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

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

On Wed, Nov 24, 2021 at 04:32:06PM +0800, Jason Wang wrote:
> On Wed, Nov 24, 2021 at 4:28 PM Jason Wang <jasowang@redhat.com> wrote:
> >
> > On Wed, Nov 24, 2021 at 4:25 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> > >
> > > On Wed, Nov 24, 2021 at 03:58:22PM +0800, Jason Wang wrote:
> > > > 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
> > >
> > > And, once the page is migrated, it's value on destination may differ
> > > from one on source.
> >
> > Right, I will fix this in the next version.
> >
> > Thanks
> 
> Btw in the get_free_page_hints() I see this:
> 
>     if (elem->in_num && dev->free_page_hint_status == FREE_PAGE_HINT_S_START) {
> qemu_guest_free_page_hint(elem->in_sg[0].iov_base,
>                                   elem->in_sg[0].iov_len);
>     }
> 
> It looks like only the first in sg is used, it looks like a bug?
> 
> Thanks

It sure does.


> >
> >
> > >
> > > > >
> > > > > > ---
> > > > > >  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
> > > > >
> > >