[PATCH] xen, blkback: fix persistent grants negotiation

[Maximilian Heyne <mheyne@amazon.de>]

Given dom0 supports persistent grants but the guest does not.
Then, when attaching a block device during runtime of the guest, dom0
will enable persistent grants for this newly attached block device:

  $ xenstore-ls -f | grep 20674 | grep persistent
  /local/domain/0/backend/vbd/20674/768/feature-persistent = "0"
  /local/domain/0/backend/vbd/20674/51792/feature-persistent = "1"

Here disk 768 was attached during guest creation while 51792 was
attached at runtime. If the guest would have advertised the persistent
grant feature, there would be a xenstore entry like:

  /local/domain/20674/device/vbd/51792/feature-persistent = "1"

Persistent grants are also used when the guest tries to access the disk
which can be seen when enabling log stats:

  $ echo 1 > /sys/module/xen_blkback/parameters/log_stats
  $ dmesg
  xen-blkback: (20674.xvdf-0): oo   0  |  rd    0  |  wr    0  |  f    0 |  ds    0 | pg:    1/1056

The "pg: 1/1056" shows that one persistent grant is used.

Before commit aac8a70db24b ("xen-blkback: add a parameter for disabling
of persistent grants") vbd->feature_gnt_persistent was set in
connect_ring. After the commit it was intended to be initialized in
xen_vbd_create and then set according to the guest feature availability
in connect_ring. However, with a running guest, connect_ring might be
called before xen_vbd_create and vbd->feature_gnt_persistent will be
incorrectly initialized. xen_vbd_create will overwrite it with the value
of feature_persistent regardless whether the guest actually supports
persistent grants.

With this commit, vbd->feature_gnt_persistent is set only in
connect_ring and this is the only use of the module parameter
feature_persistent. This avoids races when the module parameter changes
during the block attachment process.

Note that vbd->feature_gnt_persistent doesn't need to be initialized in
xen_vbd_create. It's next use is in connect which can only be called
once connect_ring has initialized the rings. xen_update_blkif_status is
checking for this.

Fixes: aac8a70db24b ("xen-blkback: add a parameter for disabling of persistent grants")
Signed-off-by: Maximilian Heyne <mheyne@amazon.de>
---
 drivers/block/xen-blkback/xenbus.c | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
index 914587aabca0c..51b6ec0380ca4 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -522,8 +522,6 @@ 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;
@@ -1090,10 +1088,9 @@ static int connect_ring(struct backend_info *be)
 		xenbus_dev_fatal(dev, err, "unknown fe protocol %s", protocol);
 		return -ENOSYS;
 	}
-	if (blkif->vbd.feature_gnt_persistent)
-		blkif->vbd.feature_gnt_persistent =
-			xenbus_read_unsigned(dev->otherend,
-					"feature-persistent", 0);
+
+	blkif->vbd.feature_gnt_persistent = feature_persistent &&
+		xenbus_read_unsigned(dev->otherend, "feature-persistent", 0);
 
 	blkif->vbd.overflow_max_grants = 0;
 
-- 
2.32.0




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