE;
 		else
 			ring_req->u.discard.flag = 0;
 	} else {
-		ring_req->u.rw.nr_segments = blk_rq_map_sg(req->q, req,
-							   info->sg);
-		BUG_ON(ring_req->u.rw.nr_segments >
-		       BLKIF_MAX_SEGMENTS_PER_REQUEST);
-
-		for_each_sg(info->sg, sg, ring_req->u.rw.nr_segments, i) {
+		BUG_ON(info->max_indirect_segments == 0 &&
+		       req->nr_phys_segments > BLKIF_MAX_SEGMENTS_PER_REQUEST);
+		BUG_ON(info->max_indirect_segments &&
+		       req->nr_phys_segments > info->max_indirect_segments);
+		nseg = blk_rq_map_sg(req->q, req, info->sg);
+		if (nseg > BLKIF_MAX_SEGMENTS_PER_REQUEST) {
+			/* Indirect OP */
+			ring_req->operation = BLKIF_OP_INDIRECT;
+			ring_req->u.indirect.indirect_op = rq_data_dir(req) ?
+				BLKIF_OP_WRITE : BLKIF_OP_READ;
+			ring_req->u.indirect.id = id;
+			ring_req->u.indirect.sector_number +				(blkif_sector_t)blk_rq_pos(req);
+			ring_req->u.indirect.handle = info->handle;
+			if (req->cmd_flags & (REQ_FLUSH | REQ_FUA)) {
+		/*
+		 * Ideally we can do an unordered flush-to-disk. In case the
+		 * backend onlysupports barriers, use that. A barrier request
+		 * a superset of FUA, so we can implement it the same
+		 * way.  (It's also a FLUSH+FUA, since it is
+		 * guaranteed ordered WRT previous writes.)
+		 */
+				ring_req->u.indirect.indirect_op +					info->flush_op;
+			}
+			ring_req->u.indirect.nr_segments = nseg;
+		} else {
+			ring_req->u.rw.id = id;
+			ring_req->u.rw.sector_number +				(blkif_sector_t)blk_rq_pos(req);
+			ring_req->u.rw.handle = info->handle;
+			ring_req->operation = rq_data_dir(req) ?
+				BLKIF_OP_WRITE : BLKIF_OP_READ;
+			if (req->cmd_flags & (REQ_FLUSH | REQ_FUA)) {
+		/*
+		 * Ideally we can do an unordered flush-to-disk. In case the
+		 * backend onlysupports barriers, use that. A barrier request
+		 * a superset of FUA, so we can implement it the same
+		 * way.  (It's also a FLUSH+FUA, since it is
+		 * guaranteed ordered WRT previous writes.)
+		 */
+				ring_req->operation = info->flush_op;
+			}
+			ring_req->u.rw.nr_segments = nseg;
+		}
+		for_each_sg(info->sg, sg, nseg, i) {
 			fsect = sg->offset >> 9;
 			lsect = fsect + (sg->length >> 9) - 1;
 
+			if ((ring_req->operation == BLKIF_OP_INDIRECT) &&
+			    (i % SEGS_PER_INDIRECT_FRAME == 0)) {
+				if (segments)
+					kunmap_atomic(segments);
+
+				n = i / SEGS_PER_INDIRECT_FRAME;
+				gnt_list_entry = get_grant(&gref_head, info);
+				info->shadow[id].indirect_grants[n] +					gnt_list_entry;
+				segments = kmap_atomic(
+					pfn_to_page(gnt_list_entry->pfn));
+				ring_req->u.indirect.indirect_grefs[n] +					gnt_list_entry->gref;
+			}
+
 			gnt_list_entry = get_grant(&gref_head, info);
 			ref = gnt_list_entry->gref;
 
@@ -461,13 +536,23 @@ static int blkif_queue_request(struct request *req)
 				kunmap_atomic(bvec_data);
 				kunmap_atomic(shared_data);
 			}
-
-			ring_req->u.rw.seg[i] -					(struct blkif_request_segment) {
-						.gref       = ref,
-						.first_sect = fsect,
-						.last_sect  = lsect };
+			if (ring_req->operation != BLKIF_OP_INDIRECT) {
+				ring_req->u.rw.seg[i] +						(struct blkif_request_segment) {
+							.gref       = ref,
+							.first_sect = fsect,
+							.last_sect  = lsect };
+			} else {
+				n = i % SEGS_PER_INDIRECT_FRAME;
+				segments[n] +					(struct blkif_request_segment_aligned) {
+							.gref       = ref,
+							.first_sect = fsect,
+							.last_sect  = lsect };
+			}
 		}
+		if (segments)
+			kunmap_atomic(segments);
 	}
 
 	info->ring.req_prod_pvt++;
@@ -542,7 +627,8 @@ wait:
 		flush_requests(info);
 }
 
-static int xlvbd_init_blk_queue(struct gendisk *gd, u16 sector_size)
+static int xlvbd_init_blk_queue(struct gendisk *gd, u16 sector_size,
+                                unsigned int segments)
 {
 	struct request_queue *rq;
 	struct blkfront_info *info = gd->private_data;
@@ -571,7 +657,7 @@ static int xlvbd_init_blk_queue(struct gendisk *gd, u16
sector_size)
 	blk_queue_max_segment_size(rq, PAGE_SIZE);
 
 	/* Ensure a merged request will fit in a single I/O ring slot. */
-	blk_queue_max_segments(rq, BLKIF_MAX_SEGMENTS_PER_REQUEST);
+	blk_queue_max_segments(rq, segments);
 
 	/* Make sure buffer addresses are sector-aligned. */
 	blk_queue_dma_alignment(rq, 511);
@@ -588,13 +674,14 @@ 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 %s\n",
+	printk(KERN_INFO "blkfront: %s: %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_persistent ? "using persistent grants" :
"");
+	       info->feature_persistent ? "using persistent grants" :
"",
+	       info->max_indirect_segments ? "using indirect descriptors"
: "");
 }
 
 static int xen_translate_vdev(int vdevice, int *minor, unsigned int *offset)
@@ -734,7 +821,9 @@ static int xlvbd_alloc_gendisk(blkif_sector_t capacity,
 	gd->driverfs_dev = &(info->xbdev->dev);
 	set_capacity(gd, capacity);
 
-	if (xlvbd_init_blk_queue(gd, sector_size)) {
+	if (xlvbd_init_blk_queue(gd, sector_size,
+	                         info->max_indirect_segments ? :
+	                         BLKIF_MAX_SEGMENTS_PER_REQUEST)) {
 		del_gendisk(gd);
 		goto release;
 	}
@@ -818,6 +907,7 @@ static void blkif_free(struct blkfront_info *info, int
suspend)
 {
 	struct grant *persistent_gnt;
 	struct grant *n;
+	int i, j, segs;
 
 	/* Prevent new requests being issued until we fix things up. */
 	spin_lock_irq(&info->io_lock);
@@ -843,6 +933,47 @@ static void blkif_free(struct blkfront_info *info, int
suspend)
 	}
 	BUG_ON(info->persistent_gnts_c != 0);
 
+	kfree(info->sg);
+	info->sg = NULL;
+	for (i = 0; i < BLK_RING_SIZE; i++) {
+		/*
+		 * Clear persistent grants present in requests already
+		 * on the shared ring
+		 */
+		if (!info->shadow[i].request)
+			goto free_shadow;
+
+		segs = info->shadow[i].req.operation == BLKIF_OP_INDIRECT ?
+		       info->shadow[i].req.u.indirect.nr_segments :
+		       info->shadow[i].req.u.rw.nr_segments;
+		for (j = 0; j < segs; j++) {
+			persistent_gnt = info->shadow[i].grants_used[j];
+			gnttab_end_foreign_access(persistent_gnt->gref, 0, 0UL);
+			__free_page(pfn_to_page(persistent_gnt->pfn));
+			kfree(persistent_gnt);
+		}
+
+		if (info->shadow[i].req.operation != BLKIF_OP_INDIRECT)
+			/*
+			 * If this is not an indirect operation don't try to
+			 * free indirect segments
+			 */
+			goto free_shadow;
+
+		for (j = 0; j < INDIRECT_GREFS(segs); j++) {
+			persistent_gnt = info->shadow[i].indirect_grants[j];
+			gnttab_end_foreign_access(persistent_gnt->gref, 0, 0UL);
+			__free_page(pfn_to_page(persistent_gnt->pfn));
+			kfree(persistent_gnt);
+		}
+
+free_shadow:
+		kfree(info->shadow[i].grants_used);
+		info->shadow[i].grants_used = NULL;
+		kfree(info->shadow[i].indirect_grants);
+		info->shadow[i].indirect_grants = NULL;
+	}
+
 	/* No more gnttab callback work. */
 	gnttab_cancel_free_callback(&info->callback);
 	spin_unlock_irq(&info->io_lock);
@@ -873,6 +1004,10 @@ static void blkif_completion(struct blk_shadow *s, struct
blkfront_info *info,
 	char *bvec_data;
 	void *shared_data;
 	unsigned int offset = 0;
+	int nseg;
+
+	nseg = s->req.operation == BLKIF_OP_INDIRECT ?
+		s->req.u.indirect.nr_segments : s->req.u.rw.nr_segments;
 
 	if (bret->operation == BLKIF_OP_READ) {
 		/*
@@ -885,7 +1020,7 @@ static void blkif_completion(struct blk_shadow *s, struct
blkfront_info *info,
 			BUG_ON((bvec->bv_offset + bvec->bv_len) > PAGE_SIZE);
 			if (bvec->bv_offset < offset)
 				i++;
-			BUG_ON(i >= s->req.u.rw.nr_segments);
+			BUG_ON(i >= nseg);
 			shared_data = kmap_atomic(
 				pfn_to_page(s->grants_used[i]->pfn));
 			bvec_data = bvec_kmap_irq(bvec, &flags);
@@ -897,10 +1032,17 @@ static void blkif_completion(struct blk_shadow *s, struct
blkfront_info *info,
 		}
 	}
 	/* Add the persistent grant into the list of free grants */
-	for (i = 0; i < s->req.u.rw.nr_segments; i++) {
+	for (i = 0; i < nseg; i++) {
 		list_add(&s->grants_used[i]->node, &info->persistent_gnts);
 		info->persistent_gnts_c++;
 	}
+	if (s->req.operation == BLKIF_OP_INDIRECT) {
+		for (i = 0; i < INDIRECT_GREFS(nseg); i++) {
+			list_add(&s->indirect_grants[i]->node,
+			         &info->persistent_gnts);
+			info->persistent_gnts_c++;
+		}
+	}
 }
 
 static irqreturn_t blkif_interrupt(int irq, void *dev_id)
@@ -1034,8 +1176,6 @@ static int setup_blkring(struct xenbus_device *dev,
 	SHARED_RING_INIT(sring);
 	FRONT_RING_INIT(&info->ring, sring, PAGE_SIZE);
 
-	sg_init_table(info->sg, BLKIF_MAX_SEGMENTS_PER_REQUEST);
-
 	err = xenbus_grant_ring(dev, virt_to_mfn(info->ring.sring));
 	if (err < 0) {
 		free_page((unsigned long)sring);
@@ -1116,12 +1256,6 @@ again:
 		goto destroy_blkring;
 	}
 
-	/* Allocate memory for grants */
-	err = fill_grant_buffer(info, BLK_RING_SIZE *
-	                              BLKIF_MAX_SEGMENTS_PER_REQUEST);
-	if (err)
-		goto out;
-
 	xenbus_switch_state(dev, XenbusStateInitialised);
 
 	return 0;
@@ -1223,13 +1357,84 @@ static int blkfront_probe(struct xenbus_device *dev,
 	return 0;
 }
 
+/*
+ * This is a clone of md_trim_bio, used to split a bio into smaller ones
+ */
+static void trim_bio(struct bio *bio, int offset, int size)
+{
+	/* 'bio' is a cloned bio which we need to trim to match
+	 * the given offset and size.
+	 * This requires adjusting bi_sector, bi_size, and bi_io_vec
+	 */
+	int i;
+	struct bio_vec *bvec;
+	int sofar = 0;
+
+	size <<= 9;
+	if (offset == 0 && size == bio->bi_size)
+		return;
+
+	bio->bi_sector += offset;
+	bio->bi_size = size;
+	offset <<= 9;
+	clear_bit(BIO_SEG_VALID, &bio->bi_flags);
+
+	while (bio->bi_idx < bio->bi_vcnt &&
+	       bio->bi_io_vec[bio->bi_idx].bv_len <= offset) {
+		/* remove this whole bio_vec */
+		offset -= bio->bi_io_vec[bio->bi_idx].bv_len;
+		bio->bi_idx++;
+	}
+	if (bio->bi_idx < bio->bi_vcnt) {
+		bio->bi_io_vec[bio->bi_idx].bv_offset += offset;
+		bio->bi_io_vec[bio->bi_idx].bv_len -= offset;
+	}
+	/* avoid any complications with bi_idx being non-zero*/
+	if (bio->bi_idx) {
+		memmove(bio->bi_io_vec, bio->bi_io_vec+bio->bi_idx,
+			(bio->bi_vcnt - bio->bi_idx) * sizeof(struct bio_vec));
+		bio->bi_vcnt -= bio->bi_idx;
+		bio->bi_idx = 0;
+	}
+	/* Make sure vcnt and last bv are not too big */
+	bio_for_each_segment(bvec, bio, i) {
+		if (sofar + bvec->bv_len > size)
+			bvec->bv_len = size - sofar;
+		if (bvec->bv_len == 0) {
+			bio->bi_vcnt = i;
+			break;
+		}
+		sofar += bvec->bv_len;
+	}
+}
+
+static void split_bio_end(struct bio *bio, int error)
+{
+	struct split_bio *split_bio = bio->bi_private;
+
+	if (error)
+		split_bio->err = error;
+
+	if (atomic_dec_and_test(&split_bio->pending)) {
+		split_bio->bio->bi_phys_segments = 0;
+		bio_endio(split_bio->bio, split_bio->err);
+		kfree(split_bio);
+	}
+	bio_put(bio);
+}
 
 static int blkif_recover(struct blkfront_info *info)
 {
 	int i;
-	struct blkif_request *req;
+	struct request *req, *n;
 	struct blk_shadow *copy;
-	int j;
+	int rc;
+	struct bio *bio, *cloned_bio;
+	struct bio_list bio_list, merge_bio;
+	unsigned int segs;
+	int pending, offset, size;
+	struct split_bio *split_bio;
+	struct list_head requests;
 
 	/* Stage 1: Make a safe copy of the shadow state. */
 	copy = kmalloc(sizeof(info->shadow),
@@ -1245,36 +1450,64 @@ static int blkif_recover(struct blkfront_info *info)
 	info->shadow_free = info->ring.req_prod_pvt;
 	info->shadow[BLK_RING_SIZE-1].req.u.rw.id = 0x0fffffff;
 
-	/* Stage 3: Find pending requests and requeue them. */
+	rc = blkfront_setup_indirect(info);
+	if (rc) {
+		kfree(copy);
+		return rc;
+	}
+
+	segs = info->max_indirect_segments ? : BLKIF_MAX_SEGMENTS_PER_REQUEST;
+	blk_queue_max_segments(info->rq, segs);
+	bio_list_init(&bio_list);
+	INIT_LIST_HEAD(&requests);
 	for (i = 0; i < BLK_RING_SIZE; i++) {
 		/* Not in use? */
 		if (!copy[i].request)
 			continue;
 
-		/* Grab a request slot and copy shadow state into it. */
-		req = RING_GET_REQUEST(&info->ring, info->ring.req_prod_pvt);
-		*req = copy[i].req;
-
-		/* We get a new request id, and must reset the shadow state. */
-		req->u.rw.id = get_id_from_freelist(info);
-		memcpy(&info->shadow[req->u.rw.id], &copy[i], sizeof(copy[i]));
-
-		if (req->operation != BLKIF_OP_DISCARD) {
-		/* Rewrite any grant references invalidated by susp/resume. */
-			for (j = 0; j < req->u.rw.nr_segments; j++)
-				gnttab_grant_foreign_access_ref(
-					req->u.rw.seg[j].gref,
-					info->xbdev->otherend_id,
-					pfn_to_mfn(copy[i].grants_used[j]->pfn),
-					0);
+		/*
+		 * Get the bios in the request so we can re-queue them.
+		 */
+		if (copy[i].request->cmd_flags &
+		    (REQ_FLUSH | REQ_FUA | REQ_DISCARD | REQ_SECURE)) {
+			/*
+			 * Flush operations don't contain bios, so
+			 * we need to requeue the whole request
+			 */
+			list_add(&copy[i].request->queuelist, &requests);
+			continue;
 		}
-		info->shadow[req->u.rw.id].req = *req;
-
-		info->ring.req_prod_pvt++;
+		merge_bio.head = copy[i].request->bio;
+		merge_bio.tail = copy[i].request->biotail;
+		bio_list_merge(&bio_list, &merge_bio);
+		copy[i].request->bio = NULL;
+		blk_put_request(copy[i].request);
 	}
 
 	kfree(copy);
 
+	/*
+	 * Empty the queue, this is important because we might have
+	 * requests in the queue with more segments than what we
+	 * can handle now.
+	 */
+	spin_lock_irq(&info->io_lock);
+	while ((req = blk_fetch_request(info->rq)) != NULL) {
+		if (req->cmd_flags &
+		    (REQ_FLUSH | REQ_FUA | REQ_DISCARD | REQ_SECURE)) {
+			list_add(&req->queuelist, &requests);
+			continue;
+		}
+		merge_bio.head = req->bio;
+		merge_bio.tail = req->biotail;
+		bio_list_merge(&bio_list, &merge_bio);
+		req->bio = NULL;
+		if (req->cmd_flags & (REQ_FLUSH | REQ_FUA))
+			pr_alert("diskcache flush request found!\n");
+		__blk_put_request(info->rq, req);
+	}
+	spin_unlock_irq(&info->io_lock);
+
 	xenbus_switch_state(info->xbdev, XenbusStateConnected);
 
 	spin_lock_irq(&info->io_lock);
@@ -1282,14 +1515,50 @@ static int blkif_recover(struct blkfront_info *info)
 	/* Now safe for us to use the shared ring */
 	info->connected = BLKIF_STATE_CONNECTED;
 
-	/* Send off requeued requests */
-	flush_requests(info);
-
 	/* Kick any other new requests queued since we resumed */
 	kick_pending_request_queues(info);
 
+	list_for_each_entry_safe(req, n, &requests, queuelist) {
+		/* Requeue pending requests (flush or discard) */
+		list_del_init(&req->queuelist);
+		BUG_ON(req->nr_phys_segments > segs);
+		blk_requeue_request(info->rq, req);
+	}
 	spin_unlock_irq(&info->io_lock);
 
+	while ((bio = bio_list_pop(&bio_list)) != NULL) {
+		/* Traverse the list of pending bios and re-queue them */
+		if (bio_segments(bio) > segs) {
+			/*
+			 * This bio has more segments than what we can
+			 * handle, we have to split it.
+			 */
+			pending = (bio_segments(bio) + segs - 1) / segs;
+			split_bio = kzalloc(sizeof(*split_bio), GFP_NOIO);
+			BUG_ON(split_bio == NULL);
+			atomic_set(&split_bio->pending, pending);
+			split_bio->bio = bio;
+			for (i = 0; i < pending; i++) {
+				offset = (i * segs * PAGE_SIZE) >> 9;
+				size = MIN((segs * PAGE_SIZE) >> 9,
+				           (bio->bi_size >> 9) - offset);
+				cloned_bio = bio_clone(bio, GFP_NOIO);
+				BUG_ON(cloned_bio == NULL);
+				trim_bio(cloned_bio, offset, size);
+				cloned_bio->bi_private = split_bio;
+				cloned_bio->bi_end_io = split_bio_end;
+				submit_bio(cloned_bio->bi_rw, cloned_bio);
+			}
+			/*
+			 * Now we have to wait for all those smaller bios to
+			 * end, so we can also end the "parent" bio.
+			 */
+			continue;
+		}
+		/* We don't need to split this bio */
+		submit_bio(bio->bi_rw, bio);
+	}
+
 	return 0;
 }
 
@@ -1309,8 +1578,12 @@ static int blkfront_resume(struct xenbus_device *dev)
 	blkif_free(info, info->connected == BLKIF_STATE_CONNECTED);
 
 	err = talk_to_blkback(dev, info);
-	if (info->connected == BLKIF_STATE_SUSPENDED && !err)
-		err = blkif_recover(info);
+
+	/*
+	 * We have to wait for the backend to switch to
+	 * connected state, since we want to read which
+	 * features it supports.
+	 */
 
 	return err;
 }
@@ -1388,6 +1661,62 @@ static void blkfront_setup_discard(struct blkfront_info
*info)
 	kfree(type);
 }
 
+static int blkfront_setup_indirect(struct blkfront_info *info)
+{
+	unsigned int indirect_segments, segs;
+	int err, i;
+
+	err = xenbus_gather(XBT_NIL, info->xbdev->otherend,
+			    "max-indirect-segments", "%u",
&indirect_segments,
+			    NULL);
+	if (err) {
+		info->max_indirect_segments = 0;
+		segs = BLKIF_MAX_SEGMENTS_PER_REQUEST;
+	} else {
+		info->max_indirect_segments = MIN(indirect_segments,
+		                                  xen_blkif_max_segments);
+		segs = info->max_indirect_segments;
+	}
+	info->sg = kzalloc(sizeof(info->sg[0]) * segs, GFP_KERNEL);
+	if (info->sg == NULL)
+		goto out_of_memory;
+	sg_init_table(info->sg, segs);
+
+	err = fill_grant_buffer(info,
+	                        (segs + INDIRECT_GREFS(segs)) * BLK_RING_SIZE);
+	if (err)
+		goto out_of_memory;
+
+	for (i = 0; i < BLK_RING_SIZE; i++) {
+		info->shadow[i].grants_used = kzalloc(
+			sizeof(info->shadow[i].grants_used[0]) * segs,
+			GFP_NOIO);
+		if (info->max_indirect_segments)
+			info->shadow[i].indirect_grants = kzalloc(
+				sizeof(info->shadow[i].indirect_grants[0]) *
+				INDIRECT_GREFS(segs),
+				GFP_NOIO);
+		if ((info->shadow[i].grants_used == NULL) ||
+		     (info->max_indirect_segments &&
+		     (info->shadow[i].indirect_grants == NULL)))
+			goto out_of_memory;
+	}
+
+
+	return 0;
+
+out_of_memory:
+	kfree(info->sg);
+	info->sg = NULL;
+	for (i = 0; i < BLK_RING_SIZE; i++) {
+		kfree(info->shadow[i].grants_used);
+		info->shadow[i].grants_used = NULL;
+		kfree(info->shadow[i].indirect_grants);
+		info->shadow[i].indirect_grants = NULL;
+	}
+	return -ENOMEM;
+}
+
 /*
  * Invoked when the backend is finally 'ready' (and has told produced
  * the details about the physical device - #sectors, size, etc).
@@ -1415,8 +1744,9 @@ static void blkfront_connect(struct blkfront_info *info)
 		set_capacity(info->gd, sectors);
 		revalidate_disk(info->gd);
 
-		/* fall through */
+		return;
 	case BLKIF_STATE_SUSPENDED:
+		blkif_recover(info);
 		return;
 
 	default:
@@ -1437,6 +1767,7 @@ static void blkfront_connect(struct blkfront_info *info)
 				 info->xbdev->otherend);
 		return;
 	}
+	info->sector_size = sector_size;
 
 	info->feature_flush = 0;
 	info->flush_op = 0;
@@ -1484,6 +1815,13 @@ static void blkfront_connect(struct blkfront_info *info)
 	else
 		info->feature_persistent = persistent;
 
+	err = blkfront_setup_indirect(info);
+	if (err) {
+		xenbus_dev_fatal(info->xbdev, err, "setup_indirect at %s",
+				 info->xbdev->otherend);
+		return;
+	}
+
 	err = xlvbd_alloc_gendisk(sectors, info, binfo, sector_size);
 	if (err) {
 		xenbus_dev_fatal(info->xbdev, err, "xlvbd_add at %s",
diff --git a/include/xen/interface/io/blkif.h b/include/xen/interface/io/blkif.h
index 01c3d62..6d99849 100644
--- a/include/xen/interface/io/blkif.h
+++ b/include/xen/interface/io/blkif.h
@@ -102,6 +102,8 @@ typedef uint64_t blkif_sector_t;
  */
 #define BLKIF_OP_DISCARD           5
 
+#define BLKIF_OP_INDIRECT          6
+
 /*
  * Maximum scatter/gather segments per request.
  * This is carefully chosen so that sizeof(struct blkif_ring) <= PAGE_SIZE.
@@ -109,6 +111,16 @@ typedef uint64_t blkif_sector_t;
  */
 #define BLKIF_MAX_SEGMENTS_PER_REQUEST 11
 
+#define BLKIF_MAX_INDIRECT_GREFS_PER_REQUEST 8
+
+struct blkif_request_segment_aligned {
+	grant_ref_t gref;        /* reference to I/O buffer frame        */
+	/* @first_sect: first sector in frame to transfer (inclusive).   */
+	/* @last_sect: last sector in frame to transfer (inclusive).     */
+	uint8_t     first_sect, last_sect;
+	uint16_t    _pad; /* padding to make it 8 bytes, so it's cache-aligned */
+} __attribute__((__packed__));
+
 struct blkif_request_rw {
 	uint8_t        nr_segments;  /* number of segments                   */
 	blkif_vdev_t   handle;       /* only for read/write requests         */
@@ -138,11 +150,24 @@ struct blkif_request_discard {
 	uint8_t        _pad3;
 } __attribute__((__packed__));
 
+struct blkif_request_indirect {
+	uint8_t        indirect_op;
+	uint16_t       nr_segments;
+#ifdef CONFIG_X86_64
+	uint32_t       _pad1;        /* offsetof(blkif_...,u.indirect.id) == 8 */
+#endif
+	uint64_t       id;
+	blkif_vdev_t   handle;
+	blkif_sector_t sector_number;
+	grant_ref_t    indirect_grefs[BLKIF_MAX_INDIRECT_GREFS_PER_REQUEST];
+} __attribute__((__packed__));
+
 struct blkif_request {
 	uint8_t        operation;    /* BLKIF_OP_???                         */
 	union {
 		struct blkif_request_rw rw;
 		struct blkif_request_discard discard;
+		struct blkif_request_indirect indirect;
 	} u;
 } __attribute__((__packed__));
 
-- 
1.7.7.5 (Apple Git-26)


_______________________________________________
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Xen-devel@lists.xen.org
http://lists.xen.org/xen-devel

>>> On 28.02.13 at 11:28, Roger Pau Monne <roger.pau@citrix.com>
wrote:
> @@ -109,6 +111,16 @@ typedef uint64_t blkif_sector_t;
>   */
>  #define BLKIF_MAX_SEGMENTS_PER_REQUEST 11
>  
> +#define BLKIF_MAX_INDIRECT_GREFS_PER_REQUEST 8
> +
> +struct blkif_request_segment_aligned {
> +	grant_ref_t gref;        /* reference to I/O buffer frame        */
> +	/* @first_sect: first sector in frame to transfer (inclusive).   */
> +	/* @last_sect: last sector in frame to transfer (inclusive).     */
> +	uint8_t     first_sect, last_sect;
> +	uint16_t    _pad; /* padding to make it 8 bytes, so it''s
cache-aligned */
> +} __attribute__((__packed__));
What''s the __packed__ for here?
> +
>  struct blkif_request_rw {
>  	uint8_t        nr_segments;  /* number of segments                   */
>  	blkif_vdev_t   handle;       /* only for read/write requests         */
> @@ -138,11 +150,24 @@ struct blkif_request_discard {
>  	uint8_t        _pad3;
>  } __attribute__((__packed__));
>  
> +struct blkif_request_indirect {
> +	uint8_t        indirect_op;
> +	uint16_t       nr_segments;
> +#ifdef CONFIG_X86_64
> +	uint32_t       _pad1;        /* offsetof(blkif_...,u.indirect.id) == 8 */
> +#endif
Either you want the structure be packed tightly (and you don''t care
about misaligned fields), in which case you shouldn''t need a padding
field. That''s even more so as there''s no padding between
indirect_op
and nr_segments, so everything is misaligned anyway, and the
comment above is wrong too (offsetof() really ought to yield 7 in
that case).

Or you want the structure fields aligned, in which case you again
ought to drop the use of the __packed__ attribute and introduce
_all_ necessary padding fields.
> +	uint64_t       id;
> +	blkif_vdev_t   handle;
> +	blkif_sector_t sector_number;
> +	grant_ref_t    indirect_grefs[BLKIF_MAX_INDIRECT_GREFS_PER_REQUEST];
> +} __attribute__((__packed__));
And then it would be quite nice for new features to no longer
require translation between a 32- and a 64-bit layout at all.

Plus, rather than introducing uninitialized padding fields, I''d
suggest using fields that are required to be zero initialized, to
allow giving them a meaning later.

Jan

On 28/02/13 12:19, Jan Beulich wrote:
>>>> On 28.02.13 at 11:28, Roger Pau Monne
<roger.pau@citrix.com> wrote:
>> @@ -109,6 +111,16 @@ typedef uint64_t blkif_sector_t;
>>   */
>>  #define BLKIF_MAX_SEGMENTS_PER_REQUEST 11
>>  
>> +#define BLKIF_MAX_INDIRECT_GREFS_PER_REQUEST 8
>> +
>> +struct blkif_request_segment_aligned {
>> +	grant_ref_t gref;        /* reference to I/O buffer frame        */
>> +	/* @first_sect: first sector in frame to transfer (inclusive).   */
>> +	/* @last_sect: last sector in frame to transfer (inclusive).     */
>> +	uint8_t     first_sect, last_sect;
>> +	uint16_t    _pad; /* padding to make it 8 bytes, so it''s
cache-aligned */
>> +} __attribute__((__packed__));
> 
> What''s the __packed__ for here?
Yes, that''s not needed.
> 
>> +
>>  struct blkif_request_rw {
>>  	uint8_t        nr_segments;  /* number of segments                  
*/
>>  	blkif_vdev_t   handle;       /* only for read/write requests        
*/
>> @@ -138,11 +150,24 @@ struct blkif_request_discard {
>>  	uint8_t        _pad3;
>>  } __attribute__((__packed__));
>>  
>> +struct blkif_request_indirect {
>> +	uint8_t        indirect_op;
>> +	uint16_t       nr_segments;
>> +#ifdef CONFIG_X86_64
>> +	uint32_t       _pad1;        /* offsetof(blkif_...,u.indirect.id) ==
8 */
>> +#endif
> 
> Either you want the structure be packed tightly (and you don''t
care
> about misaligned fields), in which case you shouldn''t need a
padding
> field. That''s even more so as there''s no padding between
indirect_op
> and nr_segments, so everything is misaligned anyway, and the
> comment above is wrong too (offsetof() really ought to yield 7 in
> that case).
This padding is because we want to have the "id" field at the same
position as blkif_request_rw, so we need to add the padding for it to
match 32 & 64 bit blkif_request_rw structures, this prevents adding some
"if (req.op == BLKIF_OP_INDIRECT)..." if we only need to get the id of
the request.

The comment is indeed wrong, I''ve copied it from blkif_request_discard
and forgot to change the offset
> 
> Or you want the structure fields aligned, in which case you again
> ought to drop the use of the __packed__ attribute and introduce
> _all_ necessary padding fields.
> 
>> +	uint64_t       id;
>> +	blkif_vdev_t   handle;
>> +	blkif_sector_t sector_number;
>> +	grant_ref_t    indirect_grefs[BLKIF_MAX_INDIRECT_GREFS_PER_REQUEST];
>> +} __attribute__((__packed__));
> 
> And then it would be quite nice for new features to no longer
> require translation between a 32- and a 64-bit layout at all.
The translation is caused by the id issue described above.
> Plus, rather than introducing uninitialized padding fields, I''d
> suggest using fields that are required to be zero initialized, to
> allow giving them a meaning later.

>>> On 28.02.13 at 13:00, Roger Pau Monné<roger.pau@citrix.com>
wrote:
> On 28/02/13 12:19, Jan Beulich wrote:
>>>>> On 28.02.13 at 11:28, Roger Pau Monne
<roger.pau@citrix.com> wrote:
>>> @@ -109,6 +111,16 @@ typedef uint64_t blkif_sector_t;
>>>   */
>>>  #define BLKIF_MAX_SEGMENTS_PER_REQUEST 11
>>>  
>>> +#define BLKIF_MAX_INDIRECT_GREFS_PER_REQUEST 8
>>> +
>>> +struct blkif_request_segment_aligned {
>>> +	grant_ref_t gref;        /* reference to I/O buffer frame       
*/
>>> +	/* @first_sect: first sector in frame to transfer (inclusive).  
*/
>>> +	/* @last_sect: last sector in frame to transfer (inclusive).    
*/
>>> +	uint8_t     first_sect, last_sect;
>>> +	uint16_t    _pad; /* padding to make it 8 bytes, so it's
cache-aligned */
>>> +} __attribute__((__packed__));
>> 
>> What's the __packed__ for here?
> 
> Yes, that's not needed.
> 
>> 
>>> +
>>>  struct blkif_request_rw {
>>>  	uint8_t        nr_segments;  /* number of segments               
*/
>>>  	blkif_vdev_t   handle;       /* only for read/write requests     
*/
>>> @@ -138,11 +150,24 @@ struct blkif_request_discard {
>>>  	uint8_t        _pad3;
>>>  } __attribute__((__packed__));
>>>  
>>> +struct blkif_request_indirect {
>>> +	uint8_t        indirect_op;
>>> +	uint16_t       nr_segments;
>>> +#ifdef CONFIG_X86_64
>>> +	uint32_t       _pad1;        /* offsetof(blkif_...,u.indirect.id)
== 8 */
>>> +#endif
>> 
>> Either you want the structure be packed tightly (and you don't care
>> about misaligned fields), in which case you shouldn't need a
padding
>> field. That's even more so as there's no padding between
indirect_op
>> and nr_segments, so everything is misaligned anyway, and the
>> comment above is wrong too (offsetof() really ought to yield 7 in
>> that case).
> 
> This padding is because we want to have the "id" field at the
same
> position as blkif_request_rw, so we need to add the padding for it to
> match 32 & 64 bit blkif_request_rw structures, this prevents adding
some
> "if (req.op == BLKIF_OP_INDIRECT)..." if we only need to get the
id of
> the request.
Oh, right, that's desirable of course.
> The comment is indeed wrong, I've copied it from blkif_request_discard
> and forgot to change the offset
But the offset stated there then is right after all - I forgot that
there is a 1-byte field outside the union (the way this is being done
in the upstream Linux header is really ugly imo, but I guess Jeremy
and/or Konrad liked it that way). That's also why the packed
attribute is needed here.

But you will probably want to switch sector_number and handle, so
that sector_number becomes aligned, and add another 16-bit
padding field between handle and indirect_grefs[].

I also wonder whether "indirect_op" wouldn't better be named
"actual_op" or just "op".

Jan

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On Thu, Feb 28, 2013 at 11:28:55AM +0100, Roger Pau Monne
wrote:
> Indirect descriptors introduce a new block operation
> (BLKIF_OP_INDIRECT) that passes grant references instead of segments
> in the request. This grant references are filled with arrays of
> blkif_request_segment_aligned, this way we can send more segments in a
> request.
> 
> The proposed implementation sets the maximum number of indirect grefs
> (frames filled with blkif_request_segment_aligned) to 256 in the
> backend and 64 in the frontend. The value in the frontend has been
> chosen experimentally, and the backend value has been set to a sane
> value that allows expanding the maximum number of indirect descriptors
> in the frontend if needed.
So we are still using a similar format of the form:

<gref, first_sec, last_sect, pad>, etc.

Why not utilize a layout that fits with the bio sg? That way
we might not even have to do the bio_alloc call and instead can
setup an bio (and bio-list) with the appropiate offsets/list?

Meaning that the format of the indirect descriptors is:

<gref, offset, next_index, pad>

We already know what the first_sec and last_sect are - they
are basically: sector_number +  nr_segments * (whatever the sector size is) +
offset


> 
> The migration code has changed from the previous implementation, in
> which we simply remapped the segments on the shared ring. Now the
> maximum number of segments allowed in a request can change depending
> on the backend, so we have to requeue all the requests in the ring and
> in the queue and split the bios in them if they are bigger than the
> new maximum number of segments.
> 
> Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> Cc: xen-devel@lists.xen.org
> ---
>  drivers/block/xen-blkback/blkback.c |  129 +++++++---
>  drivers/block/xen-blkback/common.h  |   80 ++++++-
>  drivers/block/xen-blkback/xenbus.c  |    8 +
>  drivers/block/xen-blkfront.c        |  498
+++++++++++++++++++++++++++++------
>  include/xen/interface/io/blkif.h    |   25 ++
>  5 files changed, 622 insertions(+), 118 deletions(-)
> 
> diff --git a/drivers/block/xen-blkback/blkback.c
b/drivers/block/xen-blkback/blkback.c
> index 0fa30db..98eb16b 100644
> --- a/drivers/block/xen-blkback/blkback.c
> +++ b/drivers/block/xen-blkback/blkback.c
> @@ -70,7 +70,7 @@ MODULE_PARM_DESC(reqs, "Number of blkback requests
to allocate per backend");
>   * algorithm.
>   */
>  
> -static int xen_blkif_max_pgrants = 352;
> +static int xen_blkif_max_pgrants = 1024;
>  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");
> @@ -578,10 +578,6 @@ purge_gnt_list:
>  	return 0;
>  }
>  
> -struct seg_buf {
> -	unsigned long buf;
> -	unsigned int nsec;
> -};
>  /*
>   * Unmap the grant references, and also remove the M2P over-rides
>   * used in the ''pending_req''.
> @@ -761,32 +757,79 @@ out_of_memory:
>  	return -ENOMEM;
>  }
>  
> -static int xen_blkbk_map_seg(struct blkif_request *req,
> -			     struct pending_req *pending_req,
> +static int xen_blkbk_map_seg(struct pending_req *pending_req,
>  			     struct seg_buf seg[],
>  			     struct page *pages[])
>  {
>  	int i, rc;
> -	grant_ref_t grefs[BLKIF_MAX_SEGMENTS_PER_REQUEST];
>  
> -	for (i = 0; i < req->u.rw.nr_segments; i++)
> -		grefs[i] = req->u.rw.seg[i].gref;
> -
> -	rc = xen_blkbk_map(pending_req->blkif, grefs,
> +	rc = xen_blkbk_map(pending_req->blkif, pending_req->grefs,
>  	                   pending_req->persistent_gnts,
>  	                   pending_req->grant_handles, pending_req->pages,
> -	                   req->u.rw.nr_segments,
> +	                   pending_req->nr_pages,
>  	                   (pending_req->operation != BLKIF_OP_READ));
>  	if (rc)
>  		return rc;
>  
> -	for (i = 0; i < req->u.rw.nr_segments; i++)
> -		seg[i].buf = pfn_to_mfn(page_to_pfn(pending_req->pages[i]))
> -		             << PAGE_SHIFT | (req->u.rw.seg[i].first_sect
<< 9);
> +	for (i = 0; i < pending_req->nr_pages; i++)
> +		seg[i].buf |= pfn_to_mfn(page_to_pfn(pending_req->pages[i]))
> +		             << PAGE_SHIFT;
>  
>  	return 0;
>  }
>  
> +static int xen_blkbk_parse_indirect(struct blkif_request *req,
> +                                    struct pending_req *pending_req,
> +                                    struct seg_buf seg[],
> +                                    struct phys_req *preq)
> +{
> +	struct persistent_gnt **persistent > +	
pending_req->indirect_persistent_gnts;
> +	struct 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 = (nseg + SEGS_PER_INDIRECT_FRAME - 1) /
> +		         SEGS_PER_INDIRECT_FRAME;
> +
> +	rc = xen_blkbk_map(blkif, req->u.indirect.indirect_grefs,
> +	                   persistent, pending_req->indirect_handles,
> +	                   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]);
> +		}
> +		i = n % SEGS_PER_INDIRECT_FRAME;
> +		pending_req->grefs[n] = segments[i].gref;
> +		seg[n].nsec = segments[i].last_sect -
> +			segments[i].first_sect + 1;
> +		seg[n].buf = 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, pending_req->indirect_handles,
> +                        pages, persistent, indirect_grefs);
> +	return rc;
> +}
> +
>  static int dispatch_discard_io(struct xen_blkif *blkif,
>  				struct blkif_request *req)
>  {
> @@ -980,17 +1023,21 @@ static int dispatch_rw_block_io(struct xen_blkif
*blkif,
>  				struct pending_req *pending_req)
>  {
>  	struct phys_req preq;
> -	struct seg_buf seg[BLKIF_MAX_SEGMENTS_PER_REQUEST];
> +	struct seg_buf *seg = pending_req->seg;
>  	unsigned int nseg;
>  	struct bio *bio = NULL;
> -	struct bio *biolist[BLKIF_MAX_SEGMENTS_PER_REQUEST];
> +	struct bio **biolist = pending_req->biolist;
>  	int i, nbio = 0;
>  	int operation;
>  	struct blk_plug plug;
>  	bool drain = false;
>  	struct page **pages = pending_req->pages;
> +	unsigned short req_operation;
> +
> +	req_operation = req->operation == BLKIF_OP_INDIRECT ?
> +	                req->u.indirect.indirect_op : req->operation;
>  
> -	switch (req->operation) {
> +	switch (req_operation) {
>  	case BLKIF_OP_READ:
>  		blkif->st_rd_req++;
>  		operation = READ;
> @@ -1012,33 +1059,49 @@ static int dispatch_rw_block_io(struct xen_blkif
*blkif,
>  	}
>  
>  	/* Check that the number of segments is sane. */
> -	nseg = req->u.rw.nr_segments;
> +	nseg = req->operation == BLKIF_OP_INDIRECT ?
> +	       req->u.indirect.nr_segments : req->u.rw.nr_segments;
>  
>  	if (unlikely(nseg == 0 && operation != WRITE_FLUSH) ||
> -	    unlikely(nseg > BLKIF_MAX_SEGMENTS_PER_REQUEST)) {
> +	    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.sector_number = req->u.rw.sector_number;
>  	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;
>  
> -	for (i = 0; i < nseg; i++) {
> -		seg[i].nsec = req->u.rw.seg[i].last_sect -
> -			req->u.rw.seg[i].first_sect + 1;
> -		if ((req->u.rw.seg[i].last_sect >= (PAGE_SIZE >> 9)) ||
> -		    (req->u.rw.seg[i].last_sect < req->u.rw.seg[i].first_sect))
> +	if (req->operation != BLKIF_OP_INDIRECT) {
> +		preq.dev               = req->u.rw.handle;
> +		preq.sector_number     = req->u.rw.sector_number;
> +		pending_req->id        = req->u.rw.id;
> +		pending_req->operation = req->operation;
> +		for (i = 0; i < nseg; i++) {
> +			pending_req->grefs[i] = 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].buf = 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;
> +		pending_req->id        = req->u.indirect.id;
> +		pending_req->operation = req->u.indirect.indirect_op;
> +		if (xen_blkbk_parse_indirect(req, pending_req, seg, &preq))
>  			goto fail_response;
> -		preq.nr_sects += seg[i].nsec;
> -
>  	}
>  
>  	if (xen_vbd_translate(&preq, blkif, operation) != 0) {
> @@ -1074,7 +1137,7 @@ 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_seg(req, pending_req, seg, pages))
> +	if (xen_blkbk_map_seg(pending_req, seg, pages))
>  		goto fail_flush;
>  
>  	/*
> @@ -1146,7 +1209,7 @@ static int dispatch_rw_block_io(struct xen_blkif
*blkif,
>  	                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);
> +	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;
> diff --git a/drivers/block/xen-blkback/common.h
b/drivers/block/xen-blkback/common.h
> index 0b0ad3f..d3656d2 100644
> --- a/drivers/block/xen-blkback/common.h
> +++ b/drivers/block/xen-blkback/common.h
> @@ -50,6 +50,17 @@
>  		 __func__, __LINE__, ##args)
>  
>  
> +/*
> + * This is the maximum number of segments that would be allowed in
indirect
> + * requests. This value will also be passed to the frontend.
> + */
> +#define MAX_INDIRECT_SEGMENTS 256
> +
> +#define SEGS_PER_INDIRECT_FRAME \
> +(PAGE_SIZE/sizeof(struct blkif_request_segment_aligned))
> +#define MAX_INDIRECT_GREFS \
> +((MAX_INDIRECT_SEGMENTS + SEGS_PER_INDIRECT_FRAME -
1)/SEGS_PER_INDIRECT_FRAME)
> +
>  /* Not a real protocol.  Used to generate ring structs which contain
>   * the elements common to all protocols only.  This way we get a
>   * compiler-checkable way to use common struct elements, so we can
> @@ -77,11 +88,21 @@ struct blkif_x86_32_request_discard {
>  	uint64_t       nr_sectors;
>  } __attribute__((__packed__));
>  
> +struct blkif_x86_32_request_indirect {
> +	uint8_t        indirect_op;
> +	uint16_t       nr_segments;
> +	uint64_t       id;
> +	blkif_vdev_t   handle;
> +	blkif_sector_t sector_number;
> +	grant_ref_t    indirect_grefs[BLKIF_MAX_INDIRECT_GREFS_PER_REQUEST];
> +} __attribute__((__packed__));
> +
>  struct blkif_x86_32_request {
>  	uint8_t        operation;    /* BLKIF_OP_???                         */
>  	union {
>  		struct blkif_x86_32_request_rw rw;
>  		struct blkif_x86_32_request_discard discard;
> +		struct blkif_x86_32_request_indirect indirect;
>  	} u;
>  } __attribute__((__packed__));
>  
> @@ -113,11 +134,22 @@ struct blkif_x86_64_request_discard {
>  	uint64_t       nr_sectors;
>  } __attribute__((__packed__));
>  
> +struct blkif_x86_64_request_indirect {
> +	uint8_t        indirect_op;
> +	uint16_t       nr_segments;
> +	uint32_t       _pad1;        /* offsetof(blkif_..,u.indirect.id)==8   */
> +	uint64_t       id;
> +	blkif_vdev_t   handle;
> +	blkif_sector_t sector_number;
> +	grant_ref_t    indirect_grefs[BLKIF_MAX_INDIRECT_GREFS_PER_REQUEST];
> +} __attribute__((__packed__));
> +
>  struct blkif_x86_64_request {
>  	uint8_t        operation;    /* BLKIF_OP_???                         */
>  	union {
>  		struct blkif_x86_64_request_rw rw;
>  		struct blkif_x86_64_request_discard discard;
> +		struct blkif_x86_64_request_indirect indirect;
>  	} u;
>  } __attribute__((__packed__));
>  
> @@ -235,6 +267,11 @@ struct xen_blkif {
>  	wait_queue_head_t	waiting_to_free;
>  };
>  
> +struct seg_buf {
> +	unsigned long buf;
> +	unsigned int nsec;
> +};
> +
>  /*
>   * Each outstanding request that we''ve passed to the lower device
layers has a
>   * ''pending_req'' allocated to it. Each buffer_head that
completes decrements
> @@ -249,9 +286,16 @@ struct pending_req {
>  	unsigned short		operation;
>  	int			status;
>  	struct list_head	free_list;
> -	struct persistent_gnt	*persistent_gnts[BLKIF_MAX_SEGMENTS_PER_REQUEST];
> -	struct page		*pages[BLKIF_MAX_SEGMENTS_PER_REQUEST];
> -	grant_handle_t		grant_handles[BLKIF_MAX_SEGMENTS_PER_REQUEST];
> +	struct persistent_gnt	*persistent_gnts[MAX_INDIRECT_SEGMENTS];
> +	struct page		*pages[MAX_INDIRECT_SEGMENTS];
> +	grant_handle_t		grant_handles[MAX_INDIRECT_SEGMENTS];
> +	grant_ref_t		grefs[MAX_INDIRECT_SEGMENTS];
> +	/* Indirect descriptors */
> +	struct persistent_gnt	*indirect_persistent_gnts[MAX_INDIRECT_GREFS];
> +	struct page		*indirect_pages[MAX_INDIRECT_GREFS];
> +	grant_handle_t		indirect_handles[MAX_INDIRECT_GREFS];
> +	struct seg_buf		seg[MAX_INDIRECT_SEGMENTS];
> +	struct bio		*biolist[MAX_INDIRECT_SEGMENTS];
>  };
>  
>  
> @@ -289,7 +333,7 @@ struct xenbus_device *xen_blkbk_xenbus(struct
backend_info *be);
>  static inline void blkif_get_x86_32_req(struct blkif_request *dst,
>  					struct blkif_x86_32_request *src)
>  {
> -	int i, n = BLKIF_MAX_SEGMENTS_PER_REQUEST;
> +	int i, n = BLKIF_MAX_SEGMENTS_PER_REQUEST, j = MAX_INDIRECT_GREFS;
>  	dst->operation = src->operation;
>  	switch (src->operation) {
>  	case BLKIF_OP_READ:
> @@ -312,6 +356,19 @@ static inline void blkif_get_x86_32_req(struct
blkif_request *dst,
>  		dst->u.discard.sector_number = src->u.discard.sector_number;
>  		dst->u.discard.nr_sectors = src->u.discard.nr_sectors;
>  		break;
> +	case BLKIF_OP_INDIRECT:
> +		dst->u.indirect.indirect_op = src->u.indirect.indirect_op;
> +		dst->u.indirect.nr_segments = src->u.indirect.nr_segments;
> +		dst->u.indirect.handle = src->u.indirect.handle;
> +		dst->u.indirect.id = src->u.indirect.id;
> +		dst->u.indirect.sector_number = src->u.indirect.sector_number;
> +		barrier();
> +		if (j > dst->u.indirect.nr_segments)
> +			j = dst->u.indirect.nr_segments;
> +		for (i = 0; i < j; i++)
> +			dst->u.indirect.indirect_grefs[i] > +			
src->u.indirect.indirect_grefs[i];
> +		break;
>  	default:
>  		break;
>  	}
> @@ -320,7 +377,7 @@ static inline void blkif_get_x86_32_req(struct
blkif_request *dst,
>  static inline void blkif_get_x86_64_req(struct blkif_request *dst,
>  					struct blkif_x86_64_request *src)
>  {
> -	int i, n = BLKIF_MAX_SEGMENTS_PER_REQUEST;
> +	int i, n = BLKIF_MAX_SEGMENTS_PER_REQUEST, j = MAX_INDIRECT_GREFS;
>  	dst->operation = src->operation;
>  	switch (src->operation) {
>  	case BLKIF_OP_READ:
> @@ -343,6 +400,19 @@ static inline void blkif_get_x86_64_req(struct
blkif_request *dst,
>  		dst->u.discard.sector_number = src->u.discard.sector_number;
>  		dst->u.discard.nr_sectors = src->u.discard.nr_sectors;
>  		break;
> +	case BLKIF_OP_INDIRECT:
> +		dst->u.indirect.indirect_op = src->u.indirect.indirect_op;
> +		dst->u.indirect.nr_segments = src->u.indirect.nr_segments;
> +		dst->u.indirect.handle = src->u.indirect.handle;
> +		dst->u.indirect.id = src->u.indirect.id;
> +		dst->u.indirect.sector_number = src->u.indirect.sector_number;
> +		barrier();
> +		if (j > dst->u.indirect.nr_segments)
> +			j = dst->u.indirect.nr_segments;
> +		for (i = 0; i < j; i++)
> +			dst->u.indirect.indirect_grefs[i] > +			
src->u.indirect.indirect_grefs[i];
> +		break;
>  	default:
>  		break;
>  	}
> diff --git a/drivers/block/xen-blkback/xenbus.c
b/drivers/block/xen-blkback/xenbus.c
> index 8f929cb..9e16abb 100644
> --- a/drivers/block/xen-blkback/xenbus.c
> +++ b/drivers/block/xen-blkback/xenbus.c
> @@ -700,6 +700,14 @@ again:
>  		goto abort;
>  	}
>  
> +	err = xenbus_printf(xbt, dev->nodename,
"max-indirect-segments", "%u",
> +	                    MAX_INDIRECT_SEGMENTS);
> +	if (err) {
> +		xenbus_dev_fatal(dev, err, "writing %s/max-indirect-segments",
> +				 dev->nodename);
> +		goto abort;
> +	}
> +
>  	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 4d81fcc..074d302 100644
> --- a/drivers/block/xen-blkfront.c
> +++ b/drivers/block/xen-blkfront.c
> @@ -74,12 +74,30 @@ struct grant {
>  struct blk_shadow {
>  	struct blkif_request req;
>  	struct request *request;
> -	struct grant *grants_used[BLKIF_MAX_SEGMENTS_PER_REQUEST];
> +	struct grant **grants_used;
> +	struct grant **indirect_grants;
> +};
> +
> +struct split_bio {
> +	struct bio *bio;
> +	atomic_t pending;
> +	int err;
>  };
>  
>  static DEFINE_MUTEX(blkfront_mutex);
>  static const struct block_device_operations xlvbd_block_fops;
>  
> +/*
> + * Maximum number of segments in indirect requests, the actual value used
by
> + * the frontend driver is the minimum of this value and the value provided
> + * by the backend driver.
> + */
> +
> +static int xen_blkif_max_segments = 64;
> +module_param_named(max_segments, xen_blkif_max_segments, int, 0);
> +MODULE_PARM_DESC(max_segments,
> +"Maximum number of segments in indirect requests");
> +
>  #define BLK_RING_SIZE __CONST_RING_SIZE(blkif, PAGE_SIZE)
>  
>  /*
> @@ -98,7 +116,7 @@ struct blkfront_info
>  	enum blkif_state connected;
>  	int ring_ref;
>  	struct blkif_front_ring ring;
> -	struct scatterlist sg[BLKIF_MAX_SEGMENTS_PER_REQUEST];
> +	struct scatterlist *sg;
>  	unsigned int evtchn, irq;
>  	struct request_queue *rq;
>  	struct work_struct work;
> @@ -114,6 +132,8 @@ struct blkfront_info
>  	unsigned int discard_granularity;
>  	unsigned int discard_alignment;
>  	unsigned int feature_persistent:1;
> +	unsigned int max_indirect_segments;
> +	unsigned int sector_size;
>  	int is_ready;
>  };
>  
> @@ -142,6 +162,14 @@ static DEFINE_SPINLOCK(minor_lock);
>  
>  #define DEV_NAME	"xvd"	/* name in /dev */
>  
> +#define SEGS_PER_INDIRECT_FRAME \
> +	(PAGE_SIZE/sizeof(struct blkif_request_segment_aligned))
> +#define INDIRECT_GREFS(_segs) \
> +	((_segs + SEGS_PER_INDIRECT_FRAME - 1)/SEGS_PER_INDIRECT_FRAME)
> +#define MIN(_a, _b) ((_a) < (_b) ? (_a) : (_b))
> +
> +static int blkfront_setup_indirect(struct blkfront_info *info);
> +
>  static int get_id_from_freelist(struct blkfront_info *info)
>  {
>  	unsigned long free = info->shadow_free;
> @@ -358,7 +386,8 @@ static int blkif_queue_request(struct request *req)
>  	struct blkif_request *ring_req;
>  	unsigned long id;
>  	unsigned int fsect, lsect;
> -	int i, ref;
> +	int i, ref, n;
> +	struct blkif_request_segment_aligned *segments = NULL;
>  
>  	/*
>  	 * Used to store if we are able to queue the request by just using
> @@ -369,21 +398,27 @@ static int blkif_queue_request(struct request *req)
>  	grant_ref_t gref_head;
>  	struct grant *gnt_list_entry = NULL;
>  	struct scatterlist *sg;
> +	int nseg, max_grefs;
>  
>  	if (unlikely(info->connected != BLKIF_STATE_CONNECTED))
>  		return 1;
>  
> -	/* Check if we have enought grants to allocate a requests */
> -	if (info->persistent_gnts_c < BLKIF_MAX_SEGMENTS_PER_REQUEST) {
> +	max_grefs = info->max_indirect_segments ?
> +	            info->max_indirect_segments +
> +	            INDIRECT_GREFS(info->max_indirect_segments) :
> +	            BLKIF_MAX_SEGMENTS_PER_REQUEST;
> +
> +	/* Check if we have enough grants to allocate a requests */
> +	if (info->persistent_gnts_c < max_grefs) {
>  		new_persistent_gnts = 1;
>  		if (gnttab_alloc_grant_references(
> -		    BLKIF_MAX_SEGMENTS_PER_REQUEST - info->persistent_gnts_c,
> +		    max_grefs - info->persistent_gnts_c,
>  		    &gref_head) < 0) {
>  			gnttab_request_free_callback(
>  				&info->callback,
>  				blkif_restart_queue_callback,
>  				info,
> -				BLKIF_MAX_SEGMENTS_PER_REQUEST);
> +				max_grefs);
>  			return 1;
>  		}
>  	} else
> @@ -394,42 +429,82 @@ static int blkif_queue_request(struct request *req)
>  	id = get_id_from_freelist(info);
>  	info->shadow[id].request = req;
>  
> -	ring_req->u.rw.id = id;
> -	ring_req->u.rw.sector_number = (blkif_sector_t)blk_rq_pos(req);
> -	ring_req->u.rw.handle = info->handle;
> -
> -	ring_req->operation = rq_data_dir(req) ?
> -		BLKIF_OP_WRITE : BLKIF_OP_READ;
> -
> -	if (req->cmd_flags & (REQ_FLUSH | REQ_FUA)) {
> -		/*
> -		 * Ideally we can do an unordered flush-to-disk. In case the
> -		 * backend onlysupports barriers, use that. A barrier request
> -		 * a superset of FUA, so we can implement it the same
> -		 * way.  (It''s also a FLUSH+FUA, since it is
> -		 * guaranteed ordered WRT previous writes.)
> -		 */
> -		ring_req->operation = info->flush_op;
> -	}
> -
>  	if (unlikely(req->cmd_flags & (REQ_DISCARD | REQ_SECURE))) {
>  		/* id, sector_number and handle are set above. */
>  		ring_req->operation = BLKIF_OP_DISCARD;
>  		ring_req->u.discard.nr_sectors = blk_rq_sectors(req);
> +		ring_req->u.discard.id = id;
> +		ring_req->u.discard.sector_number > +		
(blkif_sector_t)blk_rq_pos(req);
>  		if ((req->cmd_flags & REQ_SECURE) &&
info->feature_secdiscard)
>  			ring_req->u.discard.flag = BLKIF_DISCARD_SECURE;
>  		else
>  			ring_req->u.discard.flag = 0;
>  	} else {
> -		ring_req->u.rw.nr_segments = blk_rq_map_sg(req->q, req,
> -							   info->sg);
> -		BUG_ON(ring_req->u.rw.nr_segments >
> -		       BLKIF_MAX_SEGMENTS_PER_REQUEST);
> -
> -		for_each_sg(info->sg, sg, ring_req->u.rw.nr_segments, i) {
> +		BUG_ON(info->max_indirect_segments == 0 &&
> +		       req->nr_phys_segments > BLKIF_MAX_SEGMENTS_PER_REQUEST);
> +		BUG_ON(info->max_indirect_segments &&
> +		       req->nr_phys_segments > info->max_indirect_segments);
> +		nseg = blk_rq_map_sg(req->q, req, info->sg);
> +		if (nseg > BLKIF_MAX_SEGMENTS_PER_REQUEST) {
> +			/* Indirect OP */
> +			ring_req->operation = BLKIF_OP_INDIRECT;
> +			ring_req->u.indirect.indirect_op = rq_data_dir(req) ?
> +				BLKIF_OP_WRITE : BLKIF_OP_READ;
> +			ring_req->u.indirect.id = id;
> +			ring_req->u.indirect.sector_number > +			
(blkif_sector_t)blk_rq_pos(req);
> +			ring_req->u.indirect.handle = info->handle;
> +			if (req->cmd_flags & (REQ_FLUSH | REQ_FUA)) {
> +		/*
> +		 * Ideally we can do an unordered flush-to-disk. In case the
> +		 * backend onlysupports barriers, use that. A barrier request
> +		 * a superset of FUA, so we can implement it the same
> +		 * way.  (It''s also a FLUSH+FUA, since it is
> +		 * guaranteed ordered WRT previous writes.)
> +		 */
> +				ring_req->u.indirect.indirect_op > +					info->flush_op;
> +			}
> +			ring_req->u.indirect.nr_segments = nseg;
> +		} else {
> +			ring_req->u.rw.id = id;
> +			ring_req->u.rw.sector_number > +			
(blkif_sector_t)blk_rq_pos(req);
> +			ring_req->u.rw.handle = info->handle;
> +			ring_req->operation = rq_data_dir(req) ?
> +				BLKIF_OP_WRITE : BLKIF_OP_READ;
> +			if (req->cmd_flags & (REQ_FLUSH | REQ_FUA)) {
> +		/*
> +		 * Ideally we can do an unordered flush-to-disk. In case the
> +		 * backend onlysupports barriers, use that. A barrier request
> +		 * a superset of FUA, so we can implement it the same
> +		 * way.  (It''s also a FLUSH+FUA, since it is
> +		 * guaranteed ordered WRT previous writes.)
> +		 */
> +				ring_req->operation = info->flush_op;
> +			}
> +			ring_req->u.rw.nr_segments = nseg;
> +		}
> +		for_each_sg(info->sg, sg, nseg, i) {
>  			fsect = sg->offset >> 9;
>  			lsect = fsect + (sg->length >> 9) - 1;
>  
> +			if ((ring_req->operation == BLKIF_OP_INDIRECT) &&
> +			    (i % SEGS_PER_INDIRECT_FRAME == 0)) {
> +				if (segments)
> +					kunmap_atomic(segments);
> +
> +				n = i / SEGS_PER_INDIRECT_FRAME;
> +				gnt_list_entry = get_grant(&gref_head, info);
> +				info->shadow[id].indirect_grants[n] > +					gnt_list_entry;
> +				segments = kmap_atomic(
> +					pfn_to_page(gnt_list_entry->pfn));
> +				ring_req->u.indirect.indirect_grefs[n] > +				
gnt_list_entry->gref;
> +			}
> +
>  			gnt_list_entry = get_grant(&gref_head, info);
>  			ref = gnt_list_entry->gref;
>  
> @@ -461,13 +536,23 @@ static int blkif_queue_request(struct request *req)
>  				kunmap_atomic(bvec_data);
>  				kunmap_atomic(shared_data);
>  			}
> -
> -			ring_req->u.rw.seg[i] > -					(struct blkif_request_segment) {
> -						.gref       = ref,
> -						.first_sect = fsect,
> -						.last_sect  = lsect };
> +			if (ring_req->operation != BLKIF_OP_INDIRECT) {
> +				ring_req->u.rw.seg[i] > +						(struct blkif_request_segment) {
> +							.gref       = ref,
> +							.first_sect = fsect,
> +							.last_sect  = lsect };
> +			} else {
> +				n = i % SEGS_PER_INDIRECT_FRAME;
> +				segments[n] > +					(struct blkif_request_segment_aligned) {
> +							.gref       = ref,
> +							.first_sect = fsect,
> +							.last_sect  = lsect };
> +			}
>  		}
> +		if (segments)
> +			kunmap_atomic(segments);
>  	}
>  
>  	info->ring.req_prod_pvt++;
> @@ -542,7 +627,8 @@ wait:
>  		flush_requests(info);
>  }
>  
> -static int xlvbd_init_blk_queue(struct gendisk *gd, u16 sector_size)
> +static int xlvbd_init_blk_queue(struct gendisk *gd, u16 sector_size,
> +                                unsigned int segments)
>  {
>  	struct request_queue *rq;
>  	struct blkfront_info *info = gd->private_data;
> @@ -571,7 +657,7 @@ static int xlvbd_init_blk_queue(struct gendisk *gd, u16
sector_size)
>  	blk_queue_max_segment_size(rq, PAGE_SIZE);
>  
>  	/* Ensure a merged request will fit in a single I/O ring slot. */
> -	blk_queue_max_segments(rq, BLKIF_MAX_SEGMENTS_PER_REQUEST);
> +	blk_queue_max_segments(rq, segments);
>  
>  	/* Make sure buffer addresses are sector-aligned. */
>  	blk_queue_dma_alignment(rq, 511);
> @@ -588,13 +674,14 @@ 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 %s\n",
> +	printk(KERN_INFO "blkfront: %s: %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_persistent ? "using persistent grants"
: "");
> +	       info->feature_persistent ? "using persistent grants"
: "",
> +	       info->max_indirect_segments ? "using indirect
descriptors" : "");
>  }
>  
>  static int xen_translate_vdev(int vdevice, int *minor, unsigned int
*offset)
> @@ -734,7 +821,9 @@ static int xlvbd_alloc_gendisk(blkif_sector_t capacity,
>  	gd->driverfs_dev = &(info->xbdev->dev);
>  	set_capacity(gd, capacity);
>  
> -	if (xlvbd_init_blk_queue(gd, sector_size)) {
> +	if (xlvbd_init_blk_queue(gd, sector_size,
> +	                         info->max_indirect_segments ? :
> +	                         BLKIF_MAX_SEGMENTS_PER_REQUEST)) {
>  		del_gendisk(gd);
>  		goto release;
>  	}
> @@ -818,6 +907,7 @@ static void blkif_free(struct blkfront_info *info, int
suspend)
>  {
>  	struct grant *persistent_gnt;
>  	struct grant *n;
> +	int i, j, segs;
>  
>  	/* Prevent new requests being issued until we fix things up. */
>  	spin_lock_irq(&info->io_lock);
> @@ -843,6 +933,47 @@ static void blkif_free(struct blkfront_info *info, int
suspend)
>  	}
>  	BUG_ON(info->persistent_gnts_c != 0);
>  
> +	kfree(info->sg);
> +	info->sg = NULL;
> +	for (i = 0; i < BLK_RING_SIZE; i++) {
> +		/*
> +		 * Clear persistent grants present in requests already
> +		 * on the shared ring
> +		 */
> +		if (!info->shadow[i].request)
> +			goto free_shadow;
> +
> +		segs = info->shadow[i].req.operation == BLKIF_OP_INDIRECT ?
> +		       info->shadow[i].req.u.indirect.nr_segments :
> +		       info->shadow[i].req.u.rw.nr_segments;
> +		for (j = 0; j < segs; j++) {
> +			persistent_gnt = info->shadow[i].grants_used[j];
> +			gnttab_end_foreign_access(persistent_gnt->gref, 0, 0UL);
> +			__free_page(pfn_to_page(persistent_gnt->pfn));
> +			kfree(persistent_gnt);
> +		}
> +
> +		if (info->shadow[i].req.operation != BLKIF_OP_INDIRECT)
> +			/*
> +			 * If this is not an indirect operation don''t try to
> +			 * free indirect segments
> +			 */
> +			goto free_shadow;
> +
> +		for (j = 0; j < INDIRECT_GREFS(segs); j++) {
> +			persistent_gnt = info->shadow[i].indirect_grants[j];
> +			gnttab_end_foreign_access(persistent_gnt->gref, 0, 0UL);
> +			__free_page(pfn_to_page(persistent_gnt->pfn));
> +			kfree(persistent_gnt);
> +		}
> +
> +free_shadow:
> +		kfree(info->shadow[i].grants_used);
> +		info->shadow[i].grants_used = NULL;
> +		kfree(info->shadow[i].indirect_grants);
> +		info->shadow[i].indirect_grants = NULL;
> +	}
> +
>  	/* No more gnttab callback work. */
>  	gnttab_cancel_free_callback(&info->callback);
>  	spin_unlock_irq(&info->io_lock);
> @@ -873,6 +1004,10 @@ static void blkif_completion(struct blk_shadow *s,
struct blkfront_info *info,
>  	char *bvec_data;
>  	void *shared_data;
>  	unsigned int offset = 0;
> +	int nseg;
> +
> +	nseg = s->req.operation == BLKIF_OP_INDIRECT ?
> +		s->req.u.indirect.nr_segments : s->req.u.rw.nr_segments;
>  
>  	if (bret->operation == BLKIF_OP_READ) {
>  		/*
> @@ -885,7 +1020,7 @@ static void blkif_completion(struct blk_shadow *s,
struct blkfront_info *info,
>  			BUG_ON((bvec->bv_offset + bvec->bv_len) > PAGE_SIZE);
>  			if (bvec->bv_offset < offset)
>  				i++;
> -			BUG_ON(i >= s->req.u.rw.nr_segments);
> +			BUG_ON(i >= nseg);
>  			shared_data = kmap_atomic(
>  				pfn_to_page(s->grants_used[i]->pfn));
>  			bvec_data = bvec_kmap_irq(bvec, &flags);
> @@ -897,10 +1032,17 @@ static void blkif_completion(struct blk_shadow *s,
struct blkfront_info *info,
>  		}
>  	}
>  	/* Add the persistent grant into the list of free grants */
> -	for (i = 0; i < s->req.u.rw.nr_segments; i++) {
> +	for (i = 0; i < nseg; i++) {
>  		list_add(&s->grants_used[i]->node,
&info->persistent_gnts);
>  		info->persistent_gnts_c++;
>  	}
> +	if (s->req.operation == BLKIF_OP_INDIRECT) {
> +		for (i = 0; i < INDIRECT_GREFS(nseg); i++) {
> +			list_add(&s->indirect_grants[i]->node,
> +			         &info->persistent_gnts);
> +			info->persistent_gnts_c++;
> +		}
> +	}
>  }
>  
>  static irqreturn_t blkif_interrupt(int irq, void *dev_id)
> @@ -1034,8 +1176,6 @@ static int setup_blkring(struct xenbus_device *dev,
>  	SHARED_RING_INIT(sring);
>  	FRONT_RING_INIT(&info->ring, sring, PAGE_SIZE);
>  
> -	sg_init_table(info->sg, BLKIF_MAX_SEGMENTS_PER_REQUEST);
> -
>  	err = xenbus_grant_ring(dev, virt_to_mfn(info->ring.sring));
>  	if (err < 0) {
>  		free_page((unsigned long)sring);
> @@ -1116,12 +1256,6 @@ again:
>  		goto destroy_blkring;
>  	}
>  
> -	/* Allocate memory for grants */
> -	err = fill_grant_buffer(info, BLK_RING_SIZE *
> -	                              BLKIF_MAX_SEGMENTS_PER_REQUEST);
> -	if (err)
> -		goto out;
> -
>  	xenbus_switch_state(dev, XenbusStateInitialised);
>  
>  	return 0;
> @@ -1223,13 +1357,84 @@ static int blkfront_probe(struct xenbus_device
*dev,
>  	return 0;
>  }
>  
> +/*
> + * This is a clone of md_trim_bio, used to split a bio into smaller ones
> + */
> +static void trim_bio(struct bio *bio, int offset, int size)
> +{
> +	/* ''bio'' is a cloned bio which we need to trim to match
> +	 * the given offset and size.
> +	 * This requires adjusting bi_sector, bi_size, and bi_io_vec
> +	 */
> +	int i;
> +	struct bio_vec *bvec;
> +	int sofar = 0;
> +
> +	size <<= 9;
> +	if (offset == 0 && size == bio->bi_size)
> +		return;
> +
> +	bio->bi_sector += offset;
> +	bio->bi_size = size;
> +	offset <<= 9;
> +	clear_bit(BIO_SEG_VALID, &bio->bi_flags);
> +
> +	while (bio->bi_idx < bio->bi_vcnt &&
> +	       bio->bi_io_vec[bio->bi_idx].bv_len <= offset) {
> +		/* remove this whole bio_vec */
> +		offset -= bio->bi_io_vec[bio->bi_idx].bv_len;
> +		bio->bi_idx++;
> +	}
> +	if (bio->bi_idx < bio->bi_vcnt) {
> +		bio->bi_io_vec[bio->bi_idx].bv_offset += offset;
> +		bio->bi_io_vec[bio->bi_idx].bv_len -= offset;
> +	}
> +	/* avoid any complications with bi_idx being non-zero*/
> +	if (bio->bi_idx) {
> +		memmove(bio->bi_io_vec, bio->bi_io_vec+bio->bi_idx,
> +			(bio->bi_vcnt - bio->bi_idx) * sizeof(struct bio_vec));
> +		bio->bi_vcnt -= bio->bi_idx;
> +		bio->bi_idx = 0;
> +	}
> +	/* Make sure vcnt and last bv are not too big */
> +	bio_for_each_segment(bvec, bio, i) {
> +		if (sofar + bvec->bv_len > size)
> +			bvec->bv_len = size - sofar;
> +		if (bvec->bv_len == 0) {
> +			bio->bi_vcnt = i;
> +			break;
> +		}
> +		sofar += bvec->bv_len;
> +	}
> +}
> +
> +static void split_bio_end(struct bio *bio, int error)
> +{
> +	struct split_bio *split_bio = bio->bi_private;
> +
> +	if (error)
> +		split_bio->err = error;
> +
> +	if (atomic_dec_and_test(&split_bio->pending)) {
> +		split_bio->bio->bi_phys_segments = 0;
> +		bio_endio(split_bio->bio, split_bio->err);
> +		kfree(split_bio);
> +	}
> +	bio_put(bio);
> +}
>  
>  static int blkif_recover(struct blkfront_info *info)
>  {
>  	int i;
> -	struct blkif_request *req;
> +	struct request *req, *n;
>  	struct blk_shadow *copy;
> -	int j;
> +	int rc;
> +	struct bio *bio, *cloned_bio;
> +	struct bio_list bio_list, merge_bio;
> +	unsigned int segs;
> +	int pending, offset, size;
> +	struct split_bio *split_bio;
> +	struct list_head requests;
>  
>  	/* Stage 1: Make a safe copy of the shadow state. */
>  	copy = kmalloc(sizeof(info->shadow),
> @@ -1245,36 +1450,64 @@ static int blkif_recover(struct blkfront_info
*info)
>  	info->shadow_free = info->ring.req_prod_pvt;
>  	info->shadow[BLK_RING_SIZE-1].req.u.rw.id = 0x0fffffff;
>  
> -	/* Stage 3: Find pending requests and requeue them. */
> +	rc = blkfront_setup_indirect(info);
> +	if (rc) {
> +		kfree(copy);
> +		return rc;
> +	}
> +
> +	segs = info->max_indirect_segments ? : BLKIF_MAX_SEGMENTS_PER_REQUEST;
> +	blk_queue_max_segments(info->rq, segs);
> +	bio_list_init(&bio_list);
> +	INIT_LIST_HEAD(&requests);
>  	for (i = 0; i < BLK_RING_SIZE; i++) {
>  		/* Not in use? */
>  		if (!copy[i].request)
>  			continue;
>  
> -		/* Grab a request slot and copy shadow state into it. */
> -		req = RING_GET_REQUEST(&info->ring, info->ring.req_prod_pvt);
> -		*req = copy[i].req;
> -
> -		/* We get a new request id, and must reset the shadow state. */
> -		req->u.rw.id = get_id_from_freelist(info);
> -		memcpy(&info->shadow[req->u.rw.id], &copy[i],
sizeof(copy[i]));
> -
> -		if (req->operation != BLKIF_OP_DISCARD) {
> -		/* Rewrite any grant references invalidated by susp/resume. */
> -			for (j = 0; j < req->u.rw.nr_segments; j++)
> -				gnttab_grant_foreign_access_ref(
> -					req->u.rw.seg[j].gref,
> -					info->xbdev->otherend_id,
> -					pfn_to_mfn(copy[i].grants_used[j]->pfn),
> -					0);
> +		/*
> +		 * Get the bios in the request so we can re-queue them.
> +		 */
> +		if (copy[i].request->cmd_flags &
> +		    (REQ_FLUSH | REQ_FUA | REQ_DISCARD | REQ_SECURE)) {
> +			/*
> +			 * Flush operations don''t contain bios, so
> +			 * we need to requeue the whole request
> +			 */
> +			list_add(&copy[i].request->queuelist, &requests);
> +			continue;
>  		}
> -		info->shadow[req->u.rw.id].req = *req;
> -
> -		info->ring.req_prod_pvt++;
> +		merge_bio.head = copy[i].request->bio;
> +		merge_bio.tail = copy[i].request->biotail;
> +		bio_list_merge(&bio_list, &merge_bio);
> +		copy[i].request->bio = NULL;
> +		blk_put_request(copy[i].request);
>  	}
>  
>  	kfree(copy);
>  
> +	/*
> +	 * Empty the queue, this is important because we might have
> +	 * requests in the queue with more segments than what we
> +	 * can handle now.
> +	 */
> +	spin_lock_irq(&info->io_lock);
> +	while ((req = blk_fetch_request(info->rq)) != NULL) {
> +		if (req->cmd_flags &
> +		    (REQ_FLUSH | REQ_FUA | REQ_DISCARD | REQ_SECURE)) {
> +			list_add(&req->queuelist, &requests);
> +			continue;
> +		}
> +		merge_bio.head = req->bio;
> +		merge_bio.tail = req->biotail;
> +		bio_list_merge(&bio_list, &merge_bio);
> +		req->bio = NULL;
> +		if (req->cmd_flags & (REQ_FLUSH | REQ_FUA))
> +			pr_alert("diskcache flush request found!\n");
> +		__blk_put_request(info->rq, req);
> +	}
> +	spin_unlock_irq(&info->io_lock);
> +
>  	xenbus_switch_state(info->xbdev, XenbusStateConnected);
>  
>  	spin_lock_irq(&info->io_lock);
> @@ -1282,14 +1515,50 @@ static int blkif_recover(struct blkfront_info
*info)
>  	/* Now safe for us to use the shared ring */
>  	info->connected = BLKIF_STATE_CONNECTED;
>  
> -	/* Send off requeued requests */
> -	flush_requests(info);
> -
>  	/* Kick any other new requests queued since we resumed */
>  	kick_pending_request_queues(info);
>  
> +	list_for_each_entry_safe(req, n, &requests, queuelist) {
> +		/* Requeue pending requests (flush or discard) */
> +		list_del_init(&req->queuelist);
> +		BUG_ON(req->nr_phys_segments > segs);
> +		blk_requeue_request(info->rq, req);
> +	}
>  	spin_unlock_irq(&info->io_lock);
>  
> +	while ((bio = bio_list_pop(&bio_list)) != NULL) {
> +		/* Traverse the list of pending bios and re-queue them */
> +		if (bio_segments(bio) > segs) {
> +			/*
> +			 * This bio has more segments than what we can
> +			 * handle, we have to split it.
> +			 */
> +			pending = (bio_segments(bio) + segs - 1) / segs;
> +			split_bio = kzalloc(sizeof(*split_bio), GFP_NOIO);
> +			BUG_ON(split_bio == NULL);
> +			atomic_set(&split_bio->pending, pending);
> +			split_bio->bio = bio;
> +			for (i = 0; i < pending; i++) {
> +				offset = (i * segs * PAGE_SIZE) >> 9;
> +				size = MIN((segs * PAGE_SIZE) >> 9,
> +				           (bio->bi_size >> 9) - offset);
> +				cloned_bio = bio_clone(bio, GFP_NOIO);
> +				BUG_ON(cloned_bio == NULL);
> +				trim_bio(cloned_bio, offset, size);
> +				cloned_bio->bi_private = split_bio;
> +				cloned_bio->bi_end_io = split_bio_end;
> +				submit_bio(cloned_bio->bi_rw, cloned_bio);
> +			}
> +			/*
> +			 * Now we have to wait for all those smaller bios to
> +			 * end, so we can also end the "parent" bio.
> +			 */
> +			continue;
> +		}
> +		/* We don''t need to split this bio */
> +		submit_bio(bio->bi_rw, bio);
> +	}
> +
>  	return 0;
>  }
>  
> @@ -1309,8 +1578,12 @@ static int blkfront_resume(struct xenbus_device
*dev)
>  	blkif_free(info, info->connected == BLKIF_STATE_CONNECTED);
>  
>  	err = talk_to_blkback(dev, info);
> -	if (info->connected == BLKIF_STATE_SUSPENDED && !err)
> -		err = blkif_recover(info);
> +
> +	/*
> +	 * We have to wait for the backend to switch to
> +	 * connected state, since we want to read which
> +	 * features it supports.
> +	 */
>  
>  	return err;
>  }
> @@ -1388,6 +1661,62 @@ static void blkfront_setup_discard(struct
blkfront_info *info)
>  	kfree(type);
>  }
>  
> +static int blkfront_setup_indirect(struct blkfront_info *info)
> +{
> +	unsigned int indirect_segments, segs;
> +	int err, i;
> +
> +	err = xenbus_gather(XBT_NIL, info->xbdev->otherend,
> +			    "max-indirect-segments", "%u",
&indirect_segments,
> +			    NULL);
> +	if (err) {
> +		info->max_indirect_segments = 0;
> +		segs = BLKIF_MAX_SEGMENTS_PER_REQUEST;
> +	} else {
> +		info->max_indirect_segments = MIN(indirect_segments,
> +		                                  xen_blkif_max_segments);
> +		segs = info->max_indirect_segments;
> +	}
> +	info->sg = kzalloc(sizeof(info->sg[0]) * segs, GFP_KERNEL);
> +	if (info->sg == NULL)
> +		goto out_of_memory;
> +	sg_init_table(info->sg, segs);
> +
> +	err = fill_grant_buffer(info,
> +	                        (segs + INDIRECT_GREFS(segs)) * BLK_RING_SIZE);
> +	if (err)
> +		goto out_of_memory;
> +
> +	for (i = 0; i < BLK_RING_SIZE; i++) {
> +		info->shadow[i].grants_used = kzalloc(
> +			sizeof(info->shadow[i].grants_used[0]) * segs,
> +			GFP_NOIO);
> +		if (info->max_indirect_segments)
> +			info->shadow[i].indirect_grants = kzalloc(
> +				sizeof(info->shadow[i].indirect_grants[0]) *
> +				INDIRECT_GREFS(segs),
> +				GFP_NOIO);
> +		if ((info->shadow[i].grants_used == NULL) ||
> +		     (info->max_indirect_segments &&
> +		     (info->shadow[i].indirect_grants == NULL)))
> +			goto out_of_memory;
> +	}
> +
> +
> +	return 0;
> +
> +out_of_memory:
> +	kfree(info->sg);
> +	info->sg = NULL;
> +	for (i = 0; i < BLK_RING_SIZE; i++) {
> +		kfree(info->shadow[i].grants_used);
> +		info->shadow[i].grants_used = NULL;
> +		kfree(info->shadow[i].indirect_grants);
> +		info->shadow[i].indirect_grants = NULL;
> +	}
> +	return -ENOMEM;
> +}
> +
>  /*
>   * Invoked when the backend is finally ''ready'' (and has
told produced
>   * the details about the physical device - #sectors, size, etc).
> @@ -1415,8 +1744,9 @@ static void blkfront_connect(struct blkfront_info
*info)
>  		set_capacity(info->gd, sectors);
>  		revalidate_disk(info->gd);
>  
> -		/* fall through */
> +		return;
>  	case BLKIF_STATE_SUSPENDED:
> +		blkif_recover(info);
>  		return;
>  
>  	default:
> @@ -1437,6 +1767,7 @@ static void blkfront_connect(struct blkfront_info
*info)
>  				 info->xbdev->otherend);
>  		return;
>  	}
> +	info->sector_size = sector_size;
>  
>  	info->feature_flush = 0;
>  	info->flush_op = 0;
> @@ -1484,6 +1815,13 @@ static void blkfront_connect(struct blkfront_info
*info)
>  	else
>  		info->feature_persistent = persistent;
>  
> +	err = blkfront_setup_indirect(info);
> +	if (err) {
> +		xenbus_dev_fatal(info->xbdev, err, "setup_indirect at %s",
> +				 info->xbdev->otherend);
> +		return;
> +	}
> +
>  	err = xlvbd_alloc_gendisk(sectors, info, binfo, sector_size);
>  	if (err) {
>  		xenbus_dev_fatal(info->xbdev, err, "xlvbd_add at %s",
> diff --git a/include/xen/interface/io/blkif.h
b/include/xen/interface/io/blkif.h
> index 01c3d62..6d99849 100644
> --- a/include/xen/interface/io/blkif.h
> +++ b/include/xen/interface/io/blkif.h
> @@ -102,6 +102,8 @@ typedef uint64_t blkif_sector_t;
>   */
>  #define BLKIF_OP_DISCARD           5
>  
> +#define BLKIF_OP_INDIRECT          6
> +
>  /*
>   * Maximum scatter/gather segments per request.
>   * This is carefully chosen so that sizeof(struct blkif_ring) <=
PAGE_SIZE.
> @@ -109,6 +111,16 @@ typedef uint64_t blkif_sector_t;
>   */
>  #define BLKIF_MAX_SEGMENTS_PER_REQUEST 11
>  
> +#define BLKIF_MAX_INDIRECT_GREFS_PER_REQUEST 8
> +
> +struct blkif_request_segment_aligned {
> +	grant_ref_t gref;        /* reference to I/O buffer frame        */
> +	/* @first_sect: first sector in frame to transfer (inclusive).   */
> +	/* @last_sect: last sector in frame to transfer (inclusive).     */
> +	uint8_t     first_sect, last_sect;
> +	uint16_t    _pad; /* padding to make it 8 bytes, so it''s
cache-aligned */
> +} __attribute__((__packed__));
> +
>  struct blkif_request_rw {
>  	uint8_t        nr_segments;  /* number of segments                   */
>  	blkif_vdev_t   handle;       /* only for read/write requests         */
> @@ -138,11 +150,24 @@ struct blkif_request_discard {
>  	uint8_t        _pad3;
>  } __attribute__((__packed__));
>  
> +struct blkif_request_indirect {
> +	uint8_t        indirect_op;
> +	uint16_t       nr_segments;
> +#ifdef CONFIG_X86_64
> +	uint32_t       _pad1;        /* offsetof(blkif_...,u.indirect.id) == 8 */
> +#endif
> +	uint64_t       id;
> +	blkif_vdev_t   handle;
> +	blkif_sector_t sector_number;
> +	grant_ref_t    indirect_grefs[BLKIF_MAX_INDIRECT_GREFS_PER_REQUEST];
> +} __attribute__((__packed__));
> +
>  struct blkif_request {
>  	uint8_t        operation;    /* BLKIF_OP_???                         */
>  	union {
>  		struct blkif_request_rw rw;
>  		struct blkif_request_discard discard;
> +		struct blkif_request_indirect indirect;
>  	} u;
>  } __attribute__((__packed__));
>  
> -- 
> 1.7.7.5 (Apple Git-26)
>

On Thu, Feb 28, 2013 at 01:28:48PM +0000, Jan Beulich
wrote:
> >>> On 28.02.13 at 13:00, Roger Pau
Monné<roger.pau@citrix.com> wrote:
> > On 28/02/13 12:19, Jan Beulich wrote:
> >>>>> On 28.02.13 at 11:28, Roger Pau Monne
<roger.pau@citrix.com> wrote:
> >>> @@ -109,6 +111,16 @@ typedef uint64_t blkif_sector_t;
> >>>   */
> >>>  #define BLKIF_MAX_SEGMENTS_PER_REQUEST 11
> >>>  
> >>> +#define BLKIF_MAX_INDIRECT_GREFS_PER_REQUEST 8
> >>> +
> >>> +struct blkif_request_segment_aligned {
> >>> +	grant_ref_t gref;        /* reference to I/O buffer frame   
*/
> >>> +	/* @first_sect: first sector in frame to transfer
(inclusive).   */
> >>> +	/* @last_sect: last sector in frame to transfer (inclusive).
*/
> >>> +	uint8_t     first_sect, last_sect;
> >>> +	uint16_t    _pad; /* padding to make it 8 bytes, so
it''s cache-aligned */
> >>> +} __attribute__((__packed__));
> >> 
> >> What''s the __packed__ for here?
> > 
> > Yes, that''s not needed.
> > 
> >> 
> >>> +
> >>>  struct blkif_request_rw {
> >>>  	uint8_t        nr_segments;  /* number of segments          
*/
> >>>  	blkif_vdev_t   handle;       /* only for read/write requests
*/
> >>> @@ -138,11 +150,24 @@ struct blkif_request_discard {
> >>>  	uint8_t        _pad3;
> >>>  } __attribute__((__packed__));
> >>>  
> >>> +struct blkif_request_indirect {
> >>> +	uint8_t        indirect_op;
> >>> +	uint16_t       nr_segments;
> >>> +#ifdef CONFIG_X86_64
> >>> +	uint32_t       _pad1;        /*
offsetof(blkif_...,u.indirect.id) == 8 */
> >>> +#endif
> >> 
> >> Either you want the structure be packed tightly (and you
don''t care
> >> about misaligned fields), in which case you shouldn''t
need a padding
> >> field. That''s even more so as there''s no padding
between indirect_op
> >> and nr_segments, so everything is misaligned anyway, and the
> >> comment above is wrong too (offsetof() really ought to yield 7 in
> >> that case).
> > 
> > This padding is because we want to have the "id" field at
the same
> > position as blkif_request_rw, so we need to add the padding for it to
> > match 32 & 64 bit blkif_request_rw structures, this prevents
adding some
> > "if (req.op == BLKIF_OP_INDIRECT)..." if we only need to get
the id of
> > the request.
> 
> Oh, right, that''s desirable of course.
> 
> > The comment is indeed wrong, I''ve copied it from
blkif_request_discard
> > and forgot to change the offset
> 
> But the offset stated there then is right after all - I forgot that
> there is a 1-byte field outside the union (the way this is being done
> in the upstream Linux header is really ugly imo, but I guess Jeremy
> and/or Konrad liked it that way). That''s also why the packed
> attribute is needed here.
I am not particularly found as I keep on forgetting about the 1-byte field
as well. If you have a patch to clean it up would love to see it.
> 
> But you will probably want to switch sector_number and handle, so
> that sector_number becomes aligned, and add another 16-bit
> padding field between handle and indirect_grefs[].
> 
> I also wonder whether "indirect_op" wouldn''t better be
named
> "actual_op" or just "op".
<nods> ''op'' sounds good. With a comment saying it can do
all of the BLKIF_OPS_..
except the BLKIF_OP_INDIRECT one. Thought one could in theory chain
it that way for fun.
> 
> Jan

>>> On 04.03.13 at 21:44, Konrad Rzeszutek Wilk
<konrad.wilk@oracle.com> wrote:
> <nods> ''op'' sounds good. With a comment saying it
can do all of the
> BLKIF_OPS_..
> except the BLKIF_OP_INDIRECT one. Thought one could in theory chain
> it that way for fun.
In fact I''d like to exclude chaining as well as BLKIF_OP_DISCARD here.
The former should - if useful for anything - be controlled by a
separate feature flag, and the latter is plain pointless to indirect.
And I reckon the same would apply to BLKIF_OP_FLUSH_DISKCACHE
and BLKIF_OP_RESERVED_1 - i.e. it might be better to state that
indirection is only permitted for normal I/O (read/write) ops.

Jan

On Tue, Mar 05, 2013 at 08:11:19AM +0000, Jan Beulich
wrote:
> >>> On 04.03.13 at 21:44, Konrad Rzeszutek Wilk
<konrad.wilk@oracle.com> wrote:
> > <nods> ''op'' sounds good. With a comment saying
it can do all of the
> > BLKIF_OPS_..
> > except the BLKIF_OP_INDIRECT one. Thought one could in theory chain
> > it that way for fun.
> 
> In fact I''d like to exclude chaining as well as BLKIF_OP_DISCARD
here.
> The former should - if useful for anything - be controlled by a
> separate feature flag, and the latter is plain pointless to indirect.
> And I reckon the same would apply to BLKIF_OP_FLUSH_DISKCACHE
> and BLKIF_OP_RESERVED_1 - i.e. it might be better to state that
> indirection is only permitted for normal I/O (read/write) ops.
<nods> That makes sense. And also of course the new BLKIF_OP should
be documented in the Xen tree as well.
> 
> Jan
>

On 05/03/13 15:16, Konrad Rzeszutek Wilk wrote:
> On Tue, Mar 05, 2013 at 08:11:19AM +0000, Jan Beulich wrote:
>>>>> On 04.03.13 at 21:44, Konrad Rzeszutek Wilk
<konrad.wilk@oracle.com> wrote:
>>> <nods> ''op'' sounds good. With a comment
saying it can do all of the
>>> BLKIF_OPS_..
>>> except the BLKIF_OP_INDIRECT one. Thought one could in theory chain
>>> it that way for fun.
>>
>> In fact I''d like to exclude chaining as well as
BLKIF_OP_DISCARD here.
>> The former should - if useful for anything - be controlled by a
>> separate feature flag, and the latter is plain pointless to indirect.
>> And I reckon the same would apply to BLKIF_OP_FLUSH_DISKCACHE
>> and BLKIF_OP_RESERVED_1 - i.e. it might be better to state that
>> indirection is only permitted for normal I/O (read/write) ops.
> 
> <nods> That makes sense. And also of course the new BLKIF_OP should
> be documented in the Xen tree as well.
The only ops that can be done indirectly are _READ, _WRITE and
_BARRIER/_FLUSH. From the implementation in blkfront it seems like
_FLUSH/_BARRIER requests can indeed contain segments, but I haven''t
been
able to spot any _FLUSH op with segments on it. Can you confirm FLUSH
requests never contain bios?

On 04/03/13 21:41, Konrad Rzeszutek Wilk wrote:
> On Thu, Feb 28, 2013 at 11:28:55AM +0100, Roger Pau Monne wrote:
>> Indirect descriptors introduce a new block operation
>> (BLKIF_OP_INDIRECT) that passes grant references instead of segments
>> in the request. This grant references are filled with arrays of
>> blkif_request_segment_aligned, this way we can send more segments in a
>> request.
>>
>> The proposed implementation sets the maximum number of indirect grefs
>> (frames filled with blkif_request_segment_aligned) to 256 in the
>> backend and 64 in the frontend. The value in the frontend has been
>> chosen experimentally, and the backend value has been set to a sane
>> value that allows expanding the maximum number of indirect descriptors
>> in the frontend if needed.
> 
> So we are still using a similar format of the form:
> 
> <gref, first_sec, last_sect, pad>, etc.
> 
> Why not utilize a layout that fits with the bio sg? That way
> we might not even have to do the bio_alloc call and instead can
> setup an bio (and bio-list) with the appropiate offsets/list?
> 
> Meaning that the format of the indirect descriptors is:
> 
> <gref, offset, next_index, pad>
> 
> We already know what the first_sec and last_sect are - they
> are basically: sector_number +  nr_segments * (whatever the sector size is)
+ offset
This will of course be suitable for Linux, but what about other OSes, I
know they support the traditional first_sec, last_sect (because it''s
already implemented), but I don''t know how much work will it be for
them
to adopt this. If we have to do such a change I will have to check first
that other frontend/backend can handle this easily also, I wouldn''t
like
to simplify this for Linux by making it more difficult to implement in
other OSes...

On Tue, Mar 05, 2013 at 06:00:51PM +0100, Roger Pau Monné
wrote:
> On 05/03/13 15:16, Konrad Rzeszutek Wilk wrote:
> > On Tue, Mar 05, 2013 at 08:11:19AM +0000, Jan Beulich wrote:
> >>>>> On 04.03.13 at 21:44, Konrad Rzeszutek Wilk
<konrad.wilk@oracle.com> wrote:
> >>> <nods> ''op'' sounds good. With a
comment saying it can do all of the
> >>> BLKIF_OPS_..
> >>> except the BLKIF_OP_INDIRECT one. Thought one could in theory
chain
> >>> it that way for fun.
> >>
> >> In fact I''d like to exclude chaining as well as
BLKIF_OP_DISCARD here.
> >> The former should - if useful for anything - be controlled by a
> >> separate feature flag, and the latter is plain pointless to
indirect.
> >> And I reckon the same would apply to BLKIF_OP_FLUSH_DISKCACHE
> >> and BLKIF_OP_RESERVED_1 - i.e. it might be better to state that
> >> indirection is only permitted for normal I/O (read/write) ops.
> > 
> > <nods> That makes sense. And also of course the new BLKIF_OP
should
> > be documented in the Xen tree as well.
> 
> The only ops that can be done indirectly are _READ, _WRITE and
> _BARRIER/_FLUSH. From the implementation in blkfront it seems like
> _FLUSH/_BARRIER requests can indeed contain segments, but I
haven''t been
> able to spot any _FLUSH op with segments on it. Can you confirm FLUSH
> requests never contain bios?
Not FLUSH per say. But the FUA should be able to provide data and the
flush semantics with it. Except we don''t support FUA so this is
irrelevant
 - unless in the future we want to intrduce FUA or WRITE with some extra
flags.

On Tue, Mar 05, 2013 at 06:07:57PM +0100, Roger Pau Monné
wrote:
> On 04/03/13 21:41, Konrad Rzeszutek Wilk wrote:
> > On Thu, Feb 28, 2013 at 11:28:55AM +0100, Roger Pau Monne wrote:
> >> Indirect descriptors introduce a new block operation
> >> (BLKIF_OP_INDIRECT) that passes grant references instead of
segments
> >> in the request. This grant references are filled with arrays of
> >> blkif_request_segment_aligned, this way we can send more segments
in a
> >> request.
> >>
> >> The proposed implementation sets the maximum number of indirect
grefs
> >> (frames filled with blkif_request_segment_aligned) to 256 in the
> >> backend and 64 in the frontend. The value in the frontend has been
> >> chosen experimentally, and the backend value has been set to a
sane
> >> value that allows expanding the maximum number of indirect
descriptors
> >> in the frontend if needed.
> > 
> > So we are still using a similar format of the form:
> > 
> > <gref, first_sec, last_sect, pad>, etc.
> > 
> > Why not utilize a layout that fits with the bio sg? That way
> > we might not even have to do the bio_alloc call and instead can
> > setup an bio (and bio-list) with the appropiate offsets/list?
> > 
> > Meaning that the format of the indirect descriptors is:
> > 
> > <gref, offset, next_index, pad>
> > 
> > We already know what the first_sec and last_sect are - they
> > are basically: sector_number +  nr_segments * (whatever the sector
size is) + offset
> 
> This will of course be suitable for Linux, but what about other OSes, I
> know they support the traditional first_sec, last_sect (because
it''s
> already implemented), but I don''t know how much work will it be
for them
> to adopt this. If we have to do such a change I will have to check first
> that other frontend/backend can handle this easily also, I
wouldn''t like
> to simplify this for Linux by making it more difficult to implement in
> other OSes...
I would think that most OSes use the same framework. The ones that
are of notable interest are the Windows and BSD. Lets CC James here
>

On 05/03/13 22:46, Konrad Rzeszutek Wilk wrote:
> On Tue, Mar 05, 2013 at 06:07:57PM +0100, Roger Pau Monné wrote:
>> On 04/03/13 21:41, Konrad Rzeszutek Wilk wrote:
>>> On Thu, Feb 28, 2013 at 11:28:55AM +0100, Roger Pau Monne wrote:
>>>> Indirect descriptors introduce a new block operation
>>>> (BLKIF_OP_INDIRECT) that passes grant references instead of
segments
>>>> in the request. This grant references are filled with arrays of
>>>> blkif_request_segment_aligned, this way we can send more
segments in a
>>>> request.
>>>>
>>>> The proposed implementation sets the maximum number of indirect
grefs
>>>> (frames filled with blkif_request_segment_aligned) to 256 in
the
>>>> backend and 64 in the frontend. The value in the frontend has
been
>>>> chosen experimentally, and the backend value has been set to a
sane
>>>> value that allows expanding the maximum number of indirect
descriptors
>>>> in the frontend if needed.
>>>
>>> So we are still using a similar format of the form:
>>>
>>> <gref, first_sec, last_sect, pad>, etc.
>>>
>>> Why not utilize a layout that fits with the bio sg? That way
>>> we might not even have to do the bio_alloc call and instead can
>>> setup an bio (and bio-list) with the appropiate offsets/list?
I think we can already do this without changing the structure of the
segments, we could just allocate a bio big enough to hold all the
segments and queue them up (provided that the underlying storage device
supports bios of this size).

bio = bio_alloc(GFP_KERNEL, nseg);
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;

for (i = 0; i < nseg; i++) {
	rc = bio_add_page(bio, pages[i], seg[i].nsec << 9,
		seg[i].buf & ~PAGE_MASK);
	if (rc == 0)
		goto fail_put_bio;
}

This seems to work with Linux blkfront/blkback, and I guess biolist in
blkback only has one bio all the time.
>>> Meaning that the format of the indirect descriptors is:
>>>
>>> <gref, offset, next_index, pad>
Don''t we need a length parameter? Also, next_index will be current+1,
because we already send the segments sorted (using for_each_sg) in blkfront.
>>>
>>> We already know what the first_sec and last_sect are - they
>>> are basically: sector_number +  nr_segments * (whatever the sector
size is) + offset
>>
>> This will of course be suitable for Linux, but what about other OSes, I
>> know they support the traditional first_sec, last_sect (because
it''s
>> already implemented), but I don''t know how much work will it
be for them
>> to adopt this. If we have to do such a change I will have to check
first
>> that other frontend/backend can handle this easily also, I
wouldn''t like
>> to simplify this for Linux by making it more difficult to implement in
>> other OSes...
> 
> I would think that most OSes use the same framework. The ones that
> are of notable interest are the Windows and BSD. Lets CC James here
Maybe I''m missing something here, but I don''t see a really big
benefit
of using this new structure for segments instead of the current one.

On 28/02/13 11:28, Roger Pau Monne wrote:
> Indirect descriptors introduce a new block operation
> (BLKIF_OP_INDIRECT) that passes grant references instead of segments
> in the request. This grant references are filled with arrays of
> blkif_request_segment_aligned, this way we can send more segments in a
> request.
> 
> The proposed implementation sets the maximum number of indirect grefs
> (frames filled with blkif_request_segment_aligned) to 256 in the
> backend and 64 in the frontend. The value in the frontend has been
> chosen experimentally, and the backend value has been set to a sane
> value that allows expanding the maximum number of indirect descriptors
> in the frontend if needed.
I''ve added some additional debugging messages in blkfront, and found
out
that the queue in blkfront is not providing request bigger than 64
segments for read requests, or 128 segments for write requests, although
I set:

blk_queue_max_segments(info->rq, 256);

Is there any other limit I''m missing on the number of segments per
request a queue can provide?

On Mon, Mar 18, 2013 at 06:06:38PM +0100, Roger Pau Monné
wrote:
> On 28/02/13 11:28, Roger Pau Monne wrote:
> > Indirect descriptors introduce a new block operation
> > (BLKIF_OP_INDIRECT) that passes grant references instead of segments
> > in the request. This grant references are filled with arrays of
> > blkif_request_segment_aligned, this way we can send more segments in a
> > request.
> > 
> > The proposed implementation sets the maximum number of indirect grefs
> > (frames filled with blkif_request_segment_aligned) to 256 in the
> > backend and 64 in the frontend. The value in the frontend has been
> > chosen experimentally, and the backend value has been set to a sane
> > value that allows expanding the maximum number of indirect descriptors
> > in the frontend if needed.
> 
> I''ve added some additional debugging messages in blkfront, and
found out
> that the queue in blkfront is not providing request bigger than 64
> segments for read requests, or 128 segments for write requests, although
> I set:
> 
> blk_queue_max_segments(info->rq, 256);
> 
> Is there any other limit I''m missing on the number of segments per
> request a queue can provide?
Martin, any ideas?
>

On Fri, Mar 08, 2013 at 06:07:08PM +0100, Roger Pau Monné
wrote:
> On 05/03/13 22:46, Konrad Rzeszutek Wilk wrote:
> > On Tue, Mar 05, 2013 at 06:07:57PM +0100, Roger Pau Monné wrote:
> >> On 04/03/13 21:41, Konrad Rzeszutek Wilk wrote:
> >>> On Thu, Feb 28, 2013 at 11:28:55AM +0100, Roger Pau Monne
wrote:
> >>>> Indirect descriptors introduce a new block operation
> >>>> (BLKIF_OP_INDIRECT) that passes grant references instead
of segments
> >>>> in the request. This grant references are filled with
arrays of
> >>>> blkif_request_segment_aligned, this way we can send more
segments in a
> >>>> request.
> >>>>
> >>>> The proposed implementation sets the maximum number of
indirect grefs
> >>>> (frames filled with blkif_request_segment_aligned) to 256
in the
> >>>> backend and 64 in the frontend. The value in the frontend
has been
> >>>> chosen experimentally, and the backend value has been set
to a sane
> >>>> value that allows expanding the maximum number of indirect
descriptors
> >>>> in the frontend if needed.
> >>>
> >>> So we are still using a similar format of the form:
> >>>
> >>> <gref, first_sec, last_sect, pad>, etc.
> >>>
> >>> Why not utilize a layout that fits with the bio sg? That way
> >>> we might not even have to do the bio_alloc call and instead
can
> >>> setup an bio (and bio-list) with the appropiate offsets/list?
> 
> I think we can already do this without changing the structure of the
> segments, we could just allocate a bio big enough to hold all the
> segments and queue them up (provided that the underlying storage device
> supports bios of this size).
> 
> bio = bio_alloc(GFP_KERNEL, nseg);
> 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;
> 
> for (i = 0; i < nseg; i++) {
> 	rc = bio_add_page(bio, pages[i], seg[i].nsec << 9,
> 		seg[i].buf & ~PAGE_MASK);
> 	if (rc == 0)
> 		goto fail_put_bio;
> }
> 
> This seems to work with Linux blkfront/blkback, and I guess biolist in
> blkback only has one bio all the time.
> 
> >>> Meaning that the format of the indirect descriptors is:
> >>>
> >>> <gref, offset, next_index, pad>
> 
> Don''t we need a length parameter? Also, next_index will be
current+1,
> because we already send the segments sorted (using for_each_sg) in
blkfront.
> 
> >>>
> >>> We already know what the first_sec and last_sect are - they
> >>> are basically: sector_number +  nr_segments * (whatever the
sector size is) + offset
> >>
> >> This will of course be suitable for Linux, but what about other
OSes, I
> >> know they support the traditional first_sec, last_sect (because
it''s
> >> already implemented), but I don''t know how much work will
it be for them
> >> to adopt this. If we have to do such a change I will have to check
first
> >> that other frontend/backend can handle this easily also, I
wouldn''t like
> >> to simplify this for Linux by making it more difficult to
implement in
> >> other OSes...
> > 
> > I would think that most OSes use the same framework. The ones that
> > are of notable interest are the Windows and BSD. Lets CC James here
> 
> Maybe I''m missing something here, but I don''t see a
really big benefit
> of using this new structure for segments instead of the current one.
The DIF/DIX requires that the bio layout going in blkfront and then
emerging on the other side in the SAS/SCSI/SATA drivers must be the same.

That means when you have a bio-vec, for example, where there are
five pages linked - the first four have 512 bytes of data (say in the middle
of the page - so 2048 -> 2560 are occupied, the rest is not). The total
is 2048 bytes, and the last page contains 32 bytes (four CRC checksums, each
8 bytes).

If we coalesce any of the five pages in one, then we need to (when we
take the request out of the ring) in the backend, to reconstruct these
five pages. 

My thought was that with the fsect, lsect as they exist now, we will be 
tempted to just colesce four sectors in a page and just make lsect = fsect + 4.

That however is _not_ what we are doing now - I think. We look to recreate
the layout exactly as the READ/WRITE requests are set to xen-blkfront.