Re: [PATCH 4/4] vhost: convert byte order on avail_event read

[Eugenio Perez Martin]

On Sat, Oct 29, 2022 at 12:53 AM Philippe Mathieu-Daudé
<philmd@linaro.org> wrote:
>
> On 28/10/22 18:02, Eugenio Pérez wrote:
> > This causes errors on virtio modern devices on big endian hosts
> >
> > Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> > ---
> >   hw/virtio/vhost-shadow-virtqueue.c | 3 ++-
> >   1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/hw/virtio/vhost-shadow-virtqueue.c 
> > b/hw/virtio/vhost-shadow-virtqueue.c
> > index 18a49e1ecb..3131903edd 100644
> > --- a/hw/virtio/vhost-shadow-virtqueue.c
> > +++ b/hw/virtio/vhost-shadow-virtqueue.c
> > @@ -231,7 +231,8 @@ static void vhost_svq_kick(VhostShadowVirtqueue *svq)
> >           size_t num = svq->vring.num;
> >           uint16_t *avail_event = (uint16_t *)&svq->vring.used->ring[num];
> >
>
>    uint16_t avail_event = virtio_lduw_p(svq->vdev,
>                                         &svq->vring.used->ring[num]);
>    needs_kick = vring_need_event(avail_event,
>                                  svq->shadow_avail_idx,
>                                  svq->shadow_avail_idx - 1);
>

It would work, but just because all vrings must be little endian for
the moment. If we support legacy drivers on a big endian host and
guest in the future, it would not work.

virtio_ld and virtio_st handle the conversions between the guest and
the emulated device in qemu, but this conversion is between qemu
shadow vring and the vdpa device (assuming modern, little endian for
the moment).

Right now the feature set must be the same, but it could not be that
way in the future.

Thanks!

> > -        needs_kick = vring_need_event(*avail_event, svq->shadow_avail_idx,
> > +        needs_kick = vring_need_event(le16_to_cpu(*avail_event),
> > +                                      svq->shadow_avail_idx,
> >                                         svq->shadow_avail_idx - 1);
> >       } else {
> >           needs_kick = !(svq->vring.used->flags & VRING_USED_F_NO_NOTIFY);
>