[PATCH v4 2/3] xen-blkfront: add a parameter for disabling of persistent grants

[SeongJae Park <sjpark@xxxxxxxxxx>]

From: SeongJae Park <sjpark@xxxxxxxxx>

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@xxxxxxxxx>
Reviewed-by: Juergen Gross <jgross@xxxxxxxx>
Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 .../ABI/testing/sysfs-driver-xen-blkfront     |  9 +++++++++
 drivers/block/xen-blkfront.c                  | 20 ++++++++++++++-----
 2 files changed, 24 insertions(+), 5 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@xxxxxxxxx>
+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..48629d3433b4 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -1866,8 +1866,8 @@ 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);
+       err = xenbus_printf(xbt, dev->nodename, "feature-persistent", "%u",
+                       info->feature_persistent);
        if (err)
                dev_warn(&dev->dev,
                         "writing persistent grants feature to xenbus");
@@ -1941,6 +1941,13 @@ static int negotiate_mq(struct blkfront_info *info)
        }
        return 0;
 }
+
+/* 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");
+
 /**
  * Entry point to this code when a new device is created.  Allocate the basic
  * structures and the ring buffer for communication with the backend, and
@@ -2007,6 +2014,8 @@ static int blkfront_probe(struct xenbus_device *dev,
        info->vdevice = vdevice;
        info->connected = BLKIF_STATE_DISCONNECTED;
 
+       info->feature_persistent = feature_persistent;
+
        /* 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);
@@ -2316,9 +2325,10 @@ 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 (info->feature_persistent)
+               info->feature_persistent =
+                       !!xenbus_read_unsigned(info->xbdev->otherend,
+                                              "feature-persistent", 0);
 
        indirect_segments = xenbus_read_unsigned(info->xbdev->otherend,
                                        "feature-max-indirect-segments", 0);
-- 
2.17.1