Commit aac8a70d authored by SeongJae Park's avatar SeongJae Park Committed by Boris Ostrovsky
Browse files

xen-blkback: add a parameter for disabling of persistent grants 

Persistent grants feature provides high scalability.  On some small
systems, however, it could incur data copy overheads[1] and thus it is
required to be disabled.  But, there is no option to disable it.  For
the reason, this commit adds a module parameter for disabling of the
feature.

[1] https://wiki.xen.org/wiki/Xen_4.3_Block_Protocol_Scalability



Signed-off-by: default avatarAnthony Liguori <aliguori@amazon.com>
Signed-off-by: default avatarSeongJae Park <sjpark@amazon.de>
Reviewed-by: default avatarJuergen Gross <jgross@suse.com>
Acked-by: default avatarRoger Pau Monné <roger.pau@citrix.com>
Link: https://lore.kernel.org/r/20200923061841.20531-2-sjpark@amazon.com


Signed-off-by: default avatarBoris Ostrovsky <boris.ostrovsky@oracle.com>
parent 5f7f7740

Documentation/ABI/testing/sysfs-driver-xen-blkback

+9 −0
Original line number Diff line number Diff line
@@ -35,3 +35,12 @@ Description: 
                controls the duration in milliseconds that blkback will not

                cache any page not backed by a grant mapping.

                The default is 10ms.



What:           /sys/module/xen_blkback/parameters/feature_persistent

Date:           September 2020

KernelVersion:  5.10

Contact:        SeongJae Park <sjpark@amazon.de>

Description:

                Whether to enable the persistent grants feature or not.  Note

                that this option only takes effect on newly created backends.

                The default is Y (enable).

drivers/block/xen-blkback/xenbus.c

+16 −6
Original line number Diff line number Diff line
@@ -473,6 +473,12 @@ static void xen_vbd_free(struct xen_vbd *vbd) 
	vbd->bdev = NULL;

}



/* Enable the persistent grants feature. */

static bool feature_persistent = true;

module_param(feature_persistent, bool, 0644);

MODULE_PARM_DESC(feature_persistent,

		"Enables the persistent grants feature");



static int xen_vbd_create(struct xen_blkif *blkif, blkif_vdev_t handle,

			  unsigned major, unsigned minor, int readonly,

			  int cdrom)
@@ -518,6 +524,8 @@ static int xen_vbd_create(struct xen_blkif *blkif, blkif_vdev_t handle, 
	if (q && blk_queue_secure_erase(q))

		vbd->discard_secure = true;



	vbd->feature_gnt_persistent = feature_persistent;



	pr_debug("Successful creation of handle=%04x (dom=%u)\n",

		handle, blkif->domid);

	return 0;
@@ -905,7 +913,8 @@ again: 


	xen_blkbk_barrier(xbt, be, be->blkif->vbd.flush_support);



	err = xenbus_printf(xbt, dev->nodename, "feature-persistent", "%u", 1);

	err = xenbus_printf(xbt, dev->nodename, "feature-persistent", "%u",

			be->blkif->vbd.feature_gnt_persistent);

	if (err) {

		xenbus_dev_fatal(dev, err, "writing %s/feature-persistent",

				 dev->nodename);
@@ -1066,7 +1075,6 @@ static int connect_ring(struct backend_info *be) 
{

	struct xenbus_device *dev = be->dev;

	struct xen_blkif *blkif = be->blkif;

	unsigned int pers_grants;

	char protocol[64] = "";

	int err, i;

	char *xspath;
@@ -1092,9 +1100,11 @@ static int connect_ring(struct backend_info *be) 
		xenbus_dev_fatal(dev, err, "unknown fe protocol %s", protocol);

		return -ENOSYS;

	}

	pers_grants = xenbus_read_unsigned(dev->otherend, "feature-persistent",

					   0);

	blkif->vbd.feature_gnt_persistent = pers_grants;

	if (blkif->vbd.feature_gnt_persistent)

		blkif->vbd.feature_gnt_persistent =

			xenbus_read_unsigned(dev->otherend,

					"feature-persistent", 0);



	blkif->vbd.overflow_max_grants = 0;



	/*
@@ -1117,7 +1127,7 @@ static int connect_ring(struct backend_info *be) 


	pr_info("%s: using %d queues, protocol %d (%s) %s\n", dev->nodename,

		 blkif->nr_rings, blkif->blk_protocol, protocol,

		 pers_grants ? "persistent grants" : "");

		 blkif->vbd.feature_gnt_persistent ? "persistent grants" : "");



	ring_page_order = xenbus_read_unsigned(dev->otherend,

					       "ring-page-order", 0);

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