Re: [PATCH v5 02/10] vhost: use SVQ element ndescs instead of opaque data for desc validation

[Jason Wang]

On Thu, Aug 4, 2022 at 1:57 PM Eugenio Perez Martin <eperezma@redhat.com> wrote:
>
> On Thu, Aug 4, 2022 at 5:01 AM Jason Wang <jasowang@redhat.com> wrote:
> >
> >
> > 在 2022/8/3 01:57, Eugenio Pérez 写道:
> > > Since we're going to allow SVQ to add elements without the guest's
> > > knowledge and without its own VirtQueueElement, it's easier to check if
> > > an element is a valid head checking a different thing than the
> > > VirtQueueElement.
> > >
> > > Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> > > ---
> >
> >
> > Patch looks good to me. But I spot several other issues:
> >
> > 1) vhost_svq_add() use size_t for in_num and out_num, is this intended?
>
> Would you prefer them to be unsigned? To me size_t fits better, but
> VirtQueueElement uses unsigned anyway.

Yes, the problem I ask is because the in_num were from
VirtQueueElement which is unsigned and cast to size_t and then cast
back to unsigned.

>
> > 2) do we need to fail vhost_svq_add() if in_num + out_num == 0?
> >
>
> We can recover from it, but it's a failure of qemu code.
> - In the case of loading the state to the destination device, we
> already know the layout (it's always 1 out, 1 in).
> - In the case of forwarding buffers, there is no way to get a
> VirtQueueElement with 0 out and 0 in descriptors, due to the virtqueue
> way of working.

So I think it's better to fail the svq_add in this case.

Thanks

>
> Would you prefer to return success in this case?
>
> Thanks!
>