Re: [PATCH 1/2] virtio_balloon: add work around for out of spec QEMU

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Jul 10, 2024 at 11:23:20AM +0800, Jason Wang wrote:
> On Fri, Jul 5, 2024 at 6:09 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > QEMU implemented the configuration
> >         VIRTIO_BALLOON_F_REPORTING && ! VIRTIO_BALLOON_F_FREE_PAGE_HINT
> > incorrectly: it then uses vq3 for reporting, spec says it is always 4.
> >
> > This is masked by a corresponding bug in driver:
> > add a work around as I'm going to try and fix the driver bug.
> >
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > ---
> >  drivers/virtio/virtio_balloon.c | 19 +++++++++++++++++--
> >  1 file changed, 17 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> > index 9a61febbd2f7..7dc3fcd56238 100644
> > --- a/drivers/virtio/virtio_balloon.c
> > +++ b/drivers/virtio/virtio_balloon.c
> > @@ -597,8 +597,23 @@ static int init_vqs(struct virtio_balloon *vb)
> >
> >         err = virtio_find_vqs(vb->vdev, VIRTIO_BALLOON_VQ_MAX, vqs,
> >                               callbacks, names, NULL);
> > -       if (err)
> > -               return err;
> > +       if (err) {
> > +               /*
> > +                * Try to work around QEMU bug which since 2020 confused vq numbers
> > +                * when VIRTIO_BALLOON_F_REPORTING but not
> > +                * VIRTIO_BALLOON_F_FREE_PAGE_HINT are offered.
> > +                */
> > +               if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_REPORTING) &&
> > +                   !virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT)) {
> > +                       names[VIRTIO_BALLOON_VQ_FREE_PAGE] = "reporting_vq";
> > +                       callbacks[VIRTIO_BALLOON_VQ_FREE_PAGE] = balloon_ack;
> > +                       err = virtio_find_vqs(vb->vdev,
> > +                                             VIRTIO_BALLOON_VQ_REPORTING, vqs, callbacks, names, NULL);
> > +               }
> > +
> > +               if (err)
> > +                       return err;
> > +       }
> >
> >         vb->inflate_vq = vqs[VIRTIO_BALLOON_VQ_INFLATE];
> >         vb->deflate_vq = vqs[VIRTIO_BALLOON_VQ_DEFLATE];
> > --
> > MST
> >
> 
> Acked-by: Jason Wang <jasowang@redhat.com>
> 
> Do we need a spec to say this is something that needs to be considered
> by the driver?
> 
> Thanks

I'd say it's a temporary situation that we won't need to bother
about in several years.

-- 
MST