Subject: Re: [PATCH RFC 2/4] virtio_net: get rid of virtio_net_hdr/skb_vnet_hdr

From: Rusty Russell <[email-masked]>
Date: Tue, 28 Oct 2014 10:57:52 +1030

"Michael S. Tsirkin" <[email-masked]> writes:
> virtio 1.0 doesn't use virtio_net_hdr anymore, and in fact, it's not
> really useful since virtio_net_hdr_mrg_rxbuf includes that as the first
> field anyway.
>
> Let's drop it, precalculate header len and store within vi instead.
>
> This way we can also remove struct skb_vnet_hdr.

Yes, this is definitely a win.

Acked-by: Rusty Russell <[email-masked]>

Thanks,
Rusty.

>
> Signed-off-by: Michael S. Tsirkin <[email-masked]>
> ---
>  drivers/net/virtio_net.c | 88 ++++++++++++++++++++++--------------------------
>  1 file changed, 40 insertions(+), 48 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 36f3dfc..a795a23 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -123,6 +123,9 @@ struct virtnet_info {
>  	/* Host can handle any s/g split between our header and packet data */
>  	bool any_header_sg;
>  
> +	/* Packet virtio header size */
> +	u8 hdr_len;
> +
>  	/* Active statistics */
>  	struct virtnet_stats __percpu *stats;
>  
> @@ -139,21 +142,14 @@ struct virtnet_info {
>  	struct notifier_block nb;
>  };
>  
> -struct skb_vnet_hdr {
> -	union {
> -		struct virtio_net_hdr hdr;
> -		struct virtio_net_hdr_mrg_rxbuf mhdr;
> -	};
> -};
> -
>  struct padded_vnet_hdr {
> -	struct virtio_net_hdr hdr;
> +	struct virtio_net_hdr_mrg_rxbuf hdr;
>  	/*
> -	 * virtio_net_hdr should be in a separated sg buffer because of a
> -	 * QEMU bug, and data sg buffer shares same page with this header sg.
> -	 * This padding makes next sg 16 byte aligned after virtio_net_hdr.
> +	 * hdr is in a separate sg buffer, and data sg buffer shares same page
> +	 * with this header sg. This padding makes next sg 16 byte aligned
> +	 * after the header.
>  	 */
> -	char padding[6];
> +	char padding[4];
>  };
>  
>  /* Converting between virtqueue no. and kernel tx/rx queue no.
> @@ -179,9 +175,9 @@ static int rxq2vq(int rxq)
>  	return rxq * 2;
>  }
>  
> -static inline struct skb_vnet_hdr *skb_vnet_hdr(struct sk_buff *skb)
> +static inline struct virtio_net_hdr_mrg_rxbuf *skb_vnet_hdr(struct sk_buff *skb)
>  {
> -	return (struct skb_vnet_hdr *)skb->cb;
> +	return (struct virtio_net_hdr_mrg_rxbuf *)skb->cb;
>  }
>  
>  /*
> @@ -247,7 +243,7 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
>  				   unsigned int len, unsigned int truesize)
>  {
>  	struct sk_buff *skb;
> -	struct skb_vnet_hdr *hdr;
> +	struct virtio_net_hdr_mrg_rxbuf *hdr;
>  	unsigned int copy, hdr_len, hdr_padded_len;
>  	char *p;
>  
> @@ -260,13 +256,11 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
>  
>  	hdr = skb_vnet_hdr(skb);
>  
> -	if (vi->mergeable_rx_bufs) {
> -		hdr_len = sizeof hdr->mhdr;
> -		hdr_padded_len = sizeof hdr->mhdr;
> -	} else {
> -		hdr_len = sizeof hdr->hdr;
> +	hdr_len = vi->hdr_len;
> +	if (vi->mergeable_rx_bufs)
> +		hdr_padded_len = sizeof *hdr;
> +	else
>  		hdr_padded_len = sizeof(struct padded_vnet_hdr);
> -	}
>  
>  	memcpy(hdr, p, hdr_len);
>  
> @@ -317,11 +311,11 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
>  	return skb;
>  }
>  
> -static struct sk_buff *receive_small(void *buf, unsigned int len)
> +static struct sk_buff *receive_small(struct virtnet_info *vi, void *buf, unsigned int len)
>  {
>  	struct sk_buff * skb = buf;
>  
> -	len -= sizeof(struct virtio_net_hdr);
> +	len -= vi->hdr_len;
>  	skb_trim(skb, len);
>  
>  	return skb;
> @@ -354,8 +348,8 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
>  					 unsigned int len)
>  {
>  	void *buf = mergeable_ctx_to_buf_address(ctx);
> -	struct skb_vnet_hdr *hdr = buf;
> -	u16 num_buf = virtio16_to_cpu(rq->vq->vdev, hdr->mhdr.num_buffers);
> +	struct virtio_net_hdr_mrg_rxbuf *hdr = buf;
> +	u16 num_buf = virtio16_to_cpu(vi->vdev, hdr->num_buffers);
>  	struct page *page = virt_to_head_page(buf);
>  	int offset = buf - page_address(page);
>  	unsigned int truesize = max(len, mergeable_ctx_to_buf_truesize(ctx));
> @@ -373,8 +367,8 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
>  		if (unlikely(!ctx)) {
>  			pr_debug("%s: rx error: %d buffers out of %d missing\n",
>  				 dev->name, num_buf,
> -				 virtio16_to_cpu(rq->vq->vdev,
> -						 hdr->mhdr.num_buffers));
> +				 virtio16_to_cpu(vi->vdev,
> +						 hdr->num_buffers));
>  			dev->stats.rx_length_errors++;
>  			goto err_buf;
>  		}
> @@ -441,7 +435,7 @@ static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq,
>  	struct net_device *dev = vi->dev;
>  	struct virtnet_stats *stats = this_cpu_ptr(vi->stats);
>  	struct sk_buff *skb;
> -	struct skb_vnet_hdr *hdr;
> +	struct virtio_net_hdr_mrg_rxbuf *hdr;
>  
>  	if (unlikely(len < sizeof(struct virtio_net_hdr) + ETH_HLEN)) {
>  		pr_debug("%s: short packet %i\n", dev->name, len);
> @@ -536,7 +530,7 @@ static int add_recvbuf_small(struct virtnet_info *vi, struct receive_queue *rq,
>  			     gfp_t gfp)
>  {
>  	struct sk_buff *skb;
> -	struct skb_vnet_hdr *hdr;
> +	struct virtio_net_hdr_mrg_rxbuf *hdr;
>  	int err;
>  
>  	skb = __netdev_alloc_skb_ip_align(vi->dev, GOOD_PACKET_LEN, gfp);
> @@ -547,7 +541,7 @@ static int add_recvbuf_small(struct virtnet_info *vi, struct receive_queue *rq,
>  
>  	hdr = skb_vnet_hdr(skb);
>  	sg_init_table(rq->sg, MAX_SKB_FRAGS + 2);
> -	sg_set_buf(rq->sg, &hdr->hdr, sizeof hdr->hdr);
> +	sg_set_buf(rq->sg, hdr, vi->hdr_len);
>  	skb_to_sgvec(skb, rq->sg + 1, 0, skb->len);
>  
>  	err = virtqueue_add_inbuf(rq->vq, rq->sg, 2, skb, gfp);
> @@ -557,7 +551,8 @@ static int add_recvbuf_small(struct virtnet_info *vi, struct receive_queue *rq,
>  	return err;
>  }
>  
> -static int add_recvbuf_big(struct receive_queue *rq, gfp_t gfp)
> +static int add_recvbuf_big(struct virtnet_info *vi, struct receive_queue *rq,
> +			   gfp_t gfp)
>  {
>  	struct page *first, *list = NULL;
>  	char *p;
> @@ -588,8 +583,8 @@ static int add_recvbuf_big(struct receive_queue *rq, gfp_t gfp)
>  	p = page_address(first);
>  
>  	/* rq->sg[0], rq->sg[1] share the same page */
> -	/* a separated rq->sg[0] for virtio_net_hdr only due to QEMU bug */
> -	sg_set_buf(&rq->sg[0], p, sizeof(struct virtio_net_hdr));
> +	/* a separated rq->sg[0] for header - required in case !any_header_sg */
> +	sg_set_buf(&rq->sg[0], p, vi->hdr_len);
>  
>  	/* rq->sg[1] for data packet, from offset */
>  	offset = sizeof(struct padded_vnet_hdr);
> @@ -668,7 +663,7 @@ static bool try_fill_recv(struct virtnet_info *vi, struct receive_queue *rq,
>  		if (vi->mergeable_rx_bufs)
>  			err = add_recvbuf_mergeable(rq, gfp);
>  		else if (vi->big_packets)
> -			err = add_recvbuf_big(rq, gfp);
> +			err = add_recvbuf_big(vi, rq, gfp);
>  		else
>  			err = add_recvbuf_small(vi, rq, gfp);
>  
> @@ -848,18 +843,14 @@ 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;
> +	struct virtio_net_hdr_mrg_rxbuf *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;
> +	unsigned hdr_len = vi->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)) &&
> @@ -867,7 +858,7 @@ static int xmit_skb(struct send_queue *sq, struct sk_buff *skb)
>  	/* 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);
> +		hdr = (struct virtio_net_hdr_mrg_rxbuf *)(skb->data - hdr_len);
>  	else
>  		hdr = skb_vnet_hdr(skb);
>  
> @@ -902,7 +893,7 @@ static int xmit_skb(struct send_queue *sq, struct sk_buff *skb)
>  	}
>  
>  	if (vi->mergeable_rx_bufs)
> -		hdr->mhdr.num_buffers = 0;
> +		hdr->num_buffers = 0;
>  
>  	sg_init_table(sq->sg, MAX_SKB_FRAGS + 2);
>  	if (can_push) {
> @@ -1773,18 +1764,19 @@ 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_MRG_RXBUF))
> +		vi->hdr_len = sizeof(struct virtio_net_hdr_mrg_rxbuf);
> +	else
> +		vi->hdr_len = sizeof(struct virtio_net_hdr);
> +
>  	if (virtio_has_feature(vdev, VIRTIO_F_ANY_LAYOUT))
>  		vi->any_header_sg = true;
>  
>  	if (virtio_has_feature(vdev, VIRTIO_NET_F_CTRL_VQ))
>  		vi->has_cvq = true;
>  
> -	if (vi->any_header_sg) {
> -		if (vi->mergeable_rx_bufs)
> -			dev->needed_headroom = sizeof(struct virtio_net_hdr_mrg_rxbuf);
> -		else
> -			dev->needed_headroom = sizeof(struct virtio_net_hdr);
> -	}
> +	if (vi->any_header_sg)
> +		dev->needed_headroom = vi->hdr_len;
>  
>  	/* Use single tx/rx queue pair as default */
>  	vi->curr_queue_pairs = 1;
> -- 
> MST