linux-kernel - [PATCH 0/2] xen-blk{front,back}: Advertise feature-persistent as user requested

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Date:   Thu, 25 Aug 2022 16:15:09 +0000
From:   SeongJae Park <sj@...nel.org>
To:     jgross@...e.com, roger.pau@...rix.com
Cc:     marmarek@...isiblethingslab.com, mheyne@...zon.de,
        xen-devel@...ts.xenproject.org, axboe@...nel.dk,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        SeongJae Park <sj@...nel.org>
Subject: [PATCH 0/2] xen-blk{front,back}: Advertise feature-persistent as user requested

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 patchset fixes the issue by making both blkback and blkfront
advertise their 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@suse.com/

SeongJae Park (2):
  xen-blkback: Advertise feature-persistent as user requested
  xen-blkfront: Advertise feature-persistent as user requested

 drivers/block/xen-blkback/common.h | 3 +++
 drivers/block/xen-blkback/xenbus.c | 6 ++++--
 drivers/block/xen-blkfront.c       | 8 ++++++--
 3 files changed, 13 insertions(+), 4 deletions(-)

-- 
2.25.1