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

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Date:	Tue, 7 Jan 2014 11:52:37 -0800
From:	Tom Herbert <therbert@...gle.com>
To:	Or Gerlitz <or.gerlitz@...il.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 11:37 AM, Or Gerlitz <or.gerlitz@...il.com> wrote:
> 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
>
These arguments may have applied twenty years ago, but I seriously
doubt this is going to outwit any modern compiler into producing more
efficient code. To me, it's just making code less readable.

>>
>> > +                       NAPI_GRO_CB(p)->same_flow = 0;
>> > +                       continue;
>> > +               }
>> > +               goto found;
>> > +       }
>
>
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