linux-kernel - Re: [PATCH V2 4/6] vhost_net: determine whether or not to use zerocopy at one time

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Date:	Fri, 30 Aug 2013 22:35:50 +0400
From:	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
To:	Jason Wang <jasowang@...hat.com>
CC:	mst@...hat.com, kvm@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2 4/6] vhost_net: determine whether or not to use zerocopy
 at one time

Hello.

On 08/30/2013 08:29 AM, 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 determine zerocopy once by checking all
> conditions at one time before.

> Signed-off-by: Jason Wang <jasowang@...hat.com>
> ---
>   drivers/vhost/net.c |   46 +++++++++++++++++++---------------------------
>   1 files changed, 19 insertions(+), 27 deletions(-)

> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 8a6dd0d..ff60c2a 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -404,43 +404,35 @@ 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
> +			&& vhost_net_tx_select_zcopy(net);

    Could you leave && on a first of two lines, matching the previous style?

>
>   		/* 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;
[...]
> +			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);

    'sizeof(ubuf)' where 'ubuf' is a pointer? Are you sure it shouldn't be 
'sizeof(*ubuf)'?

WBR, Sergei

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