[PATCH v2 1/3] xen-blkback: Advertise feature-persistent as user requested

[SeongJae Park <sj@xxxxxxxxxx>]

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
aac8a70db24b ("xen-blkback: add a parameter for disabling of persistent
grants") made a field of blkback, 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 blkback saves the
parameter value in a separate place and advertises the support based on
only the saved value.

Fixes: aac8a70db24b ("xen-blkback: add a parameter for disabling of persistent 
grants")
Cc: <stable@xxxxxxxxxxxxxxx> # 5.10.x
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