[PATCH for-stable-5.10.y] xen-blkfront: Cache feature_persistent value before advertisement

[SeongJae Park <sj@xxxxxxxxxx>]

commit fe8f65b018effbf473f53af3538d0c1878b8b329 upstream.

Xen blkfront advertises its support of the persistent grants feature
when it first setting up and when resuming in 'talk_to_blkback()'.
Then, blkback reads the advertised value when it connects with blkfront
and decides if it will use the persistent grants feature or not, and
advertises its decision to blkfront.  Blkfront reads the blkback's
decision and it also makes the decision for the use of the feature.

Commit 402c43ea6b34 ("xen-blkfront: Apply 'feature_persistent' parameter
when connect"), however, made the blkfront's read of the parameter for
disabling the advertisement, namely 'feature_persistent', to be done
when it negotiate, not when advertise.  Therefore blkfront advertises
without reading the parameter.  As the field for caching the parameter
value is zero-initialized, it always advertises as the feature is
disabled, so that the persistent grants feature becomes always disabled.

This commit fixes the issue by making the blkfront does parmeter caching
just before the advertisement.

Fixes: 402c43ea6b34 ("xen-blkfront: Apply 'feature_persistent' parameter when 
connect")
Cc: <stable@xxxxxxxxxxxxxxx> # 5.10.x
Reported-by: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
Signed-off-by: SeongJae Park <sj@xxxxxxxxxx>
Tested-by: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
Reviewed-by: Juergen Gross <jgross@xxxxxxxx>
Link: https://lore.kernel.org/r/20220831165824.94815-4-sj@xxxxxxxxxx
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---

This patch is a manual backport of the upstream commit on the 5.10.y
kernel.  Please note that this patch can be applied on the latest 5.10.y
only after the preceding patch[1] is applied.

[1] https://lore.kernel.org/stable/20220906132819.016040100@xxxxxxxxxxxxxxxxxxx/

 drivers/block/xen-blkfront.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index 9d5460f6e0ff..6f33d62331b1 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -1852,6 +1852,12 @@ static void free_info(struct blkfront_info *info)
        kfree(info);
 }
 
+/* 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");
+
 /* Common code used when first setting up, and when resuming. */
 static int talk_to_blkback(struct xenbus_device *dev,
                           struct blkfront_info *info)
@@ -1943,6 +1949,7 @@ static int talk_to_blkback(struct xenbus_device *dev,
                message = "writing protocol";
                goto abort_transaction;
        }
+       info->feature_persistent_parm = feature_persistent;
        err = xenbus_printf(xbt, dev->nodename, "feature-persistent", "%u",
                        info->feature_persistent_parm);
        if (err)
@@ -2019,12 +2026,6 @@ static int negotiate_mq(struct blkfront_info *info)
        return 0;
 }
 
-/* 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");
-
 /**
  * Entry point to this code when a new device is created.  Allocate the basic
  * structures and the ring buffer for communication with the backend, and
@@ -2394,7 +2395,6 @@ static void blkfront_gather_backend_features(struct 
blkfront_info *info)
        if (xenbus_read_unsigned(info->xbdev->otherend, "feature-discard", 0))
                blkfront_setup_discard(info);
 
-       info->feature_persistent_parm = feature_persistent;
        if (info->feature_persistent_parm)
                info->feature_persistent =
                        !!xenbus_read_unsigned(info->xbdev->otherend,
-- 
2.25.1