Re: [PATCH 2/4] vhost: toggle device callbacks using used event idx

[Jason Wang <jasowang@redhat.com>]

On Thu, Oct 20, 2022 at 11:53 PM Eugenio Pérez <eperezma@redhat.com> wrote:
>
> Actually use the new field of the used ring and tell the device if SVQ
> wants to be notified.
>
> The code is not reachable at the moment.
>
> Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> ---
>  hw/virtio/vhost-shadow-virtqueue.c | 18 +++++++++++++++---
>  1 file changed, 15 insertions(+), 3 deletions(-)
>
> diff --git a/hw/virtio/vhost-shadow-virtqueue.c b/hw/virtio/vhost-shadow-virtqueue.c
> index a518f84772..f5c0fad3fc 100644
> --- a/hw/virtio/vhost-shadow-virtqueue.c
> +++ b/hw/virtio/vhost-shadow-virtqueue.c
> @@ -369,15 +369,27 @@ static bool vhost_svq_more_used(VhostShadowVirtqueue *svq)
>   */
>  static bool vhost_svq_enable_notification(VhostShadowVirtqueue *svq)
>  {
> -    svq->vring.avail->flags &= ~cpu_to_le16(VRING_AVAIL_F_NO_INTERRUPT);
> -    /* Make sure the flag is written before the read of used_idx */
> +    if (virtio_vdev_has_feature(svq->vdev, VIRTIO_RING_F_EVENT_IDX)) {
> +        uint16_t *used_event = (uint16_t *)&svq->vring.avail->ring[svq->vring.num];
> +        *used_event = svq->shadow_used_idx;

Do we need to care about the endian here?

E.g vduse has:

    *((uint16_t *)&vq->vring.used->ring[vq->vring.num]) = htole16(val);

Thanks

> +    } else {
> +        svq->vring.avail->flags &= ~cpu_to_le16(VRING_AVAIL_F_NO_INTERRUPT);
> +    }
> +
> +    /* Make sure the event is enabled before the read of used_idx */
>      smp_mb();
>      return !vhost_svq_more_used(svq);
>  }
>
>  static void vhost_svq_disable_notification(VhostShadowVirtqueue *svq)
>  {
> -    svq->vring.avail->flags |= cpu_to_le16(VRING_AVAIL_F_NO_INTERRUPT);
> +    /*
> +     * No need to disable notification in the event idx case, since used event
> +     * index is already an index too far away.
> +     */
> +    if (!virtio_vdev_has_feature(svq->vdev, VIRTIO_RING_F_EVENT_IDX)) {
> +        svq->vring.avail->flags |= cpu_to_le16(VRING_AVAIL_F_NO_INTERRUPT);
> +    }
>  }
>
>  static uint16_t vhost_svq_last_desc_of_chain(const VhostShadowVirtqueue *svq,
> --
> 2.31.1
>