From aac8a70db24bd3916b9ddce1761e500771356b6b Mon Sep 17 00:00:00 2001
From: SeongJae Park <sjpark@amazon.de>
Date: Wed, 23 Sep 2020 08:18:39 +0200
Subject: [PATCH] xen-blkback: add a parameter for disabling of persistent
grants
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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>
Reviewed-by: Juergen Gross <jgross@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Link: https://lore.kernel.org/r/20200923061841.20531-2-sjpark@amazon.com
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
---
.../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@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 5e7c36d73dc6..f5705569e2a7 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -473,6 +473,12 @@ 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)
@@ -518,6 +524,8 @@ 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;
@@ -905,7 +913,8 @@ again:
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);
@@ -1066,7 +1075,6 @@ 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;
@@ -1092,9 +1100,11 @@ 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;
/*
@@ -1117,7 +1127,7 @@ 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);
--
2.50.1