Re: [PATCH 2/2] xen-blkfront: Advertise feature-persistent as user requested

[Maximilian Heyne <mheyne@xxxxxxxxx>]

On Thu, Aug 25, 2022 at 04:15:11PM +0000, SeongJae Park wrote:
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> 
> 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-e9b2f8c5f7cd@xxxxxxxx/
> 
> Fixes: 402c43ea6b34 ("xen-blkfront: 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-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 *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;

I think setting this here is too late because "feature-persistent" was already
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 patch
from your series yet. Will do more testing next week.

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



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