Subject: Re: [Xen-devel] [PATCHv1 net] xen-netback: use skb to determine number of required guest Rx requests

From: David Vrabel <[email-masked]>
Date: Fri, 15 Jan 2016 10:31:57 +0000

On 14/01/16 21:54, David Miller wrote:
> From: David Vrabel <[email-masked]>
> Date: Thu, 14 Jan 2016 15:18:30 +0000
> 
>> -	needed = xenvif_rx_ring_slots_needed(queue->vif);
>> +	skb = skb_peek(&queue->rx_queue);
>> +	if (!skb)
>> +		return false;
>> +
>> +	needed = DIV_ROUND_UP(skb->len, XEN_PAGE_SIZE);
>> +	if (skb_is_gso(skb))
>> +		needed++;
> 
> If I am not mistaken, we moved away from this kind of test exactly because
> it is inaccurate and may under-estimate the needs.
> 
> It is possible for an N byte SKB to require N segments.  Therefore, the:
> 
> 	DIV_ROUND_UP(skb->len, XEN_PAGE_SIZE);
> 
> calculation doesn't cut it.

After 1650d5455bd2dc6b5ee134bd6fc1a3236c266b5b (xen-netback: always
fully coalesce guest Rx packets) we always fully pack a packet into its
guest Rx slots.  Each slot has space for XEN_PAGE_SIZE bytes so this
calculation for the number of slots is correct.

Shall I resend with a more description changelog?

David