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

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

> > > +		/* 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.

Didn't understand that one...
Why you suddenly need to pad non-first segments with zeroes?
Why simply rte_raw_cksum() can't be used for multi-seg case?

> Also, mbuf is not passed down to process_inner/outer_chksum so the change will be a lot.

I also think that copying whole packet just to calculate a checksum - way too much overhead.