[PATCH v2 2/3] xen-blkfront: add a parameter for disabling of persistent grants

[SeongJae Park <sjpark@amazon.com>]

From: SeongJae Park <sjpark@amazon.de>

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.  It can be disabled from blkback side using a
module parameter, 'feature_persistent'.  But, it is impossible from
blkfront side.  For the reason, this commit adds a blkfront module
parameter for disabling of the feature.

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

Signed-off-by: SeongJae Park <sjpark@amazon.de>
---
 .../ABI/testing/sysfs-driver-xen-blkfront     |  9 ++++++
 drivers/block/xen-blkfront.c                  | 28 +++++++++++++------
 2 files changed, 29 insertions(+), 8 deletions(-)

diff --git a/Documentation/ABI/testing/sysfs-driver-xen-blkfront b/Documentation/ABI/testing/sysfs-driver-xen-blkfront
index c0a6cb7eb314..9c31334cb2e6 100644
--- a/Documentation/ABI/testing/sysfs-driver-xen-blkfront
+++ b/Documentation/ABI/testing/sysfs-driver-xen-blkfront
@@ -8,3 +8,12 @@ Description:
                 is 32 - higher value means more potential throughput but more
                 memory usage. The backend picks the minimum of the frontend
                 and its default backend value.
+
+What:           /sys/module/xen_blkfront/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 frontends.
+                The default is Y (enable).
diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index 91de2e0755ae..49c324f377de 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -149,6 +149,13 @@ static unsigned int xen_blkif_max_ring_order;
 module_param_named(max_ring_page_order, xen_blkif_max_ring_order, int, 0444);
 MODULE_PARM_DESC(max_ring_page_order, "Maximum order of pages to be used for the shared ring");
 
+/* 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");
+
+
 #define BLK_RING_SIZE(info)	\
 	__CONST_RING_SIZE(blkif, XEN_PAGE_SIZE * (info)->nr_ring_pages)
 
@@ -1866,11 +1873,13 @@ static int talk_to_blkback(struct xenbus_device *dev,
 		message = "writing protocol";
 		goto abort_transaction;
 	}
-	err = xenbus_printf(xbt, dev->nodename,
-			    "feature-persistent", "%u", 1);
-	if (err)
-		dev_warn(&dev->dev,
-			 "writing persistent grants feature to xenbus");
+	if (feature_persistent) {
+		err = xenbus_printf(xbt, dev->nodename,
+				    "feature-persistent", "%u", 1);
+		if (err)
+			dev_warn(&dev->dev,
+				 "writing persistent grants feature to xenbus");
+	}
 
 	err = xenbus_transaction_end(xbt, 0);
 	if (err) {
@@ -2316,9 +2325,12 @@ static void blkfront_gather_backend_features(struct blkfront_info *info)
 	if (xenbus_read_unsigned(info->xbdev->otherend, "feature-discard", 0))
 		blkfront_setup_discard(info);
 
-	info->feature_persistent =
-		!!xenbus_read_unsigned(info->xbdev->otherend,
-				       "feature-persistent", 0);
+	if (feature_persistent)
+		info->feature_persistent =
+			!!xenbus_read_unsigned(info->xbdev->otherend,
+					       "feature-persistent", 0);
+	else
+		info->feature_persistent = 0;
 
 	indirect_segments = xenbus_read_unsigned(info->xbdev->otherend,
 					"feature-max-indirect-segments", 0);
-- 
2.17.1