Re: [外部邮件] Re: [PATCH] virtio_ring: reset IN_ORDER state in virtqueue_init()

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

On Tue, Nov 03, 2026 at 10:36:26AM +0100, Eugenio Perez Martin wrote:
> On Fri, Sep 11, 2026 at 8:05 AM Li,Rongqing <lirongqing@baidu.com> wrote:
> >
> > > >
> > > > The IN_ORDER feature uses additional software state to track the next
> > > > available descriptor and in-flight descriptor batches.
> > > >
> > > > virtqueue_init() resets the common virtqueue state, but free_head and
> > > > batch_last.id are initialized only when the virtqueue is created. When
> > > > an IN_ORDER virtqueue is reset, these fields can retain state from the
> > > > previous queue instance.
> > > >
> > > > Reset free_head and invalidate batch_last when initializing an
> > > > IN_ORDER virtqueue. Keep this in virtqueue_init() so the state is
> > > > initialized consistently for both newly created and reset virtqueues.
> > > >
> > > > Fixes: f6a15d854986 ("virtio_ring: add in order support")
> > >
> > > I think this is needed for -stable.
> > >
> > > > Signed-off-by: Li RongQing <lirongqing@baidu.com>
> > > > ---
> > > >  drivers/virtio/virtio_ring.c | 5 +++++
> > > >  1 file changed, 5 insertions(+)
> > > >
> > > > diff --git a/drivers/virtio/virtio_ring.c
> > > > b/drivers/virtio/virtio_ring.c index db678f5..d0dc464 100644
> > > > --- a/drivers/virtio/virtio_ring.c
> > > > +++ b/drivers/virtio/virtio_ring.c
> > > > @@ -559,6 +559,11 @@ static void virtqueue_init(struct vring_virtqueue *vq,
> > > u32 num)
> > > >         vq->in_use = false;
> > > >         vq->last_add_time_valid = false;  #endif
> > > > +
> > > > +       if (virtqueue_is_in_order(vq)) {
> > > > +               vq->free_head = 0;
> > > > +               vq->batch_last.id = UINT_MAX;
> > > > +       }
> > >
> > > I would try to reuse or refactor virtqueue_vring_attach_split() to reduce code
> > > duplication.
> > >
> > > Btw have you checked Qemu shadow virtqueue as it might suffer from the same
> > > issue.
> > >
> >
> > I checked QEMU's shadow virtqueue implementation. batch_last is reinitialized in vhost_svq_start(),
> > but free_head is not. Since the SVQ object can be reused across stop/start, free_head may retain stale
> > state. So a corresponding fix should be needed
> >
> > void vhost_svq_start(VhostShadowVirtqueue *svq, VirtIODevice *vdev,
> >                       VirtQueue *vq, VhostIOVATree *iova_tree)
> >  {
> >      ...
> >      svq->shadow_avail_idx = 0;
> >      svq->shadow_used_idx = 0;
> > +    svq->free_head = 0;
> >      memset(&svq->batch_last, 0, sizeof(svq->batch_last));
> >      svq->last_used = 0;
> >      svq->last_used_idx = 0;
> >
> 
> I'll ack this patch if it is sent to the QEMU mailing list, but the
> guest is not able to reset a virtqueue without deallocating it

what would "deallocating" mean here? who's doing the deallocating?

> as far
> as I know. Do you have a reproducer or something similar?