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 03:37:49PM +0800, Jason Wang wrote:
> On Wed, Jul 10, 2024 at 2:16 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > 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.
> 
> I mean, should a newly-written virtio-balloon driver care about this?
> If not, it means it can't work for several Qemu versions.
> 
> Thanks

True - I could not find a way to make it work in a way that
would be compatible with old qemu.


> >
> > --
> > MST
> >