Re: [dpdk-dev] [PATCH] app/testpmd: fix l4 sw csum over multi segments

["Li, Xiaoyun" <xiaoyun.li@intel.com>]

Hi

> -----Original Message-----
> From: Stephen Hemminger <stephen@networkplumber.org>
> Sent: Monday, October 18, 2021 11:00
> To: Li, Xiaoyun <xiaoyun.li@intel.com>
> Cc: Yigit, Ferruh <ferruh.yigit@intel.com>; dev@dpdk.org; stable@dpdk.org
> Subject: Re: [dpdk-dev] [PATCH] app/testpmd: fix l4 sw csum over multi
> segments
> 
> On Fri, 15 Oct 2021 13:13:06 +0800
> Xiaoyun Li <xiaoyun.li@intel.com> wrote:
> 
> > +		/* When sw csum is needed, multi-segs needs a buf to contain
> > +		 * the whole packet for later UDP/TCP csum calculation.
> > +		 */
> > +		if (m->nb_segs > 1 && !(tx_ol_flags & PKT_TX_TCP_SEG) &&
> > +		    !(tx_offloads & UDP_TCP_CSUM)) {
> > +			l3_buf = rte_zmalloc("csum l3_buf",
> > +					     info.pkt_len - info.l2_len,
> > +					     RTE_CACHE_LINE_SIZE);
> > +			rte_pktmbuf_read(m, info.l2_len,
> > +					 info.pkt_len - info.l2_len, l3_buf);
> > +			l3_hdr = l3_buf;
> > +		} else
> > +			l3_hdr = (char *)eth_hdr + info.l2_len;
> >
> 
> Rather than copying whole packet, make the code handle checksum streaming.

Copying is the easiest way to do this.

The problem of handling checksum streaming is that in the first segment, l2 and l3 hdr len is 14 bytes when checksum takes 4 bytes each time.
If the datalen of the first segment is 4 bytes aligned (usual case), for the second segment and the following segments, they may need to add a special 2 bytes 0x0 at the start.
Also, mbuf is not passed down to process_inner/outer_chksum so the change will be a lot.