Date: Mon, 25 Aug 2008 09:45:42 -0400
From: Stephen Hemminger <shemminger@vyatta.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Karen Xie <kxie@chelsio.com>, netdev@vger.kernel.org,
       open-iscsi@googlegroups.com, linux-scsi@vger.kernel.org,
       linux-kernel@vger.kernel.org, jgarzik@pobox.com, davem@davemloft.net,
       michaelc@cs.wisc.edu, swise@opengridcomputing.com, rdreier@cisco.com,
       daisyc@us.ibm.com, wenxiong@us.ibm.com, bhua@us.ibm.com,
       divy@chelsio.com, dm@chelsio.com, leedom@chelsio.com
Subject: Re: [PATCH 2/4 2.6.28] cxgb3 - handle ARP replies for private iSCSI
 IP address
Message-ID: <20080825094542.27dbed2b@speedy>
In-Reply-To: <20080822121250.55949d3d.akpm@linux-foundation.org>
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	<20080822121250.55949d3d.akpm@linux-foundation.org>
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On Fri, 22 Aug 2008 12:12:50 -0700
Andrew Morton <akpm@linux-foundation.org> wrote:

> On Fri, 22 Aug 2008 11:39:08 -0700
> Karen Xie <kxie@chelsio.com> wrote:
> 
> > [PATCH 2/4 2.6.28] cxgb3 - handle ARP replies for private iSCSI IP address
> > 
> > From: Karen Xie <kxie@chelsio.com>
> > 
> > stg can be tricky ...
> > [adding @adapter in cxgb3_arp_process doxygen header]
> > The accelerated iSCSI traffic uses a private IP address unknown to the OS.
> > The driver has to reply to ARP requests dedicated to the private IP address.
> > 
> > Signed-off-by: Divy Le Ray <divy@chelsio.com>
> > ---
> > 
> >  drivers/net/cxgb3/sge.c |   71 ++++++++++++++++++++++++++++++++++++++++++++---
> >  1 files changed, 66 insertions(+), 5 deletions(-)
> > 
> > diff --git a/drivers/net/cxgb3/sge.c b/drivers/net/cxgb3/sge.c
> > index 1b0861d..d2a9285 100644
> > --- a/drivers/net/cxgb3/sge.c
> > +++ b/drivers/net/cxgb3/sge.c
> > @@ -36,6 +36,7 @@
> >  #include <linux/ip.h>
> >  #include <linux/tcp.h>
> >  #include <linux/dma-mapping.h>
> > +#include <net/arp.h>
> >  #include "common.h"
> >  #include "regs.h"
> >  #include "sge_defs.h"
> > @@ -1859,6 +1860,54 @@ static void restart_tx(struct sge_qset *qs)
> >  }
> >  
> >  /**
> > + *	cxgb3_arp_process - process an ARP request probing a private IP address
> > + *	@adapter: the adapter
> > + *	@skb: the skbuff containing the ARP request
> > + *
> > + *	Check if the ARP request is probing the private IP address
> > + *	dedicated to iSCSI, generate an ARP reply if so.
> > + */
> > +static void cxgb3_arp_process(struct adapter *adapter, struct sk_buff *skb)
> > +{
> > +	struct net_device *dev = skb->dev;
> > +	struct port_info *pi;
> > +	struct arphdr *arp;
> > +	unsigned char *arp_ptr;
> > +	unsigned char *sha;
> > +	u32 sip, tip;
> > +
> > +	if (!dev)
> > +		return;
> 
> Can this happen?
> 
> > +	skb_reset_network_header(skb);
> > +	arp = arp_hdr(skb);
> > +
> > +	if (arp->ar_op != htons(ARPOP_REQUEST))
> > +		return;
> > +
> > +	arp_ptr = (unsigned char *)(arp + 1);
> > +	sha = arp_ptr;
> > +	arp_ptr += dev->addr_len;
> > +	memcpy(&sip, arp_ptr, 4);
> > +	arp_ptr += 4;
> > +	arp_ptr += dev->addr_len;
> > +	memcpy(&tip, arp_ptr, 4);
> 
> Should arp_hdr_len() be used here?
> 
> > +	pi = netdev_priv(dev);
> > +	if (ntohl(tip) != pi->iscsi_ipaddr)
> > +		return;
> > +
> > +	arp_send(ARPOP_REPLY, ETH_P_ARP, sip, dev, tip, sha,
> > +		 dev->dev_addr, sha);
> > +
> > +}
> > +
> > +static inline int is_arp(struct sk_buff *skb)
> > +{
> > +	return skb->protocol == htons(ETH_P_ARP);
> 
> Other net code uses __constant_htons() for this.

htons() goes through several levels of indirection to make sure
that if argument is constant, no code is necessary. The practice is
reserve __constant_htons() only for those cases like initialization and case
statements where C won't allow it.
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