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Subject: Re: [PATCH 2/2] xen-blkfront: Advertise feature-persistent as user requested
Date:   Fri, 26 Aug 2022 14:26:58 +0000
From:   Maximilian Heyne <mheyne@amazon.de>
To:     SeongJae Park <sj@kernel.org>
CC:     <jgross@suse.com>, <roger.pau@citrix.com>,
        <marmarek@invisiblethingslab.com>,
        <xen-devel@lists.xenproject.org>, <axboe@kernel.dk>,
        <linux-block@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        <stable@vger.kernel.org>
Message-ID: <20220826142658.GA77627@dev-dsk-mheyne-1b-c1362c4d.eu-west-1.amazon.com>
References: <20220825161511.94922-1-sj@kernel.org>
 <20220825161511.94922-3-sj@kernel.org>
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On Thu, Aug 25, 2022 at 04:15:11PM +0000, SeongJae Park wrote:
> CAUTION: This email originated from outside of the organization. Do not c=
lick links or open attachments unless you can confirm the sender and know t=
he content is safe.
> =

> =

> =

> 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 blkfront 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-e9b2f8c5f7c=
d@suse.com/
> =

> Fixes: 402c43ea6b34 ("xen-blkfront: Apply 'feature_persistent' parameter =
when connect")
> Cc: <stable@vger.kernel.org> # 5.10.x
> Reported-by: Marek Marczykowski-G=F3recki <marmarek@invisiblethingslab.co=
m>
> Suggested-by: Juergen Gross <jgross@suse.com>
> Signed-off-by: SeongJae Park <sj@kernel.org>
> ---
>  drivers/block/xen-blkfront.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> =

> diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
> index 8e56e69fb4c4..dfae08115450 100644
> --- a/drivers/block/xen-blkfront.c
> +++ b/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 *de=
v,
>                 goto abort_transaction;
>         }
>         err =3D 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 =3D feature_persistent;

I think setting this here is too late because "feature-persistent" was alre=
ady
written to xenstore via talk_to_blkback but with default 0. So during the
connect blkback will not see that the guest supports the feature and falls =
back
to no persistent grants.

Tested only this patch with some hacky dom0 kernel that doesn't have the pa=
tch
from your series yet. Will do more testing next week.

> +       if (info->feature_persistent_parm)
>                 info->feature_persistent =3D
>                         !!xenbus_read_unsigned(info->xbdev->otherend,
>                                                "feature-persistent", 0);
> --
> 2.25.1
> =


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