Subject: Re: [PATCH] virtio-net: put virtio net header inline with data

From: Jason Wang <[email-masked]>
Date: Fri, 07 Jun 2013 10:52:01 +0800

On 06/06/2013 05:55 PM, Michael S. Tsirkin wrote:
> For small packets we can simplify xmit processing by linearizing buffers
> with the header: most packets seem to have enough head room we can use
> for this purpose.
>
> Since some older hypervisors (e.g. qemu before version 1.5)
> required that header is the first s/g element,
> we need a feature bit for this.
>
> Signed-off-by: Michael S. Tsirkin <[email-masked]>
> ---

Hi Michael:

The idea looks good, but there are some questions:

- What if we just use direct descriptors with sgs <=2 and double the
ring size?
- I believe we may add more things into vnet header in the future, so
this trick may not work because of limited header room.

Thanks
> This is a repost of my old patch, rebased to latest kernel.
>
> Before:
> Thu Jun  6 05:24:59 EDT 2013
> TCP REQUEST/RESPONSE TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to
> 11.0.0.4 (11.0.0.4) port 0 AF_INET : demo
> Local /Remote
> Socket Size   Request  Resp.   Elapsed  Trans.
> Send   Recv   Size     Size    Time     Rate         
> bytes  Bytes  bytes    bytes   secs.    per sec   
>
> 16384  87380  1        1       10.00    12931.13   
>
>
> After:
> TCP REQUEST/RESPONSE TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to
> 11.0.0.4 (11.0.0.4) port 0 AF_INET : demo
> Local /Remote
> Socket Size   Request  Resp.   Elapsed  Trans.
> Send   Recv   Size     Size    Time     Rate         
> bytes  Bytes  bytes    bytes   secs.    per sec   
>
> 16384  87380  1        1       10.00    14151.12
>
>
>  drivers/net/virtio_net.c        | 42 +++++++++++++++++++++++++++++++++--------
>  include/uapi/linux/virtio_net.h |  6 +++++-
>  2 files changed, 39 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index c9e0038..d35a097 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -106,6 +106,9 @@ struct virtnet_info {
>  	/* Has control virtqueue */
>  	bool has_cvq;
>  
> +	/* Host can handle any s/g split between our header and packet data */
> +	bool any_header_sg;
> +
>  	/* enable config space updates */
>  	bool config_enable;
>  
> @@ -668,12 +671,28 @@ static void free_old_xmit_skbs(struct send_queue *sq)
>  
>  static int xmit_skb(struct send_queue *sq, struct sk_buff *skb)
>  {
> -	struct skb_vnet_hdr *hdr = skb_vnet_hdr(skb);
> +	struct skb_vnet_hdr *hdr;
>  	const unsigned char *dest = ((struct ethhdr *)skb->data)->h_dest;
>  	struct virtnet_info *vi = sq->vq->vdev->priv;
>  	unsigned num_sg;
> +	unsigned hdr_len;
> +	bool can_push;
>  
>  	pr_debug("%s: xmit %p %pM\n", vi->dev->name, skb, dest);
> +	if (vi->mergeable_rx_bufs)
> +		hdr_len = sizeof hdr->mhdr;
> +	else
> +		hdr_len = sizeof hdr->hdr;
> +
> +	can_push = vi->any_header_sg &&
> +		!((unsigned long)skb->data & (__alignof__(*hdr) - 1)) &&
> +		!skb_header_cloned(skb) && skb_headroom(skb) >= hdr_len;
> +	/* Even if we can, don't push here yet as this would skew
> +	 * csum_start offset below. */
> +	if (can_push)
> +		hdr = (struct skb_vnet_hdr *)(skb->data - hdr_len);
> +	else
> +		hdr = skb_vnet_hdr(skb);
>  
>  	if (skb->ip_summed == CHECKSUM_PARTIAL) {
>  		hdr->hdr.flags = VIRTIO_NET_HDR_F_NEEDS_CSUM;
> @@ -702,15 +721,18 @@ static int xmit_skb(struct send_queue *sq, struct sk_buff *skb)
>  		hdr->hdr.gso_size = hdr->hdr.hdr_len = 0;
>  	}
>  
> -	hdr->mhdr.num_buffers = 0;
> -
> -	/* Encode metadata header at front. */
>  	if (vi->mergeable_rx_bufs)
> -		sg_set_buf(sq->sg, &hdr->mhdr, sizeof hdr->mhdr);
> -	else
> -		sg_set_buf(sq->sg, &hdr->hdr, sizeof hdr->hdr);
> +		hdr->mhdr.num_buffers = 0;
>  
> -	num_sg = skb_to_sgvec(skb, sq->sg + 1, 0, skb->len) + 1;
> +	if (can_push) {
> +		__skb_push(skb, hdr_len);
> +		num_sg = skb_to_sgvec(skb, sq->sg, 0, skb->len);
> +		/* Pull header back to avoid skew in tx bytes calculations. */
> +		__skb_pull(skb, hdr_len);
> +	} else {
> +		sg_set_buf(sq->sg, hdr, hdr_len);
> +		num_sg = skb_to_sgvec(skb, sq->sg + 1, 0, skb->len) + 1;
> +	}
>  	return virtqueue_add_outbuf(sq->vq, sq->sg, num_sg, skb, GFP_ATOMIC);
>  }
>  
> @@ -1554,6 +1576,9 @@ static int virtnet_probe(struct virtio_device *vdev)
>  	if (virtio_has_feature(vdev, VIRTIO_NET_F_MRG_RXBUF))
>  		vi->mergeable_rx_bufs = true;
>  
> +	if (virtio_has_feature(vdev, VIRTIO_NET_F_ANY_HEADER_SG))
> +		vi->any_header_sg = true;
> +
>  	if (virtio_has_feature(vdev, VIRTIO_NET_F_CTRL_VQ))
>  		vi->has_cvq = true;
>  
> @@ -1729,6 +1754,7 @@ static unsigned int features[] = {
>  	VIRTIO_NET_F_CTRL_RX, VIRTIO_NET_F_CTRL_VLAN,
>  	VIRTIO_NET_F_GUEST_ANNOUNCE, VIRTIO_NET_F_MQ,
>  	VIRTIO_NET_F_CTRL_MAC_ADDR,
> +	VIRTIO_NET_F_ANY_HEADER_SG,
>  };
>  
>  static struct virtio_driver virtio_net_driver = {
> diff --git a/include/uapi/linux/virtio_net.h b/include/uapi/linux/virtio_net.h
> index c520203..9c98b7d 100644
> --- a/include/uapi/linux/virtio_net.h
> +++ b/include/uapi/linux/virtio_net.h
> @@ -55,6 +55,8 @@
>  					 * Steering */
>  #define VIRTIO_NET_F_CTRL_MAC_ADDR 23	/* Set MAC address */
>  
> +#define VIRTIO_NET_F_ANY_HEADER_SG 25	/* Host can handle any header s/g */
> +
>  #define VIRTIO_NET_S_LINK_UP	1	/* Link is up */
>  #define VIRTIO_NET_S_ANNOUNCE	2	/* Announcement is needed */
>  
> @@ -70,7 +72,9 @@ struct virtio_net_config {
>  	__u16 max_virtqueue_pairs;
>  } __attribute__((packed));
>  
> -/* This is the first element of the scatter-gather list.  If you don't
> +/* This header comes first in the scatter-gather list.
> + * If VIRTIO_NET_F_ANY_HEADER_SG is not negotiated, it must
> + * be the first element of the scatter-gather list.  If you don't
>   * specify GSO or CSUM features, you can simply ignore the header. */
>  struct virtio_net_hdr {
>  #define VIRTIO_NET_HDR_F_NEEDS_CSUM	1	// Use csum_start, csum_offset