Date:	Tue, 09 Jun 2015 16:50:08 +0800
From:	Bob Liu <bob.liu@...cle.com>
To:	Bob Liu <bob.liu@...cle.com>
CC:	xen-devel@...ts.xen.org, david.vrabel@...rix.com,
	justing@...ctralogic.com, konrad.wilk@...cle.com,
	roger.pau@...rix.com, paul.durrant@...rix.com,
	julien.grall@...rix.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] xen/block: add multi-page ring support


On 06/03/2015 01:40 PM, Bob Liu wrote:
> Extend xen/block to support multi-page ring, so that more requests can be
> issued by using more than one pages as the request ring between blkfront
> and backend.
> As a result, the performance can get improved significantly.
> 
> We got some impressive improvements on our highend iscsi storage cluster
> backend. If using 64 pages as the ring, the IOPS increased about 15 times
> for the throughput testing and above doubled for the latency testing.
> 
> The reason was the limit on outstanding requests is 32 if use only one-page
> ring, but in our case the iscsi lun was spread across about 100 physical
> drives, 32 was really not enough to keep them busy.
> 
> Changes in v2:
>  - Rebased to 4.0-rc6.
>  - Document on how multi-page ring feature working to linux io/blkif.h.
> 
> Changes in v3:
>  - Remove changes to linux io/blkif.h and follow the protocol defined
>    in io/blkif.h of XEN tree.
>  - Rebased to 4.1-rc3
> 
> Changes in v4:
>  - Turn to use 'ring-page-order' and 'max-ring-page-order'.
>  - A few comments from Roger.
> 
> Changes in v5:
>  - Clarify with 4k granularity to comment
>  - Address more comments from Roger
> 
> Signed-off-by: Bob Liu <bob.liu@...cle.com>

Also tested the windows PV driver which also works fine when multi-page ring feature
was enabled in Linux backend.
http://www.xenproject.org/downloads/windows-pv-drivers.html

Regards,
-Bob

> ---
>  drivers/block/xen-blkback/blkback.c |   13 ++++
>  drivers/block/xen-blkback/common.h  |    2 +
>  drivers/block/xen-blkback/xenbus.c  |   89 +++++++++++++++++------
>  drivers/block/xen-blkfront.c        |  135 +++++++++++++++++++++++++----------
>  4 files changed, 180 insertions(+), 59 deletions(-)
> 
> diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
> index 713fc9f..2126842 100644
> --- a/drivers/block/xen-blkback/blkback.c
> +++ b/drivers/block/xen-blkback/blkback.c
> @@ -84,6 +84,13 @@ MODULE_PARM_DESC(max_persistent_grants,
>                   "Maximum number of grants to map persistently");
>  
>  /*
> + * Maximum order of pages to be used for the shared ring between front and
> + * backend, 4KB page granularity is used.
> + */
> +unsigned int xen_blkif_max_ring_order = XENBUS_MAX_RING_PAGE_ORDER;
> +module_param_named(max_ring_page_order, xen_blkif_max_ring_order, int, S_IRUGO);
> +MODULE_PARM_DESC(max_ring_page_order, "Maximum order of pages to be used for the shared ring");
> +/*
>   * The LRU mechanism to clean the lists of persistent grants needs to
>   * be executed periodically. The time interval between consecutive executions
>   * of the purge mechanism is set in ms.
> @@ -1438,6 +1445,12 @@ static int __init xen_blkif_init(void)
>  	if (!xen_domain())
>  		return -ENODEV;
>  
> +	if (xen_blkif_max_ring_order > XENBUS_MAX_RING_PAGE_ORDER) {
> +		pr_info("Invalid max_ring_order (%d), will use default max: %d.\n",
> +			xen_blkif_max_ring_order, XENBUS_MAX_RING_PAGE_ORDER);
> +		xen_blkif_max_ring_order = XENBUS_MAX_RING_PAGE_ORDER;
> +	}
> +
>  	rc = xen_blkif_interface_init();
>  	if (rc)
>  		goto failed_init;
> diff --git a/drivers/block/xen-blkback/common.h b/drivers/block/xen-blkback/common.h
> index 043f13b..8ccc49d 100644
> --- a/drivers/block/xen-blkback/common.h
> +++ b/drivers/block/xen-blkback/common.h
> @@ -44,6 +44,7 @@
>  #include <xen/interface/io/blkif.h>
>  #include <xen/interface/io/protocols.h>
>  
> +extern unsigned int xen_blkif_max_ring_order;
>  /*
>   * This is the maximum number of segments that would be allowed in indirect
>   * requests. This value will also be passed to the frontend.
> @@ -320,6 +321,7 @@ struct xen_blkif {
>  	struct work_struct	free_work;
>  	/* Thread shutdown wait queue. */
>  	wait_queue_head_t	shutdown_wq;
> +	unsigned int nr_ring_pages;
>  };
>  
>  struct seg_buf {
> diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
> index c212d41..deb3f00 100644
> --- a/drivers/block/xen-blkback/xenbus.c
> +++ b/drivers/block/xen-blkback/xenbus.c
> @@ -25,6 +25,7 @@
>  
>  /* Enlarge the array size in order to fully show blkback name. */
>  #define BLKBACK_NAME_LEN (20)
> +#define RINGREF_NAME_LEN (20)
>  
>  struct backend_info {
>  	struct xenbus_device	*dev;
> @@ -156,8 +157,8 @@ static struct xen_blkif *xen_blkif_alloc(domid_t domid)
>  	return blkif;
>  }
>  
> -static int xen_blkif_map(struct xen_blkif *blkif, grant_ref_t gref,
> -			 unsigned int evtchn)
> +static int xen_blkif_map(struct xen_blkif *blkif, grant_ref_t *gref,
> +			 unsigned int nr_grefs, unsigned int evtchn)
>  {
>  	int err;
>  
> @@ -165,7 +166,7 @@ static int xen_blkif_map(struct xen_blkif *blkif, grant_ref_t gref,
>  	if (blkif->irq)
>  		return 0;
>  
> -	err = xenbus_map_ring_valloc(blkif->be->dev, &gref, 1,
> +	err = xenbus_map_ring_valloc(blkif->be->dev, gref, nr_grefs,
>  				     &blkif->blk_ring);
>  	if (err < 0)
>  		return err;
> @@ -175,21 +176,21 @@ static int xen_blkif_map(struct xen_blkif *blkif, grant_ref_t gref,
>  	{
>  		struct blkif_sring *sring;
>  		sring = (struct blkif_sring *)blkif->blk_ring;
> -		BACK_RING_INIT(&blkif->blk_rings.native, sring, PAGE_SIZE);
> +		BACK_RING_INIT(&blkif->blk_rings.native, sring, PAGE_SIZE * nr_grefs);
>  		break;
>  	}
>  	case BLKIF_PROTOCOL_X86_32:
>  	{
>  		struct blkif_x86_32_sring *sring_x86_32;
>  		sring_x86_32 = (struct blkif_x86_32_sring *)blkif->blk_ring;
> -		BACK_RING_INIT(&blkif->blk_rings.x86_32, sring_x86_32, PAGE_SIZE);
> +		BACK_RING_INIT(&blkif->blk_rings.x86_32, sring_x86_32, PAGE_SIZE * nr_grefs);
>  		break;
>  	}
>  	case BLKIF_PROTOCOL_X86_64:
>  	{
>  		struct blkif_x86_64_sring *sring_x86_64;
>  		sring_x86_64 = (struct blkif_x86_64_sring *)blkif->blk_ring;
> -		BACK_RING_INIT(&blkif->blk_rings.x86_64, sring_x86_64, PAGE_SIZE);
> +		BACK_RING_INIT(&blkif->blk_rings.x86_64, sring_x86_64, PAGE_SIZE * nr_grefs);
>  		break;
>  	}
>  	default:
> @@ -270,7 +271,7 @@ static void xen_blkif_free(struct xen_blkif *blkif)
>  		i++;
>  	}
>  
> -	WARN_ON(i != XEN_BLKIF_REQS_PER_PAGE);
> +	WARN_ON(i != (XEN_BLKIF_REQS_PER_PAGE * blkif->nr_ring_pages));
>  
>  	kmem_cache_free(xen_blkif_cachep, blkif);
>  }
> @@ -555,6 +556,11 @@ static int xen_blkbk_probe(struct xenbus_device *dev,
>  	if (err)
>  		goto fail;
>  
> +	err = xenbus_printf(XBT_NIL, dev->nodename, "max-ring-page-order", "%u",
> +			    xen_blkif_max_ring_order);
> +	if (err)
> +		pr_warn("%s write out 'max-ring-page-order' failed\n", __func__);
> +
>  	err = xenbus_switch_state(dev, XenbusStateInitWait);
>  	if (err)
>  		goto fail;
> @@ -818,8 +824,8 @@ again:
>  static int connect_ring(struct backend_info *be)
>  {
>  	struct xenbus_device *dev = be->dev;
> -	unsigned long ring_ref;
> -	unsigned int evtchn;
> +	unsigned int ring_ref[XENBUS_MAX_RING_PAGES];
> +	unsigned int evtchn, nr_grefs, ring_page_order;
>  	unsigned int pers_grants;
>  	char protocol[64] = "";
>  	struct pending_req *req, *n;
> @@ -827,14 +833,57 @@ static int connect_ring(struct backend_info *be)
>  
>  	pr_debug("%s %s\n", __func__, dev->otherend);
>  
> -	err = xenbus_gather(XBT_NIL, dev->otherend, "ring-ref", "%lu",
> -			    &ring_ref, "event-channel", "%u", &evtchn, NULL);
> -	if (err) {
> -		xenbus_dev_fatal(dev, err,
> -				 "reading %s/ring-ref and event-channel",
> +	err = xenbus_scanf(XBT_NIL, dev->otherend, "event-channel", "%u",
> +			  &evtchn);
> +	if (err != 1) {
> +		err = -EINVAL;
> +		xenbus_dev_fatal(dev, err, "reading %s/event-channel",
>  				 dev->otherend);
>  		return err;
>  	}
> +	pr_info("event-channel %u\n", evtchn);
> +
> +	err = xenbus_scanf(XBT_NIL, dev->otherend, "ring-page-order", "%u",
> +			  &ring_page_order);
> +	if (err != 1) {
> +		err = xenbus_scanf(XBT_NIL, dev->otherend, "ring-ref",
> +				  "%u", &ring_ref[0]);
> +		if (err != 1) {
> +			err = -EINVAL;
> +			xenbus_dev_fatal(dev, err, "reading %s/ring-ref",
> +					 dev->otherend);
> +			return err;
> +		}
> +		nr_grefs = 1;
> +		pr_info("%s:using single page: ring-ref %d\n", dev->otherend,
> +			ring_ref[0]);
> +	} else {
> +		unsigned int i;
> +
> +		if (ring_page_order > xen_blkif_max_ring_order) {
> +			err = -EINVAL;
> +			xenbus_dev_fatal(dev, err, "%s/request %d ring page order exceed max:%d",
> +					 dev->otherend, ring_page_order,
> +					 xen_blkif_max_ring_order);
> +			return err;
> +		}
> +
> +		nr_grefs = 1 << ring_page_order;
> +		for (i = 0; i < nr_grefs; i++) {
> +			char ring_ref_name[RINGREF_NAME_LEN];
> +
> +			snprintf(ring_ref_name, RINGREF_NAME_LEN, "ring-ref%u", i);
> +			err = xenbus_scanf(XBT_NIL, dev->otherend, ring_ref_name,
> +					   "%u", &ring_ref[i]);
> +			if (err != 1) {
> +				err = -EINVAL;
> +				xenbus_dev_fatal(dev, err, "reading %s/%s",
> +						 dev->otherend, ring_ref_name);
> +				return err;
> +			}
> +			pr_info("ring-ref%u: %u\n", i, ring_ref[i]);
> +		}
> +	}
>  
>  	be->blkif->blk_protocol = BLKIF_PROTOCOL_DEFAULT;
>  	err = xenbus_gather(XBT_NIL, dev->otherend, "protocol",
> @@ -859,12 +908,13 @@ static int connect_ring(struct backend_info *be)
>  
>  	be->blkif->vbd.feature_gnt_persistent = pers_grants;
>  	be->blkif->vbd.overflow_max_grants = 0;
> +	be->blkif->nr_ring_pages = nr_grefs;
>  
> -	pr_info("ring-ref %ld, event-channel %d, protocol %d (%s) %s\n",
> -		ring_ref, evtchn, be->blkif->blk_protocol, protocol,
> +	pr_info("ring-pages:%d, event-channel %d, protocol %d (%s) %s\n",
> +		nr_grefs, evtchn, be->blkif->blk_protocol, protocol,
>  		pers_grants ? "persistent grants" : "");
>  
> -	for (i = 0; i < XEN_BLKIF_REQS_PER_PAGE; i++) {
> +	for (i = 0; i < nr_grefs * XEN_BLKIF_REQS_PER_PAGE; i++) {
>  		req = kzalloc(sizeof(*req), GFP_KERNEL);
>  		if (!req)
>  			goto fail;
> @@ -883,10 +933,9 @@ static int connect_ring(struct backend_info *be)
>  	}
>  
>  	/* Map the shared frame, irq etc. */
> -	err = xen_blkif_map(be->blkif, ring_ref, evtchn);
> +	err = xen_blkif_map(be->blkif, ring_ref, nr_grefs, evtchn);
>  	if (err) {
> -		xenbus_dev_fatal(dev, err, "mapping ring-ref %lu port %u",
> -				 ring_ref, evtchn);
> +		xenbus_dev_fatal(dev, err, "mapping ring-ref port %u", evtchn);
>  		return err;
>  	}
>  
> diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
> index 88e23fd..d3c1a95 100644
> --- a/drivers/block/xen-blkfront.c
> +++ b/drivers/block/xen-blkfront.c
> @@ -98,7 +98,21 @@ static unsigned int xen_blkif_max_segments = 32;
>  module_param_named(max, xen_blkif_max_segments, int, S_IRUGO);
>  MODULE_PARM_DESC(max, "Maximum amount of segments in indirect requests (default is 32)");
>  
> -#define BLK_RING_SIZE __CONST_RING_SIZE(blkif, PAGE_SIZE)
> +/*
> + * Maximum order of pages to be used for the shared ring between front and
> + * backend, 4KB page granularity is used.
> + */
> +static unsigned int xen_blkif_max_ring_order;
> +module_param_named(max_ring_page_order, xen_blkif_max_ring_order, int, S_IRUGO);
> +MODULE_PARM_DESC(max_ring_page_order, "Maximum order of pages to be used for the shared ring");
> +
> +#define BLK_RING_SIZE(info) __CONST_RING_SIZE(blkif, PAGE_SIZE * (info)->nr_ring_pages)
> +#define BLK_MAX_RING_SIZE __CONST_RING_SIZE(blkif, PAGE_SIZE * XENBUS_MAX_RING_PAGES)
> +/*
> + * ring-ref%i i=(-1UL) would take 11 characters + 'ring-ref' is 8, so 19
> + * characters are enough. Define to 20 to keep consist with backend.
> + */
> +#define RINGREF_NAME_LEN (20)
>  
>  /*
>   * We have one of these per vbd, whether ide, scsi or 'other'.  They
> @@ -114,13 +128,14 @@ struct blkfront_info
>  	int vdevice;
>  	blkif_vdev_t handle;
>  	enum blkif_state connected;
> -	int ring_ref;
> +	int ring_ref[XENBUS_MAX_RING_PAGES];
> +	unsigned int nr_ring_pages;
>  	struct blkif_front_ring ring;
>  	unsigned int evtchn, irq;
>  	struct request_queue *rq;
>  	struct work_struct work;
>  	struct gnttab_free_callback callback;
> -	struct blk_shadow shadow[BLK_RING_SIZE];
> +	struct blk_shadow shadow[BLK_MAX_RING_SIZE];
>  	struct list_head grants;
>  	struct list_head indirect_pages;
>  	unsigned int persistent_gnts_c;
> @@ -139,8 +154,6 @@ static unsigned int nr_minors;
>  static unsigned long *minors;
>  static DEFINE_SPINLOCK(minor_lock);
>  
> -#define MAXIMUM_OUTSTANDING_BLOCK_REQS \
> -	(BLKIF_MAX_SEGMENTS_PER_REQUEST * BLK_RING_SIZE)
>  #define GRANT_INVALID_REF	0
>  
>  #define PARTS_PER_DISK		16
> @@ -170,7 +183,7 @@ 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;
> -	BUG_ON(free >= BLK_RING_SIZE);
> +	BUG_ON(free >= BLK_RING_SIZE(info));
>  	info->shadow_free = info->shadow[free].req.u.rw.id;
>  	info->shadow[free].req.u.rw.id = 0x0fffffee; /* debug */
>  	return free;
> @@ -983,7 +996,7 @@ static void blkif_free(struct blkfront_info *info, int suspend)
>  		}
>  	}
>  
> -	for (i = 0; i < BLK_RING_SIZE; i++) {
> +	for (i = 0; i < BLK_RING_SIZE(info); i++) {
>  		/*
>  		 * Clear persistent grants present in requests already
>  		 * on the shared ring
> @@ -1033,12 +1046,15 @@ free_shadow:
>  	flush_work(&info->work);
>  
>  	/* Free resources associated with old device channel. */
> -	if (info->ring_ref != GRANT_INVALID_REF) {
> -		gnttab_end_foreign_access(info->ring_ref, 0,
> -					  (unsigned long)info->ring.sring);
> -		info->ring_ref = GRANT_INVALID_REF;
> -		info->ring.sring = NULL;
> +	for (i = 0; i < info->nr_ring_pages; i++) {
> +		if (info->ring_ref[i] != GRANT_INVALID_REF) {
> +			gnttab_end_foreign_access(info->ring_ref[i], 0, 0);
> +			info->ring_ref[i] = GRANT_INVALID_REF;
> +		}
>  	}
> +	free_pages((unsigned long)info->ring.sring, get_order(info->nr_ring_pages * PAGE_SIZE));
> +	info->ring.sring = NULL;
> +
>  	if (info->irq)
>  		unbind_from_irqhandler(info->irq, info);
>  	info->evtchn = info->irq = 0;
> @@ -1157,7 +1173,7 @@ static irqreturn_t blkif_interrupt(int irq, void *dev_id)
>  		 * never have given to it (we stamp it up to BLK_RING_SIZE -
>  		 * look in get_id_from_freelist.
>  		 */
> -		if (id >= BLK_RING_SIZE) {
> +		if (id >= BLK_RING_SIZE(info)) {
>  			WARN(1, "%s: response to %s has incorrect id (%ld)\n",
>  			     info->gd->disk_name, op_name(bret->operation), id);
>  			/* We can't safely get the 'struct request' as
> @@ -1245,26 +1261,30 @@ static int setup_blkring(struct xenbus_device *dev,
>  			 struct blkfront_info *info)
>  {
>  	struct blkif_sring *sring;
> -	grant_ref_t gref;
> -	int err;
> +	int err, i;
> +	unsigned long ring_size = info->nr_ring_pages * PAGE_SIZE;
> +	grant_ref_t gref[XENBUS_MAX_RING_PAGES];
>  
> -	info->ring_ref = GRANT_INVALID_REF;
> +	for (i = 0; i < info->nr_ring_pages; i++)
> +		info->ring_ref[i] = GRANT_INVALID_REF;
>  
> -	sring = (struct blkif_sring *)__get_free_page(GFP_NOIO | __GFP_HIGH);
> +	sring = (struct blkif_sring *)__get_free_pages(GFP_NOIO | __GFP_HIGH,
> +						       get_order(ring_size));
>  	if (!sring) {
>  		xenbus_dev_fatal(dev, -ENOMEM, "allocating shared ring");
>  		return -ENOMEM;
>  	}
>  	SHARED_RING_INIT(sring);
> -	FRONT_RING_INIT(&info->ring, sring, PAGE_SIZE);
> +	FRONT_RING_INIT(&info->ring, sring, ring_size);
>  
> -	err = xenbus_grant_ring(dev, info->ring.sring, 1, &gref);
> +	err = xenbus_grant_ring(dev, info->ring.sring, info->nr_ring_pages, gref);
>  	if (err < 0) {
> -		free_page((unsigned long)sring);
> +		free_pages((unsigned long)sring, get_order(ring_size));
>  		info->ring.sring = NULL;
>  		goto fail;
>  	}
> -	info->ring_ref = gref;
> +	for (i = 0; i < info->nr_ring_pages; i++)
> +		info->ring_ref[i] = gref[i];
>  
>  	err = xenbus_alloc_evtchn(dev, &info->evtchn);
>  	if (err)
> @@ -1292,7 +1312,18 @@ static int talk_to_blkback(struct xenbus_device *dev,
>  {
>  	const char *message = NULL;
>  	struct xenbus_transaction xbt;
> -	int err;
> +	int err, i;
> +	unsigned int max_page_order = 0;
> +	unsigned int ring_page_order = 0;
> +
> +	err = xenbus_scanf(XBT_NIL, info->xbdev->otherend,
> +			   "max-ring-page-order", "%u", &max_page_order);
> +	if (err != 1)
> +		info->nr_ring_pages = 1;
> +	else {
> +		ring_page_order = min(xen_blkif_max_ring_order, max_page_order);
> +		info->nr_ring_pages = 1 << ring_page_order;
> +	}
>  
>  	/* Create shared ring, alloc event channel. */
>  	err = setup_blkring(dev, info);
> @@ -1306,11 +1337,32 @@ again:
>  		goto destroy_blkring;
>  	}
>  
> -	err = xenbus_printf(xbt, dev->nodename,
> -			    "ring-ref", "%u", info->ring_ref);
> -	if (err) {
> -		message = "writing ring-ref";
> -		goto abort_transaction;
> +	if (info->nr_ring_pages == 1) {
> +		err = xenbus_printf(xbt, dev->nodename,
> +				    "ring-ref", "%u", info->ring_ref[0]);
> +		if (err) {
> +			message = "writing ring-ref";
> +			goto abort_transaction;
> +		}
> +	} else {
> +		err = xenbus_printf(xbt, dev->nodename,
> +				    "ring-page-order", "%u", ring_page_order);
> +		if (err) {
> +			message = "writing ring-page-order";
> +			goto abort_transaction;
> +		}
> +
> +		for (i = 0; i < info->nr_ring_pages; i++) {
> +			char ring_ref_name[RINGREF_NAME_LEN];
> +
> +			snprintf(ring_ref_name, RINGREF_NAME_LEN, "ring-ref%u", i);
> +			err = xenbus_printf(xbt, dev->nodename, ring_ref_name,
> +					    "%u", info->ring_ref[i]);
> +			if (err) {
> +				message = "writing ring-ref";
> +				goto abort_transaction;
> +			}
> +		}
>  	}
>  	err = xenbus_printf(xbt, dev->nodename,
>  			    "event-channel", "%u", info->evtchn);
> @@ -1338,6 +1390,9 @@ again:
>  		goto destroy_blkring;
>  	}
>  
> +	for (i = 0; i < BLK_RING_SIZE(info); i++)
> +		info->shadow[i].req.u.rw.id = i+1;
> +	info->shadow[BLK_RING_SIZE(info)-1].req.u.rw.id = 0x0fffffff;
>  	xenbus_switch_state(dev, XenbusStateInitialised);
>  
>  	return 0;
> @@ -1361,7 +1416,7 @@ again:
>  static int blkfront_probe(struct xenbus_device *dev,
>  			  const struct xenbus_device_id *id)
>  {
> -	int err, vdevice, i;
> +	int err, vdevice;
>  	struct blkfront_info *info;
>  
>  	/* FIXME: Use dynamic device id if this is not set. */
> @@ -1422,10 +1477,6 @@ static int blkfront_probe(struct xenbus_device *dev,
>  	info->connected = BLKIF_STATE_DISCONNECTED;
>  	INIT_WORK(&info->work, blkif_restart_queue);
>  
> -	for (i = 0; i < BLK_RING_SIZE; i++)
> -		info->shadow[i].req.u.rw.id = i+1;
> -	info->shadow[BLK_RING_SIZE-1].req.u.rw.id = 0x0fffffff;
> -
>  	/* Front end dir is a number, which is used as the id. */
>  	info->handle = simple_strtoul(strrchr(dev->nodename, '/')+1, NULL, 0);
>  	dev_set_drvdata(&dev->dev, info);
> @@ -1469,10 +1520,10 @@ static int blkif_recover(struct blkfront_info *info)
>  
>  	/* Stage 2: Set up free list. */
>  	memset(&info->shadow, 0, sizeof(info->shadow));
> -	for (i = 0; i < BLK_RING_SIZE; i++)
> +	for (i = 0; i < BLK_RING_SIZE(info); i++)
>  		info->shadow[i].req.u.rw.id = i+1;
>  	info->shadow_free = info->ring.req_prod_pvt;
> -	info->shadow[BLK_RING_SIZE-1].req.u.rw.id = 0x0fffffff;
> +	info->shadow[BLK_RING_SIZE(info)-1].req.u.rw.id = 0x0fffffff;
>  
>  	rc = blkfront_setup_indirect(info);
>  	if (rc) {
> @@ -1484,7 +1535,7 @@ static int blkif_recover(struct blkfront_info *info)
>  	blk_queue_max_segments(info->rq, segs);
>  	bio_list_init(&bio_list);
>  	INIT_LIST_HEAD(&requests);
> -	for (i = 0; i < BLK_RING_SIZE; i++) {
> +	for (i = 0; i < BLK_RING_SIZE(info); i++) {
>  		/* Not in use? */
>  		if (!copy[i].request)
>  			continue;
> @@ -1690,7 +1741,7 @@ static int blkfront_setup_indirect(struct blkfront_info *info)
>  		segs = info->max_indirect_segments;
>  	}
>  
> -	err = fill_grant_buffer(info, (segs + INDIRECT_GREFS(segs)) * BLK_RING_SIZE);
> +	err = fill_grant_buffer(info, (segs + INDIRECT_GREFS(segs)) * BLK_RING_SIZE(info));
>  	if (err)
>  		goto out_of_memory;
>  
> @@ -1700,7 +1751,7 @@ static int blkfront_setup_indirect(struct blkfront_info *info)
>  		 * grants, we need to allocate a set of pages that can be
>  		 * used for mapping indirect grefs
>  		 */
> -		int num = INDIRECT_GREFS(segs) * BLK_RING_SIZE;
> +		int num = INDIRECT_GREFS(segs) * BLK_RING_SIZE(info);
>  
>  		BUG_ON(!list_empty(&info->indirect_pages));
>  		for (i = 0; i < num; i++) {
> @@ -1711,7 +1762,7 @@ static int blkfront_setup_indirect(struct blkfront_info *info)
>  		}
>  	}
>  
> -	for (i = 0; i < BLK_RING_SIZE; i++) {
> +	for (i = 0; i < BLK_RING_SIZE(info); i++) {
>  		info->shadow[i].grants_used = kzalloc(
>  			sizeof(info->shadow[i].grants_used[0]) * segs,
>  			GFP_NOIO);
> @@ -1733,7 +1784,7 @@ static int blkfront_setup_indirect(struct blkfront_info *info)
>  	return 0;
>  
>  out_of_memory:
> -	for (i = 0; i < BLK_RING_SIZE; i++) {
> +	for (i = 0; i < BLK_RING_SIZE(info); i++) {
>  		kfree(info->shadow[i].grants_used);
>  		info->shadow[i].grants_used = NULL;
>  		kfree(info->shadow[i].sg);
> @@ -2089,6 +2140,12 @@ static int __init xlblk_init(void)
>  	if (!xen_domain())
>  		return -ENODEV;
>  
> +	if (xen_blkif_max_ring_order > XENBUS_MAX_RING_PAGE_ORDER) {
> +		pr_info("Invalid max_ring_order (%d), will use default max: %d.\n",
> +			xen_blkif_max_ring_order, XENBUS_MAX_RING_PAGE_ORDER);
> +		xen_blkif_max_ring_order = 0;
> +	}
> +
>  	if (!xen_has_pv_disk_devices())
>  		return -ENODEV;
>  
> 
--
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