Subject: Re: [PATCH net-next 2/3] vhost_net: support in order feature

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 23 Nov 2018 10:49:13 -0500

On Fri, Nov 23, 2018 at 11:00:15AM +0800, Jason Wang wrote:
> This makes vhost_net to support in order feature. This is as simple as
> use datacopy path when it was negotiated. An alternative is not to
> advertise in order when zerocopy is enabled which tends to be
> suboptimal consider zerocopy may suffer from e.g HOL issues.

Well IIRC vhost_zerocopy_signal_used is used to
actually reorder used ring to match available ring.
So with a big comment explaining why it is so,
we could just enable IN_ORDER there too.

> 
> Signed-off-by: Jason Wang <[email-masked]>
> ---
>  drivers/vhost/net.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index d919284f103b..bdf5de5a7eb2 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -74,7 +74,8 @@ enum {
>  	VHOST_NET_FEATURES = VHOST_FEATURES |
>  			 (1ULL << VHOST_NET_F_VIRTIO_NET_HDR) |
>  			 (1ULL << VIRTIO_NET_F_MRG_RXBUF) |
> -			 (1ULL << VIRTIO_F_IOMMU_PLATFORM)
> +			 (1ULL << VIRTIO_F_IOMMU_PLATFORM) |
> +	                 (1ULL << VIRTIO_F_IN_ORDER)
>  };
>  
>  enum {
> @@ -971,7 +972,8 @@ static void handle_tx(struct vhost_net *net)
>  	vhost_disable_notify(&net->dev, vq);
>  	vhost_net_disable_vq(net, vq);
>  
> -	if (vhost_sock_zcopy(sock))
> +	if (vhost_sock_zcopy(sock) &&
> +	    !vhost_has_feature(vq, VIRTIO_F_IN_ORDER))
>  		handle_tx_zerocopy(net, sock);
>  	else
>  		handle_tx_copy(net, sock);
> -- 
> 2.17.1