Re: [Qemu-devel] [PATCH v4 09/11] virtio-net: update the head descriptor in a chain lastly

[Jason Wang <jasowang@redhat.com>]

On 2019/2/14 下午12:26, wexu@redhat.com wrote:
> From: Wei Xu <wexu@redhat.com>
>
> This is a helper for packed ring.
>
> To support packed ring, the head descriptor in a chain should be updated
> lastly since no 'avail_idx' like in packed ring to explicitly tell the
> driver side that all payload is ready after having done the chain, so
> the head is always visible immediately.
>
> This patch fills the header after done all the other ones.
>
> Signed-off-by: Wei Xu <wexu@redhat.com>


It's really odd to workaround API issue in the implementation of device. 
Please introduce batched used updating helpers instead.

Thanks


> ---
>   hw/net/virtio-net.c | 11 ++++++++++-
>   1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> index 3f319ef..330abea 100644
> --- a/hw/net/virtio-net.c
> +++ b/hw/net/virtio-net.c
> @@ -1251,6 +1251,8 @@ static ssize_t virtio_net_receive_rcu(NetClientState *nc, const uint8_t *buf,
>       struct virtio_net_hdr_mrg_rxbuf mhdr;
>       unsigned mhdr_cnt = 0;
>       size_t offset, i, guest_offset;
> +    VirtQueueElement head;
> +    int head_len = 0;
>   
>       if (!virtio_net_can_receive(nc)) {
>           return -1;
> @@ -1328,7 +1330,13 @@ static ssize_t virtio_net_receive_rcu(NetClientState *nc, const uint8_t *buf,
>           }
>   
>           /* signal other side */
> -        virtqueue_fill(q->rx_vq, elem, total, i++);
> +        if (i == 0) {
> +            head_len = total;
> +            head = *elem;
> +        } else {
> +            virtqueue_fill(q->rx_vq, elem, len, i);
> +        }
> +        i++;
>           g_free(elem);
>       }
>   
> @@ -1339,6 +1347,7 @@ static ssize_t virtio_net_receive_rcu(NetClientState *nc, const uint8_t *buf,
>                        &mhdr.num_buffers, sizeof mhdr.num_buffers);
>       }
>   
> +    virtqueue_fill(q->rx_vq, &head, head_len, 0);
>       virtqueue_flush(q->rx_vq, i);
>       virtio_notify(vdev, q->rx_vq);
>