Re: [Xen-devel] [PATCH v2 1/1] xen/netback: correctly calculate required slots of skb.

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Wed, 2013-07-10 at 17:15 +0800, Annie Li wrote:
> +static int netbk_count_slots(struct xenvif *vif, struct sk_buff *skb,
> +                         int *copy_off, unsigned long size,
> +                         unsigned long offset, int *head)
>  {
> -     unsigned int count;
> -     int i, copy_off;
> +     unsigned long bytes;
> +     int count = 0;
>  
> -     count = DIV_ROUND_UP(skb_headlen(skb), PAGE_SIZE);
> +     offset &= ~PAGE_MASK;
>  
> -     copy_off = skb_headlen(skb) % PAGE_SIZE;
> +     while (size > 0) {
> +             BUG_ON(offset >= PAGE_SIZE);
> +             BUG_ON(*copy_off > MAX_BUFFER_OFFSET);
>  
> -     if (skb_shinfo(skb)->gso_size)
> -             count++;
> +             bytes = PAGE_SIZE - offset;
>  
> -     for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
> -             unsigned long size = skb_frag_size(&skb_shinfo(skb)->frags[i]);
> -             unsigned long offset = skb_shinfo(skb)->frags[i].page_offset;
> -             unsigned long bytes;
> +             if (bytes > size)
> +                     bytes = size;
>  
> -             offset &= ~PAGE_MASK;
> +             if (start_new_rx_buffer(*copy_off, bytes, *head)) {
> +                     count++;
> +                     *copy_off = 0;
> +             }
>  
> -             while (size > 0) {
> -                     BUG_ON(offset >= PAGE_SIZE);
> -                     BUG_ON(copy_off > MAX_BUFFER_OFFSET);
> +             if (*copy_off + bytes > MAX_BUFFER_OFFSET)
> +                     bytes = MAX_BUFFER_OFFSET - *copy_off;
>  
> -                     bytes = PAGE_SIZE - offset;
> +             *copy_off += bytes;
>  
> -                     if (bytes > size)
> -                             bytes = size;
> +             offset += bytes;
> +             size -= bytes;
>  
> -                     if (start_new_rx_buffer(copy_off, bytes, 0)) {
> -                             count++;
> -                             copy_off = 0;
> -                     }
> +             /* Next frame */
> +             if (offset == PAGE_SIZE && size)
> +                     offset = 0;
> +
> +             if (*head)
> +                     count++;

This little bit corresponds to the "/* Leave a gap for the GSO
descriptor. */" in gop_frag_copy?

If so then it would be useful to duplicate the comment, but more
importantly the condition on gop_frag_copy is:
        (head && skb_shinfo(skb)->gso_size && !vif->gso_prefix)
why the difference?

> +             *head = 0; /* There must be something in this buffer now. */
> +     }
> +
> +     return count;
> +}



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