Subject: Re: [PATCH V3 4/6] vhost_net: determine whether or not to use zerocopy at one time

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 4 Sep 2013 14:59:29 +0300

On Mon, Sep 02, 2013 at 04:40:59PM +0800, Jason Wang wrote:
> Currently, even if the packet length is smaller than VHOST_GOODCOPY_LEN, if
> upend_idx != done_idx we still set zcopy_used to true and rollback this choice
> later. This could be avoided by determining zerocopy once by checking all
> conditions at one time before.
> 
> Signed-off-by: Jason Wang <[email-masked]>
> ---
>  drivers/vhost/net.c |   47 ++++++++++++++++++++---------------------------
>  1 files changed, 20 insertions(+), 27 deletions(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 8a6dd0d..3f89dea 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -404,43 +404,36 @@ static void handle_tx(struct vhost_net *net)
>  			       iov_length(nvq->hdr, s), hdr_size);
>  			break;
>  		}
> -		zcopy_used = zcopy && (len >= VHOST_GOODCOPY_LEN ||
> -				       nvq->upend_idx != nvq->done_idx);
> +
> +		zcopy_used = zcopy && len >= VHOST_GOODCOPY_LEN
> +				   && (nvq->upend_idx + 1) % UIO_MAXIOV !=
> +				      nvq->done_idx

Thinking about this, this looks strange.
The original idea was that once we start doing zcopy, we keep
using the heads ring even for short packets until no zcopy is outstanding.

What's the logic behind (nvq->upend_idx + 1) % UIO_MAXIOV != nvq->done_idx
here?

> +				   && vhost_net_tx_select_zcopy(net);
>  
>  		/* use msg_control to pass vhost zerocopy ubuf info to skb */
>  		if (zcopy_used) {
> +			struct ubuf_info *ubuf;
> +			ubuf = nvq->ubuf_info + nvq->upend_idx;
> +
>  			vq->heads[nvq->upend_idx].id = head;
> -			if (!vhost_net_tx_select_zcopy(net) ||
> -			    len < VHOST_GOODCOPY_LEN) {
> -				/* copy don't need to wait for DMA done */
> -				vq->heads[nvq->upend_idx].len =
> -							VHOST_DMA_DONE_LEN;
> -				msg.msg_control = NULL;
> -				msg.msg_controllen = 0;
> -				ubufs = NULL;
> -			} else {
> -				struct ubuf_info *ubuf;
> -				ubuf = nvq->ubuf_info + nvq->upend_idx;
> -
> -				vq->heads[nvq->upend_idx].len =
> -					VHOST_DMA_IN_PROGRESS;
> -				ubuf->callback = vhost_zerocopy_callback;
> -				ubuf->ctx = nvq->ubufs;
> -				ubuf->desc = nvq->upend_idx;
> -				msg.msg_control = ubuf;
> -				msg.msg_controllen = sizeof(ubuf);
> -				ubufs = nvq->ubufs;
> -				kref_get(&ubufs->kref);
> -			}
> +			vq->heads[nvq->upend_idx].len = VHOST_DMA_IN_PROGRESS;
> +			ubuf->callback = vhost_zerocopy_callback;
> +			ubuf->ctx = nvq->ubufs;
> +			ubuf->desc = nvq->upend_idx;
> +			msg.msg_control = ubuf;
> +			msg.msg_controllen = sizeof(ubuf);
> +			ubufs = nvq->ubufs;
> +			kref_get(&ubufs->kref);
>  			nvq->upend_idx = (nvq->upend_idx + 1) % UIO_MAXIOV;
> -		} else
> +		} else {
>  			msg.msg_control = NULL;
> +			ubufs = NULL;
> +		}
>  		/* TODO: Check specific error and bomb out unless ENOBUFS? */
>  		err = sock->ops->sendmsg(NULL, sock, &msg, len);
>  		if (unlikely(err < 0)) {
>  			if (zcopy_used) {
> -				if (ubufs)
> -					vhost_net_ubuf_put(ubufs);
> +				vhost_net_ubuf_put(ubufs);
>  				nvq->upend_idx = ((unsigned)nvq->upend_idx - 1)
>  					% UIO_MAXIOV;
>  			}
> -- 
> 1.7.1