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

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Date:	Fri, 15 Jan 2016 10:31:57 +0000
From:	David Vrabel <david.vrabel@...rix.com>
To:	David Miller <davem@...emloft.net>, <david.vrabel@...rix.com>
CC:	<netdev@...r.kernel.org>, <wei.liu2@...rix.com>,
	<ian.campbell@...rix.com>, <xen-devel@...ts.xenproject.org>
Subject: Re: [Xen-devel] [PATCHv1 net] xen-netback: use skb to determine number
 of required guest Rx requests

On 14/01/16 21:54, David Miller wrote:
> From: David Vrabel <david.vrabel@...rix.com>
> 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