Re: [RFC v3 0/5] virtio: support packed ring

[Jason Wang]

On 2018å04æ27æ 12:18, Michael S. Tsirkin wrote:
> On Fri, Apr 27, 2018 at 11:56:05AM +0800, Jason Wang wrote:
> > On 2018å04æ25æ 13:15, Tiwei Bie wrote:
> > > Hello everyone,
> > >
> > > This RFC implements packed ring support in virtio driver.
> > >
> > > Some simple functional tests have been done with Jason's
> > > packed ring implementation in vhost:
> > >
> > > https://lkml.org/lkml/2018/4/23/12
> > >
> > > Both of ping and netperf worked as expected (with EVENT_IDX
> > > disabled). But there are below known issues:
> > >
> > > 1. Reloading the guest driver will break the Tx/Rx;
> > Will have a look at this issue.
> >
> > > 2. Zeroing the flags when detaching a used desc will
> > >      break the guest -> host path.
> > I still think zeroing flags is unnecessary or even a bug. At host, I track
> > last observed avail wrap counter and detect avail like (what is suggested in
> > the example code in the spec):
> >
> > static bool desc_is_avail(struct vhost_virtqueue *vq, __virtio16 flags)
> > {
> >  ÂÂÂÂÂÂ bool avail = flags & cpu_to_vhost16(vq, DESC_AVAIL);
> >
> >  ÂÂÂÂÂÂ return avail == vq->avail_wrap_counter;
> > }
> >
> > So zeroing wrap can not work with this obviously.
> >
> > Thanks
> I agree. I think what one should do is flip the available bit.

But is this flipping a must?

Thanks