[dpdk-dev] [PATCH v3 2/5] gso: add TCP/IPv4 GSO support

[Hu, Jiayu]

Hi Konstantin,

> -----Original Message-----
> From: Ananyev, Konstantin
> Sent: Wednesday, September 13, 2017 11:13 PM
> To: Kavanagh, Mark B <mark.b.kavanagh at intel.com>; Hu, Jiayu
> <jiayu.hu at intel.com>
> Cc: dev at dpdk.org; Tan, Jianfeng <jianfeng.tan at intel.com>
> Subject: RE: [PATCH v3 2/5] gso: add TCP/IPv4 GSO support
> 
> Hi Mark,
> 
> > -----Original Message-----
> > From: Kavanagh, Mark B
> > Sent: Wednesday, September 13, 2017 3:52 PM
> > To: Ananyev, Konstantin <konstantin.ananyev at intel.com>; Hu, Jiayu
> <jiayu.hu at intel.com>
> > Cc: dev at dpdk.org; Tan, Jianfeng <jianfeng.tan at intel.com>
> > Subject: RE: [PATCH v3 2/5] gso: add TCP/IPv4 GSO support
> >
> > >From: Ananyev, Konstantin
> > >Sent: Wednesday, September 13, 2017 10:38 AM
> > >To: Hu, Jiayu <jiayu.hu at intel.com>
> > >Cc: dev at dpdk.org; Kavanagh, Mark B <mark.b.kavanagh at intel.com>;
> Tan, Jianfeng
> > ><jianfeng.tan at intel.com>
> > >Subject: RE: [PATCH v3 2/5] gso: add TCP/IPv4 GSO support
> > >
> > >
> > >
> > >> > > +
> > >> > > +int
> > >> > > +gso_tcp4_segment(struct rte_mbuf *pkt,
> > >> > > +		uint16_t gso_size,
> > >> > > +		uint8_t ipid_delta,
> > >> > > +		struct rte_mempool *direct_pool,
> > >> > > +		struct rte_mempool *indirect_pool,
> > >> > > +		struct rte_mbuf **pkts_out,
> > >> > > +		uint16_t nb_pkts_out)
> > >> > > +{
> > >> > > +	struct ipv4_hdr *ipv4_hdr;
> > >> > > +	uint16_t tcp_dl;
> > >> > > +	uint16_t pyld_unit_size;
> > >> > > +	uint16_t hdr_offset;
> > >> > > +	int ret = 1;
> > >> > > +
> > >> > > +	ipv4_hdr = (struct ipv4_hdr *)(rte_pktmbuf_mtod(pkt, char *)
> +
> > >> > > +			pkt->l2_len);
> > >> > > +	/* Don't process the fragmented packet */
> > >> > > +	if (unlikely((ipv4_hdr->fragment_offset & rte_cpu_to_be_16(
> > >> > > +
> 	IPV4_HDR_DF_MASK)) == 0)) {
> > >> >
> > >> >
> > >> > It is not a check for fragmented packet - it is a check that
> fragmentation
> > >is allowed for that packet.
> > >> > Should be IPV4_HDR_DF_MASK - 1,  I think.
> > >
> > >DF bit doesn't indicate is packet fragmented or not.
> > >It forbids to fragment packet any further.
> > >To check is packet already fragmented or not, you have to check MF bit
> and
> > >frag_offset.
> > >Both have to be zero for un-fragmented packets.
> > >
> > >>
> > >> IMO, IPV4_HDR_DF_MASK whose value is (1 << 14) is used to get DF bit.
> It's a
> > >> little-endian value. But ipv4_hdr->fragment_offset is big-endian order.
> > >> So the value of DF bit should be "ipv4_hdr->fragment_offset &
> > >rte_cpu_to_be_16(
> > >> IPV4_HDR_DF_MASK)". If this value is 0, the input packet is fragmented.
> > >>
> > >> >
> > >> > > +		pkts_out[0] = pkt;
> > >> > > +		return ret;
> > >> > > +	}
> > >> > > +
> > >> > > +	tcp_dl = rte_be_to_cpu_16(ipv4_hdr->total_length) - pkt-
> >l3_len -
> > >> > > +		pkt->l4_len;
> > >> >
> > >> > Why not use pkt->pkt_len - pkt->l2_len -pkt_l3_len - pkt_l4_len?
> > >>
> > >> Yes, we can use pkt->pkt_len - pkt->l2_len -pkt_l3_len - pkt_l4_len here.
> > >>
> > >> >
> > >> > > +	/* Don't process the packet without data */
> > >> > > +	if (unlikely(tcp_dl == 0)) {
> > >> > > +		pkts_out[0] = pkt;
> > >> > > +		return ret;
> > >> > > +	}
> > >> > > +
> > >> > > +	hdr_offset = pkt->l2_len + pkt->l3_len + pkt->l4_len;
> > >> > > +	pyld_unit_size = gso_size - hdr_offset - ETHER_CRC_LEN;
> > >> >
> > >> > Hmm, why do we need to count CRC_LEN here?
> > >>
> > >> Yes, we shouldn't count ETHER_CRC_LEN here. Its length should be
> > >> included in gso_size.
> > >
> > >Why?
> > >What is the point to account crc len into this computation?
> > >Why not just assume that gso_size is already a max_frame_size - crc_len
> > >As I remember, when we RX packet crc bytes will be already stripped,
> > >when user populates the packet, he doesn't care about crc bytes too.
> >
> > Hi Konstantin,
> >
> > When packet is tx'd, the 4B for CRC are added back into the packet; if the
> payload is already at max capacity, then the actual segment size
> > will be 4B larger than expected (e.g. 1522B, as opposed to 1518B).
> > To prevent that from happening, we account for the CRC len in this
> calculation.
> 
> 
> Ok, and what prevents you to set gso_ctx.gso_size = 1514;  /*ether frame
> size without crc bytes */
> ?

Exactly, applications can set 1514 to gso_segsz instead of 1518, if the lower layer
will add CRC to the packet.

Jiayu

> Konstantin
> 
> >
> > If I've missed anything, please do let me know!
> >
> > Thanks,
> > Mark
> >
> > >
> > >Konstantin