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

[SeongJae Park <sj@xxxxxxxxxx>]

Hi Max,

On Fri, 26 Aug 2022 14:26:58 +0000 Maximilian Heyne <mheyne@xxxxxxxxx> wrote:

> On Thu, Aug 25, 2022 at 04:15:11PM +0000, SeongJae Park wrote:
> > 
> > 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.

Appreciate for your test!  And you're right, this patch is not fixing the issue
completely.  That is, commit 402c43ea6b34 ("xen-blkfront: Apply
'feature_persistent' parameter when connect") introduced two bugs.  One is the
misuse of the semantic of the advertisement.  It's fixed by this patch.  The
second bug, which you found here, is caching the parameter in a wrong place.

In detail, blkfront does the advertisement before connect (for init and resume)
and then negotiation after connected.  And the blkback does the negotiation
first, and then the advertisement during the establishing the connection.
Hence, blkback should cache the parameter just before the negotiation logic
while blkfront should do that just before the advertisement logic.

The blkback behavior change commit (e94c6101e151) did the work in the right
place, but the blkfront behavior change commit didn't.

So, I guess below change would fix the issue entirely when applied together
with this patch.  Any opinion, please?


diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index dfae08115450..7d3bde271e69 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -1850,6 +1850,7 @@ static int talk_to_blkback(struct xenbus_device *dev,
                message = "writing protocol";
                goto abort_transaction;
        }
+       info->feature_persistent_parm = feature_persistent;
        err = xenbus_printf(xbt, dev->nodename, "feature-persistent", "%u",
                        info->feature_persistent_parm);
        if (err)
@@ -2284,7 +2285,6 @@ static void blkfront_gather_backend_features(struct 
blkfront_info *info)
        if (xenbus_read_unsigned(info->xbdev->otherend, "feature-discard", 0))
                blkfront_setup_discard(info);

-       info->feature_persistent_parm = feature_persistent;
        if (info->feature_persistent_parm)
                info->feature_persistent =
                        !!xenbus_read_unsigned(info->xbdev->otherend,


Thanks,
SJ


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