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

[Eugenio Perez Martin]

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.

> 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.

Would you prefer to return success in this case?

Thanks!