From mboxrd@z Thu Jan  1 00:00:00 1970
From: Stephen Hemminger <stephen@networkplumber.org>
Subject: Re: [PATCH v2 12/12] virtio: add Tso support
Date: Thu, 13 Oct 2016 16:33:30 -0700
Message-ID: <20161013163330.584b05db@xeon-e3>
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 <20161013081839.GT16751@yliu-dev.sh.intel.com>
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Cc: Olivier Matz <olivier.matz@6wind.com>, dev@dpdk.org,
 konstantin.ananyev@intel.com, sugesh.chandran@intel.com,
 bruce.richardson@intel.com, jianfeng.tan@intel.com, helin.zhang@intel.com,
 adrien.mazarguil@6wind.com, dprovan@bivio.net, xiao.w.wang@intel.com
To: Yuanhan Liu <yuanhan.liu@linux.intel.com>
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On Thu, 13 Oct 2016 16:18:39 +0800
Yuanhan Liu <yuanhan.liu@linux.intel.com> wrote:

> On Mon, Oct 03, 2016 at 11:00:23AM +0200, Olivier Matz wrote:
> > +/* When doing TSO, the IP length is not included in the pseudo header
> > + * checksum of the packet given to the PMD, but for virtio it is
> > + * expected.
> > + */
> > +static void
> > +virtio_tso_fix_cksum(struct rte_mbuf *m)
> > +{
> > +	/* common case: header is not fragmented */
> > +	if (likely(rte_pktmbuf_data_len(m) >= m->l2_len + m->l3_len +
> > +			m->l4_len)) {  
> ...
> > +		/* replace it in the packet */
> > +		th->cksum = new_cksum;
> > +	} else {  
> ...
> > +		/* replace it in the packet */
> > +		*rte_pktmbuf_mtod_offset(m, uint8_t *,
> > +			m->l2_len + m->l3_len + 16) = new_cksum.u8[0];
> > +		*rte_pktmbuf_mtod_offset(m, uint8_t *,
> > +			m->l2_len + m->l3_len + 17) = new_cksum.u8[1];
> > +	}  
> 
> The tcp header will always be in the mbuf, right? Otherwise, you can't
> update the cksum field here. What's the point of introducing the "else
> clause" then?
> 
> 	--yliu

You need to check the reference count before updating any data in mbuf.