Re: [PATCH v2] vhost: Get vring base from vq, not svq

[Jason Wang]

On Mon, Jul 18, 2022 at 8:05 PM Eugenio Pérez <eperezma@redhat.com> wrote:
>
> The SVQ vring used idx usually match with the guest visible one, as long
> as all the guest buffers (GPA) maps to exactly one buffer within qemu's
> VA. However, as we can see in virtqueue_map_desc, a single guest buffer
> could map to many buffers in SVQ vring.
>
> Also, its also a mistake to rewind them at the source of migration.
> Since VirtQueue is able to migrate the inflight descriptors, its
> responsability of the destination to perform the rewind just in case it
> cannot report the inflight descriptors to the device.
>
> This makes easier to migrate between backends or to recover them in
> vhost devices that support set in flight descriptors.
>
> Fixes: 6d0b22266633 ("vdpa: Adapt vhost_vdpa_get_vring_base to SVQ")
> Signed-off-by: Eugenio Pérez <eperezma@redhat.com>

Acked-by: Jason Wang <jasowang@redhat.com>

>
> --
> v2: Squash both fixes in one.
> ---
>  hw/virtio/vhost-vdpa.c | 24 ++++++++++++------------
>  1 file changed, 12 insertions(+), 12 deletions(-)
>
> diff --git a/hw/virtio/vhost-vdpa.c b/hw/virtio/vhost-vdpa.c
> index 795ed5a049..4458c8d23e 100644
> --- a/hw/virtio/vhost-vdpa.c
> +++ b/hw/virtio/vhost-vdpa.c
> @@ -1178,7 +1178,18 @@ static int vhost_vdpa_set_vring_base(struct vhost_dev 
> *dev,
>                                         struct vhost_vring_state *ring)
>  {
>      struct vhost_vdpa *v = dev->opaque;
> +    VirtQueue *vq = virtio_get_queue(dev->vdev, ring->index);
>
> +    /*
> +     * vhost-vdpa devices does not support in-flight requests. Set all of 
> them
> +     * as available.
> +     *
> +     * TODO: This is ok for networking, but other kinds of devices might
> +     * have problems with these retransmissions.
> +     */
> +    while (virtqueue_rewind(vq, 1)) {
> +        continue;
> +    }
>      if (v->shadow_vqs_enabled) {
>          /*
>           * Device vring base was set at device start. SVQ base is handled by
> @@ -1194,21 +1205,10 @@ static int vhost_vdpa_get_vring_base(struct vhost_dev 
> *dev,
>                                         struct vhost_vring_state *ring)
>  {
>      struct vhost_vdpa *v = dev->opaque;
> -    int vdpa_idx = ring->index - dev->vq_index;
>      int ret;
>
>      if (v->shadow_vqs_enabled) {
> -        VhostShadowVirtqueue *svq = g_ptr_array_index(v->shadow_vqs, 
> vdpa_idx);
> -
> -        /*
> -         * Setting base as last used idx, so destination will see as 
> available
> -         * all the entries that the device did not use, including the 
> in-flight
> -         * processing ones.
> -         *
> -         * TODO: This is ok for networking, but other kinds of devices might
> -         * have problems with these retransmissions.
> -         */
> -        ring->num = svq->last_used_idx;
> +        ring->num = virtio_queue_get_last_avail_idx(dev->vdev, ring->index);
>          return 0;
>      }
>
> --
> 2.31.1
>