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

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Message-Id: <20160115.113430.2270570646723842525.davem@davemloft.net>
Date:	Fri, 15 Jan 2016 11:34:30 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	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

From: David Vrabel <david.vrabel@...rix.com>
Date: Fri, 15 Jan 2016 10:31:57 +0000

> 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?

Yeah that would help a lot.