Subject: Re: [PATCH net-next 2/5] virtio-net: pack headroom into ctx for mergeable buffer

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 18 Jul 2017 21:59:33 +0300

On Mon, Jul 17, 2017 at 08:43:58PM +0800, Jason Wang wrote:
> Pack headroom into ctx, then during XDP set, we could know the size of
> headroom and copy if needed. This is required for avoiding reset on
> XDP.

Not really when XDP is set - it's when buffers are used.

virtio-net: pack headroom into ctx for mergeable buffers

Pack headroom into ctx - this way when we get a buffer we can figure out
the actual headroom that was allocated for the buffer. Will be helpful
to optimize switching between XDP and non-XDP modes which have different
headroom requirements.

> 
> Signed-off-by: Jason Wang <[email-masked]>
> ---
>  drivers/net/virtio_net.c | 29 ++++++++++++++++++++++++-----
>  1 file changed, 24 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 1f8c15c..8fae9a8 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -270,6 +270,23 @@ static void skb_xmit_done(struct virtqueue *vq)
>  		netif_wake_subqueue(vi->dev, vq2txq(vq));
>  }
>  
> +#define MRG_CTX_HEADER_SHIFT 22
> +static void *mergeable_len_to_ctx(unsigned int truesize,
> +				  unsigned int headroom)
> +{
> +	return (void *)(unsigned long)((headroom << MRG_CTX_HEADER_SHIFT) | truesize);
> +}
> +
> +static unsigned int mergeable_ctx_to_headroom(void *mrg_ctx)
> +{
> +	return (unsigned long)mrg_ctx >> MRG_CTX_HEADER_SHIFT;
> +}
> +
> +static unsigned int mergeable_ctx_to_truesize(void *mrg_ctx)
> +{
> +	return (unsigned long)mrg_ctx & ((1 << MRG_CTX_HEADER_SHIFT) - 1);
> +}
> +
>  /* Called from bottom half context */
>  static struct sk_buff *page_to_skb(struct virtnet_info *vi,
>  				   struct receive_queue *rq,
> @@ -639,13 +656,14 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
>  	}
>  	rcu_read_unlock();
>  
> -	if (unlikely(len > (unsigned long)ctx)) {
> +	truesize = mergeable_ctx_to_truesize(ctx);
> +	if (unlikely(len > truesize)) {
>  		pr_debug("%s: rx error: len %u exceeds truesize %lu\n",
>  			 dev->name, len, (unsigned long)ctx);
>  		dev->stats.rx_length_errors++;
>  		goto err_skb;
>  	}
> -	truesize = (unsigned long)ctx;
> +
>  	head_skb = page_to_skb(vi, rq, page, offset, len, truesize);
>  	curr_skb = head_skb;
>  
> @@ -665,13 +683,14 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
>  		}
>  
>  		page = virt_to_head_page(buf);
> -		if (unlikely(len > (unsigned long)ctx)) {
> +
> +		truesize = mergeable_ctx_to_truesize(ctx);
> +		if (unlikely(len > truesize)) {
>  			pr_debug("%s: rx error: len %u exceeds truesize %lu\n",
>  				 dev->name, len, (unsigned long)ctx);
>  			dev->stats.rx_length_errors++;
>  			goto err_skb;
>  		}
> -		truesize = (unsigned long)ctx;
>  
>  		num_skb_frags = skb_shinfo(curr_skb)->nr_frags;
>  		if (unlikely(num_skb_frags == MAX_SKB_FRAGS)) {
> @@ -889,7 +908,7 @@ static int add_recvbuf_mergeable(struct virtnet_info *vi,
>  
>  	buf = (char *)page_address(alloc_frag->page) + alloc_frag->offset;
>  	buf += headroom; /* advance address leaving hole at front of pkt */
> -	ctx = (void *)(unsigned long)len;
> +	ctx = mergeable_len_to_ctx(len, headroom);
>  	get_page(alloc_frag->page);
>  	alloc_frag->offset += len + headroom;
>  	hole = alloc_frag->size - alloc_frag->offset;
> -- 
> 2.7.4