xen-blkfront: Advertise feature-persistent as user requested

The advertisement of the persistent grants feature (writing
'feature-persistent' to xenbus) should mean not the decision for using
the feature but only the availability of the feature.  However, commit
74a85247 ("xen-blkfront: add a parameter for disabling of persistent
grants") made a field of blkfront, which was a place for saving only the
negotiation result, to be used for yet another purpose: caching of the
'feature_persistent' parameter value.  As a result, the advertisement,
which should follow only the parameter value, becomes inconsistent.

This commit fixes the misuse of the semantic by making blkfront saves
the parameter value in a separate place and advertises the support based
on only the saved value.

Fixes: 74a85247 ("xen-blkfront: add a parameter for disabling of persistent grants")
Cc: <stable@vger.kernel.org> # 5.10.x
Suggested-by: Juergen Gross <jgross@suse.com>
Signed-off-by: SeongJae Park <sj@kernel.org>
Tested-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20220831165824.94815-3-sj@kernel.orgSigned-off-by: Juergen Gross <jgross@suse.com>

drivers/block/xen-blkfront.c

@@ -213,6 +213,9 @@ struct blkfront_info
 	unsigned int feature_fua:1;
 	unsigned int feature_discard:1;
 	unsigned int feature_secdiscard:1;
+	/* Connect-time cached feature_persistent parameter */
+	unsigned int feature_persistent_parm:1;
+	/* Persistent grants feature negotiation result */
 	unsigned int feature_persistent:1;
 	unsigned int bounce:1;
 	unsigned int discard_granularity;
@@ -1848,7 +1851,7 @@ static int talk_to_blkback(struct xenbus_device *dev,
 		goto abort_transaction;
 	}
 	err = xenbus_printf(xbt, dev->nodename, "feature-persistent", "%u",
-			info->feature_persistent);
+			info->feature_persistent_parm);
 	if (err)
 		dev_warn(&dev->dev,
 			 "writing persistent grants feature to xenbus");
@@ -2281,7 +2284,8 @@ static void blkfront_gather_backend_features(struct blkfront_info *info)
 	if (xenbus_read_unsigned(info->xbdev->otherend, "feature-discard", 0))
 		blkfront_setup_discard(info);
 
-	if (feature_persistent)
+	info->feature_persistent_parm = feature_persistent;
+	if (info->feature_persistent_parm)
 		info->feature_persistent =
 			!!xenbus_read_unsigned(info->xbdev->otherend,
 					       "feature-persistent", 0);