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Subject[PATCH v2 1/3] xen-blkback: add a parameter for disabling of persistent grants
Date
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. 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 | 9 ++++++
drivers/block/xen-blkback/xenbus.c | 28 ++++++++++++++-----
2 files changed, 30 insertions(+), 7 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@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).
diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
index b9aa5d1ac10b..8a95ddd08b13 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 bool feature_persistent = true;
+module_param(feature_persistent, bool, 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.
@@ -906,11 +912,15 @@ 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);
- if (err) {
- xenbus_dev_fatal(dev, err, "writing %s/feature-persistent",
- dev->nodename);
- goto abort;
+ if (feature_persistent) {
+ err = xenbus_printf(xbt, dev->nodename, "feature-persistent",
+ "%u", feature_persistent);
+ if (err) {
+ xenbus_dev_fatal(dev, err,
+ "writing %s/feature-persistent",
+ dev->nodename);
+ goto abort;
+ }
}

err = xenbus_printf(xbt, dev->nodename, "sectors", "%llu",
@@ -1093,8 +1103,12 @@ 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
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