[PATCH 1/2] xen-blkback: Advertise feature-persistent as user requested

[SeongJae Park <sj@xxxxxxxxxx>]

Commit e94c6101e151 ("xen-blkback: Apply 'feature_persistent' parameter
when connect") made blkback to advertise its support of the persistent
grants feature only if the user sets the 'feature_persistent' parameter
of the driver and the frontend advertised its support of the feature.
However, following commit 402c43ea6b34 ("xen-blkfront: Apply
'feature_persistent' parameter when connect") made the blkfront to work
in the same way.  That is, blkfront also advertises its support of the
persistent grants feature only if the user sets the 'feature_persistent'
parameter of the driver and the backend advertised its support of the
feature.

Hence blkback and blkfront will never advertise their support of the
feature but wait until the other advertises the support, even though
users set the 'feature_persistent' parameters of the drivers.  As a
result, the persistent grants feature is disabled always regardless of
the 'feature_persistent' values[1].

The problem comes from the misuse of the semantic of the advertisement
of the feature.  The advertisement of the feature should means only
availability of the feature not the decision for using the feature.
However, current behavior is working in the wrong way.

This commit fixes the issue by making the blkback advertises its support
of the feature as user requested via 'feature_persistent' parameter
regardless of the otherend's support of the feature.

[1] 
https://lore.kernel.org/xen-devel/bd818aba-4857-bc07-dc8a-e9b2f8c5f7cd@xxxxxxxx/

Fixes: e94c6101e151 ("xen-blkback: Apply 'feature_persistent' parameter when 
connect")
Cc: <stable@xxxxxxxxxxxxxxx> # 5.10.x
Reported-by: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
Suggested-by: Juergen Gross <jgross@xxxxxxxx>
Signed-off-by: SeongJae Park <sj@xxxxxxxxxx>
---
 drivers/block/xen-blkback/common.h | 3 +++
 drivers/block/xen-blkback/xenbus.c | 6 ++++--
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/drivers/block/xen-blkback/common.h 
b/drivers/block/xen-blkback/common.h
index bda5c815e441..a28473470e66 100644
--- a/drivers/block/xen-blkback/common.h
+++ b/drivers/block/xen-blkback/common.h
@@ -226,6 +226,9 @@ struct xen_vbd {
        sector_t                size;
        unsigned int            flush_support:1;
        unsigned int            discard_secure:1;
+       /* Connect-time cached feature_persistent parameter value */
+       unsigned int            feature_gnt_persistent_parm:1;
+       /* Persistent grants feature negotiation result */
        unsigned int            feature_gnt_persistent:1;
        unsigned int            overflow_max_grants:1;
 };
diff --git a/drivers/block/xen-blkback/xenbus.c 
b/drivers/block/xen-blkback/xenbus.c
index ee7ad2fb432d..c0227dfa4688 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -907,7 +907,7 @@ static void connect(struct backend_info *be)
        xen_blkbk_barrier(xbt, be, be->blkif->vbd.flush_support);
 
        err = xenbus_printf(xbt, dev->nodename, "feature-persistent", "%u",
-                       be->blkif->vbd.feature_gnt_persistent);
+                       be->blkif->vbd.feature_gnt_persistent_parm);
        if (err) {
                xenbus_dev_fatal(dev, err, "writing %s/feature-persistent",
                                 dev->nodename);
@@ -1085,7 +1085,9 @@ static int connect_ring(struct backend_info *be)
                return -ENOSYS;
        }
 
-       blkif->vbd.feature_gnt_persistent = feature_persistent &&
+       blkif->vbd.feature_gnt_persistent_parm = feature_persistent;
+       blkif->vbd.feature_gnt_persistent =
+               blkif->vbd.feature_gnt_persistent_parm &&
                xenbus_read_unsigned(dev->otherend, "feature-persistent", 0);
 
        blkif->vbd.overflow_max_grants = 0;
-- 
2.25.1