Re: [PATCH 2/4] vhost: convert byte order on SVQ used event write

[Michael S. Tsirkin]

On Sat, Oct 29, 2022 at 12:48:43AM +0200, Philippe Mathieu-Daudé wrote:
> On 28/10/22 18:02, Eugenio Pérez wrote:
> > This causes errors on virtio modern devices on big endian hosts
> > 
> > Fixes: 01f8beacea2a ("vhost: toggle device callbacks using used event idx")
> > Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> > ---
> >   hw/virtio/vhost-shadow-virtqueue.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/hw/virtio/vhost-shadow-virtqueue.c 
> > b/hw/virtio/vhost-shadow-virtqueue.c
> > index 70766ea740..467099f5d9 100644
> > --- a/hw/virtio/vhost-shadow-virtqueue.c
> > +++ b/hw/virtio/vhost-shadow-virtqueue.c
> > @@ -382,7 +382,7 @@ static bool 
> > vhost_svq_enable_notification(VhostShadowVirtqueue *svq)
> >   {
> >       if (virtio_vdev_has_feature(svq->vdev, VIRTIO_RING_F_EVENT_IDX)) {
> >           uint16_t *used_event = &svq->vring.avail->ring[svq->vring.num];
> > -        *used_event = svq->shadow_used_idx;
> > +        *used_event = cpu_to_le16(svq->shadow_used_idx);
> 
> This looks correct, but what about:
> 
>            virtio_stw_p(svq->vdev, used_event, svq->shadow_used_idx);


Philippe thanks for review but this comment isn't all that clear.
I think you meant something like:
        this won't handle endian-ness for legacy virtio devices
        on BE correctly. I think  virtio_stw_p would be better.

which would make sense.

Yes contributors should document motivation for changes but so
should reviewers.
        

> >       } else {
> >           svq->vring.avail->flags &= 
> > ~cpu_to_le16(VRING_AVAIL_F_NO_INTERRUPT);
> >       }