netdev - Re: [PATCH net-next V2 3/3] net: Add GRO support for vxlan traffic

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Date:	Tue, 7 Jan 2014 21:43:06 +0200
From:	Or Gerlitz <or.gerlitz@...il.com>
To:	Tom Herbert <therbert@...gle.com>
Cc:	Or Gerlitz <ogerlitz@...lanox.com>, Jerry Chu <hkchu@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Linux Netdev List <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	Yan Burman <yanb@...lanox.com>,
	Shlomo Pongratz <shlomop@...lanox.com>
Subject: Re: [PATCH net-next V2 3/3] net: Add GRO support for vxlan traffic

On Tue, Jan 7, 2014 at 8:08 PM, Tom Herbert <therbert@...gle.com> wrote:
> On Tue, Jan 7, 2014 at 7:29 AM, Or Gerlitz <ogerlitz@...lanox.com> wrote:
>> +static struct sk_buff **vxlan_gro_receive(struct sk_buff **head, struct sk_buff *skb)
>> +{
>> +       struct sk_buff *p, **pp = NULL;
>> +       struct vxlanhdr *vh, *vh2;
>> +       struct ethhdr *eh;
>> +       unsigned int hlen, off, off_eth;
>> +       const struct packet_offload *ptype;
>> +       __be16 type;
>> +       int flush = 1;
>> +
>> +       off  = skb_gro_offset(skb);
>> +       hlen = off + sizeof(*vh);
>> +       vh   = skb_gro_header_fast(skb, off);
>> +       if (skb_gro_header_hard(skb, hlen)) {
>> +               vh = skb_gro_header_slow(skb, hlen, off);
>> +               if (unlikely(!vh))
>> +                       goto out;
>> +       }
>> +
>> +       flush = 0;
>> +
>> +       for (p = *head; p; p = p->next) {
>> +               if (!NAPI_GRO_CB(p)->same_flow)
>> +                       continue;
>> +
>> +               vh2 = (struct vxlanhdr   *)(p->data + off);
>> +               if (vh->vx_vni ^ vh2->vx_vni) {
>
> Why ^ instead of != ?

The XOR approach is very popular in the GRO stack, e.g see the IPv4 chain
of inet_gro_receive() && tcp_gro_receive(), I guess this might relates
to more efficient assembly code for ^ vs. != and/or the fast/elegant
transitive nature of that operator
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