Re: [Qemu-devel] [[RFC v3 09/12] virtio-net: fill head desc after done all in a chain

[Jason Wang]

On 2018年10月11日 22:08, address@hidden wrote:
> From: Wei Xu <address@hidden>
>
> With the support of marking a descriptor used/unused in 'flags'
> field for 1.1, the current way of filling a chained descriptors
> does not work since driver side may get the wrong 'num_buffer'
> information in case of the head descriptor has been filled in
> while the subsequent ones are still in processing in device side.
>
> This patch fills the head one after done all the others one.
>
> Signed-off-by: Wei Xu <address@hidden>
> ---
>   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 4bdd5b8..186c86cd2 100644
> --- a/hw/net/virtio-net.c
> +++ b/hw/net/virtio-net.c
> @@ -1198,6 +1198,8 @@ 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;
> @@ -1275,7 +1277,13 @@ 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);
>       }
>   
> @@ -1286,6 +1294,7 @@ 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);

It's not a good idea to fix API in device implementation. Let's 
introduce new API and fix it there.

E.g virtqueue_fill_n() and update the flag of first elem at the last step.

Thanks

>       virtqueue_flush(q->rx_vq, i);
>       virtio_notify(vdev, q->rx_vq);
>