[PATCH v4 1/3] xen-blkback: 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.  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@xxxxxxxxxx>
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-blkback      |  9 ++++++++
 drivers/block/xen-blkback/xenbus.c            | 22 ++++++++++++++-----
 2 files changed, 25 insertions(+), 6 deletions(-)

diff --git a/Documentation/ABI/testing/sysfs-driver-xen-blkback 
b/Documentation/ABI/testing/sysfs-driver-xen-blkback
index ecb7942ff146..ac2947b98950 100644
--- a/Documentation/ABI/testing/sysfs-driver-xen-blkback
+++ b/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@xxxxxxxxx>
+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).
diff --git a/drivers/block/xen-blkback/xenbus.c 
b/drivers/block/xen-blkback/xenbus.c
index b9aa5d1ac10b..8fc34211dc8b 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -474,6 +474,12 @@ 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)
@@ -519,6 +525,8 @@ 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;
@@ -906,7 +914,8 @@ 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);
@@ -1067,7 +1076,6 @@ 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;
@@ -1093,9 +1101,11 @@ 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;
 
        /*
@@ -1118,7 +1128,7 @@ 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);
-- 
2.17.1