Re: [PATCH net-next] xen-netback: fix xenvif_count_skb_slots()

[Matt Wilson <msw@linux.com>]

On Fri, Oct 04, 2013 at 05:26:23PM +0100, Paul Durrant wrote:
> Commit 4f0581d25827d5e864bcf07b05d73d0d12a20a5c introduced an error into
> xenvif_count_skb_slots() for skbs with a linear area spanning a page
> boundary. The alignment of skb->data needs to be taken into account, not
> just the head length. This patch fixes the issue by dry-running the code
> from xenvif_gop_skb() (and adjusting the comment above the function to note
> that).
> 
> Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
> Cc: Xi Xiong <xixiong@amazon.com>
> Cc: Matt Wilson <msw@amazon.com>
> Cc: Annie Li <annie.li@oracle.com>
> Cc: Wei Liu <wei.liu2@citrix.com>
> Cc: Ian Campbell <Ian.Campbell@citrix.com>

Paul, can you reconcile this change with the one made by Simon in cs
8f985b4f7a5394c8f8725a5109451a541ddb9eea?

--msw

> ---
>  drivers/net/xen-netback/netback.c |   17 +++++++++++++++--
>  1 file changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
> index d0b0feb..6f680f4 100644
> --- a/drivers/net/xen-netback/netback.c
> +++ b/drivers/net/xen-netback/netback.c
> @@ -223,15 +223,28 @@ static bool start_new_rx_buffer(int offset, unsigned long size, int head)
>  /*
>   * Figure out how many ring slots we're going to need to send @skb to
>   * the guest. This function is essentially a dry run of
> - * xenvif_gop_frag_copy.
> + * xenvif_gop_skb.
>   */
>  unsigned int xenvif_count_skb_slots(struct xenvif *vif, struct sk_buff *skb)
>  {
> +	unsigned char *data;
>  	unsigned int count;
>  	int i, copy_off;
>  	struct skb_cb_overlay *sco;
>  
> -	count = DIV_ROUND_UP(skb_headlen(skb), PAGE_SIZE);
> +	count = 0;
> +
> +	data = skb->data;
> +	while (data < skb_tail_pointer(skb)) {
> +		unsigned int offset = offset_in_page(data);
> +		unsigned int len = PAGE_SIZE - offset;
> +
> +		if (data + len > skb_tail_pointer(skb))
> +			len = skb_tail_pointer(skb) - data;
> +
> +		count++;
> +		data += len;
> +	}
>  
>  	copy_off = skb_headlen(skb) % PAGE_SIZE;
>