[PATCH] xen-blkback: 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 overhead[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: Anthony Liguori <aliguori@amazon.com>
Signed-off-by: SeongJae Park <sjpark@amazon.de>
---
 .../ABI/testing/sysfs-driver-xen-blkback        |  8 ++++++++
 drivers/block/xen-blkback/xenbus.c              | 17 ++++++++++++++---
 2 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/Documentation/ABI/testing/sysfs-driver-xen-blkback b/Documentation/ABI/testing/sysfs-driver-xen-blkback
index ecb7942ff146..0c42285c75ee 100644
--- a/Documentation/ABI/testing/sysfs-driver-xen-blkback
+++ b/Documentation/ABI/testing/sysfs-driver-xen-blkback
@@ -35,3 +35,11 @@ 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.
+                The default is 1 (enable).
diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
index b9aa5d1ac10b..9c03d70469f4 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -879,6 +879,12 @@ static void reclaim_memory(struct xenbus_device *dev)
 
 /* ** Connection ** */
 
+/* Enable the persistent grants feature. */
+static unsigned int feature_persistent = 1;
+module_param_named(feature_persistent, feature_persistent, int, 0644);
+MODULE_PARM_DESC(feature_persistent,
+		"Enables the persistent grants feature");
+
 /*
  * Write the physical details regarding the block device to the store, and
  * switch to Connected state.
@@ -906,7 +912,8 @@ 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",
+			feature_persistent ? 1 : 0);
 	if (err) {
 		xenbus_dev_fatal(dev, err, "writing %s/feature-persistent",
 				 dev->nodename);
@@ -1093,8 +1100,12 @@ 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);
+	if (feature_persistent)
+		pers_grants = xenbus_read_unsigned(dev->otherend,
+				"feature-persistent", 0);
+	else
+		pers_grants = 0;
+
 	blkif->vbd.feature_gnt_persistent = pers_grants;
 	blkif->vbd.overflow_max_grants = 0;
 
-- 
2.17.1