Re: [PATCH] virtio-balloon: Fix page-poison subsection name

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Daniel P. Berrangé (berrange@redhat.com) wrote:
> On Tue, Sep 14, 2021 at 02:30:06PM +0100, Dr. David Alan Gilbert wrote:
> > * David Hildenbrand (david@redhat.com) wrote:
> > > On 14.09.21 15:17, Dr. David Alan Gilbert (git) wrote:
> > > > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > > > 
> > > > The subsection name for page-poison was typo'd as:
> > > > 
> > > >    vitio-balloon-device/page-poison
> > > > 
> > > > Note the missing 'r' in virtio.
> > > > 
> > > > When we have a machine type that enables page poison, and the guest
> > > > enables it (which needs a new kernel), things fail rather unpredictably.
> > > > 
> > > > The fallout from this is that most of the other subsections fail to
> > > > load, including things like the feature bits in the device, one
> > > > possible fallout is that the physical addresses of the queues
> > > > then get aligned differently and we fail with an error about
> > > > last_avail_idx being wrong.
> > > > It's not obvious to me why this doesn't produce a more obvious failure,
> > > > but virtio's vmstate loading is a bit open-coded.
> > > > 
> > > > Fixes: 7483cbbaf82 ("virtio-balloon: Implement support for page poison reporting feature")
> > > > bz: https://bugzilla.redhat.com/show_bug.cgi?id=1984401
> > > > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > > > ---
> > > >   hw/virtio/virtio-balloon.c | 2 +-
> > > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > > 
> > > > diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
> > > > index 5a69dce35d..c6962fcbfe 100644
> > > > --- a/hw/virtio/virtio-balloon.c
> > > > +++ b/hw/virtio/virtio-balloon.c
> > > > @@ -852,7 +852,7 @@ static const VMStateDescription vmstate_virtio_balloon_free_page_hint = {
> > > >   };
> > > >   static const VMStateDescription vmstate_virtio_balloon_page_poison = {
> > > > -    .name = "vitio-balloon-device/page-poison",
> > > > +    .name = "virtio-balloon-device/page-poison",
> > > >       .version_id = 1,
> > > >       .minimum_version_id = 1,
> > > >       .needed = virtio_balloon_page_poison_support,
> > > > 
> > > 
> > > Oh, that's very subtle. I wasn't even aware that the prefix really has to
> > > match the actual device ... I thought the whole idea of the prefix here was
> > > just to make the string unique ...
> > 
> > Subsection naming is *very* critical; the logic is something like:
> >   'we're loading the X device'
> > a subsection arrives for 'N/P'
> > if 'X==N' then it looks in X for subsection P.
> > If 'X!=N' then it assumes we've finished loading X
> > and P is really for an outer device that X is part of.
> > This is horrible.
> 
> Is there value in making this more explicit via a code convention
> for .name field initializers. eg instead of
> 
>    .name = "virtio-balloon-device/page-poison",
> 
> Prefer
> 
>    .name = TYPE_VIRTIO_BALLOON "/page-poison"
> 
> ?

I think it might be better to see if the vmstate code can check it and
error during saving; certainly this case feels detectable, but I need
to keep an eye open for all the other weird cases.

Dave

> 
> Regards,
> Daniel
> -- 
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-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK