[PATCH 1/2] xen, blkback: fix persistent grants negotiation

[SeongJae Park <sj@xxxxxxxxxx>]

From: Maximilian Heyne <mheyne@xxxxxxxxx>

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.

Please also note that this commit makes a behavioral change of the
parameter.  That is, the parameter made effect on only newly created
backends before this commit, but it makes the effect for every new
connection after this commit.  Therefore, this commit also updates the
document.

Fixes: aac8a70db24b ("xen-blkback: add a parameter for disabling of persistent 
grants")
Cc: <stable@xxxxxxxxxxxxxxx> # 5.10.x
Signed-off-by: Maximilian Heyne <mheyne@xxxxxxxxx>
Signed-off-by: SeongJae Park <sj@xxxxxxxxxx>
---
 Documentation/ABI/testing/sysfs-driver-xen-blkback | 2 +-
 drivers/block/xen-blkback/xenbus.c                 | 9 +++------
 2 files changed, 4 insertions(+), 7 deletions(-)

diff --git a/Documentation/ABI/testing/sysfs-driver-xen-blkback 
b/Documentation/ABI/testing/sysfs-driver-xen-blkback
index 7faf719af165..fac0f429a869 100644
--- a/Documentation/ABI/testing/sysfs-driver-xen-blkback
+++ b/Documentation/ABI/testing/sysfs-driver-xen-blkback
@@ -42,5 +42,5 @@ KernelVersion:  5.10
 Contact:        Maximilian Heyne <mheyne@xxxxxxxxx>
 Description:
                 Whether to enable the persistent grants feature or not.  Note
-                that this option only takes effect on newly created backends.
+                that this option only takes effect on newly connected backends.
                 The default is Y (enable).
diff --git a/drivers/block/xen-blkback/xenbus.c 
b/drivers/block/xen-blkback/xenbus.c
index 97de13b14175..874b846fb622 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -520,8 +520,6 @@ static int xen_vbd_create(struct xen_blkif *blkif, 
blkif_vdev_t handle,
        if (bdev_max_secure_erase_sectors(bdev))
                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;
@@ -1087,10 +1085,9 @@ 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;
 
-- 
2.25.1