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_ScalabilitySigned-off-by: Anthony Liguori <aliguori@amazon.com>
Signed-off-by: SeongJae Park <sjpark@amazon.de>
Reviewed-by: Juergen Gross <jgross@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Link: https://lore.kernel.org/r/20200923061841.20531-2-sjpark@amazon.comSigned-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

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

@@ -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

@@ -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 @@ static void connect(struct backend_info *be)
 
 	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);