Re: [PATCH] virtio-balloon: correct used length

[Jason Wang <jasowang@redhat.com>]

On Wed, Nov 24, 2021 at 3:03 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Wed, Nov 24, 2021 at 12:32:55PM +0800, Jason Wang wrote:
> > Spec said:
> >
> > "and len the total of bytes written into the buffer."
> >
> > For inflateq, deflateq and statsq, we don't process in_sg so the used
> > length should be zero. For free_page_vq, though the free pages are
> > supplied via in_sgs, zero used length should still be fine since
> > anyway driver is expected to use the length in this case and it
> > simplifies the error handling path.
> >
> > Signed-off-by: Jason Wang <jasowang@redhat.com>
>
> I think for free page vq the point is that the pages are
> zeroed by hypervisor, so we must set used len accordingly. No?

I may miss something, I think it is used for clearing the dirty
bitmaps to avoid migrating unused pages, the zeroing is not needed.

Thanks

>
> > ---
> >  hw/virtio/virtio-balloon.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
> > index c6962fcbfe..3e52daa793 100644
> > --- a/hw/virtio/virtio-balloon.c
> > +++ b/hw/virtio/virtio-balloon.c
> > @@ -231,7 +231,7 @@ static void balloon_stats_poll_cb(void *opaque)
> >          return;
> >      }
> >
> > -    virtqueue_push(s->svq, s->stats_vq_elem, s->stats_vq_offset);
> > +    virtqueue_push(s->svq, s->stats_vq_elem, 0);
> >      virtio_notify(vdev, s->svq);
> >      g_free(s->stats_vq_elem);
> >      s->stats_vq_elem = NULL;
> > @@ -438,7 +438,7 @@ static void virtio_balloon_handle_output(VirtIODevice *vdev, VirtQueue *vq)
> >              memory_region_unref(section.mr);
> >          }
> >
> > -        virtqueue_push(vq, elem, offset);
> > +        virtqueue_push(vq, elem, 0);
> >          virtio_notify(vdev, vq);
> >          g_free(elem);
> >          virtio_balloon_pbp_free(&pbp);
> > @@ -549,7 +549,7 @@ static bool get_free_page_hints(VirtIOBalloon *dev)
> >      }
> >
> >  out:
> > -    virtqueue_push(vq, elem, 1);
> > +    virtqueue_push(vq, elem, 0);
> >      g_free(elem);
> >      return ret;
> >  }
> > --
> > 2.25.1
>