Re: [dpdk-dev] [PATCH v1] gso: fix marking TCP checksum flag in TCP segments

[Ophir Munk <ophirmu@mellanox.com>]

Hi Jiayu,
Please find comments inline

> -----Original Message-----
> From: Hu, Jiayu [mailto:jiayu.hu@intel.com]
> Sent: Monday, April 23, 2018 7:14 AM
> To: Ophir Munk <ophirmu@mellanox.com>; dev@dpdk.org; Ananyev,
> Konstantin <konstantin.ananyev@intel.com>
> Cc: Thomas Monjalon <thomas@monjalon.net>; Olga Shern
> <olgas@mellanox.com>; Pascal Mazon <pascal.mazon@6wind.com>;
> stable@dpdk.org
> Subject: RE: [PATCH v1] gso: fix marking TCP checksum flag in TCP segments
> 
> Hi Ophir,
> 
> In the GSO design, the GSO library doesn't care about checksums, which
> means it doesn't check if input packets have correct checksums, and it
> doesn't do any checksum related work for the output GSO segments. It
> depends on the callers to use HW or SW checksum calculation for output
> packets. This is why the GSO library doesn't set PKT_TX_TCP_CKSUM. So I
> don't think it's a bug.
> 

Can you please reconsider this design? I think the GSO library should imitate the HW behavior where TCP segments checksum is automatically calculated without explicitly requesting it. I am not saying that GSO library itself should calculate the checksums - but at least it should mark each segment as requiring this calculation.

> In my opinion, it's not a good idea to enable HW TCP checksum calculation
> silently, and without the aware of the caller. In fact, the caller always know it
> does SW TSO (i.e. GSO), instead of real HW TSO. 

This is not correct. Consider net_failsafe with 2 sub-devices: one is a HW PCI device, the other one is a SW TAP device. Failsafe must work transparently with these two sub-devices and the caller cannot tell if TSO is done in SW or HW. 

> If the caller wants HW
> checksum calculation, it can add PKT_TX_TCP_CKSUM to ol_flags before or
> after calling the GSO library.
> 

FYI - TAP TSO patches were submitted to dpdk.org mailing list. These patches use the GSO library.
https://dpdk.org/dev/patchwork/patch/38666/
https://dpdk.org/dev/patchwork/patch/38667/

Running testpmd with TAP TSO is currently broken without the suggested librte_gso patch. 
Please note testpmd implementation (app/test-pmd/csumonly.c b/app/test-pmd/csumonly.c) in case *both* TSO and TCP CKSUM are configured:

  if (tso_segsz)
      ol_flags |= PKT_TX_TCP_SEG;    // *** if TSO is applicable - the packet flags are only marked with PKT_TX_TCP_SEG and no PKT_TX_TCP_CKSUM ***
   else if (tx_offloads & DEV_TX_OFFLOAD_TCP_CKSUM) 
       ol_flags |= PKT_TX_TCP_CKSUM;     // *** PKT_TX_TCP_CKSUM is marked only if TSO is not applicable ***
  else {
      tcp_hdr->cksum =
         get_udptcp_checksum(l3_hdr, tcp_hdr,

In other words - testpmd does not set TCP_CKSUM along with TCP_SEG therefore using testpmd with TAP/TSO will result in TCP segments with 0 (incorrect) TCP checksums.

In addition - please note the comments in lib/librte_mbuf/rte_mbuf.h which specify that PKT_TX_TCP_SEG flag implies the PKT_TX_TCP_CKSUM (hence it is not required to be explicitly set by the caller)

/**
* TCP segmentation offload. To enable this offload feature for a
* packet to be transmitted on hardware supporting TSO:
*  - set the PKT_TX_TCP_SEG flag in mbuf->ol_flags (this flag implies
*    PKT_TX_TCP_CKSUM)
...

> Add Konstantin for more suggestions.
> 
> Thanks,
> Jiayu
> 
> > -----Original Message-----
> > From: Ophir Munk [mailto:ophirmu@mellanox.com]
> > Sent: Sunday, April 22, 2018 10:21 PM
> > To: dev@dpdk.org; Hu, Jiayu <jiayu.hu@intel.com>
> > Cc: Thomas Monjalon <thomas@monjalon.net>; Olga Shern
> > <olgas@mellanox.com>; Pascal Mazon <pascal.mazon@6wind.com>;
> Ophir
> > Munk <ophirmu@mellanox.com>; stable@dpdk.org
> > Subject: [PATCH v1] gso: fix marking TCP checksum flag in TCP segments
> >
> > Large TCP packets which are marked with PKT_TX_TCP_SEG flag are
> > segmented and the flag is cleared in the resulting segments, however,
> > the segments checksum is not updated. It is therefore required to set
> > the PKT_TX_TCP_CKSUM flag in each TCP segment in order to mark for the
> > sending driver the need to update the TCP checksum before transmitting
> > the segment.
> >
> > Fixes: 119583797b6a ("gso: support TCP/IPv4 GSO")
> > Cc: stable@dpdk.org
> >
> > Signed-off-by: Ophir Munk <ophirmu@mellanox.com>
> > ---
> >  lib/librte_gso/rte_gso.c | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/lib/librte_gso/rte_gso.c b/lib/librte_gso/rte_gso.c index
> > a44e3d4..e9ce9ce 100644
> > --- a/lib/librte_gso/rte_gso.c
> > +++ b/lib/librte_gso/rte_gso.c
> > @@ -50,12 +50,14 @@ rte_gso_segment(struct rte_mbuf *pkt,
> >  			((IS_IPV4_GRE_TCP4(pkt->ol_flags) &&
> >  			 (gso_ctx->gso_types &
> > DEV_TX_OFFLOAD_GRE_TNL_TSO)))) {
> >  		pkt->ol_flags &= (~PKT_TX_TCP_SEG);
> > +		pkt->ol_flags |= PKT_TX_TCP_CKSUM;
> >  		ret = gso_tunnel_tcp4_segment(pkt, gso_size, ipid_delta,
> >  				direct_pool, indirect_pool,
> >  				pkts_out, nb_pkts_out);
> >  	} else if (IS_IPV4_TCP(pkt->ol_flags) &&
> >  			(gso_ctx->gso_types &
> > DEV_TX_OFFLOAD_TCP_TSO)) {
> >  		pkt->ol_flags &= (~PKT_TX_TCP_SEG);
> > +		pkt->ol_flags |= PKT_TX_TCP_CKSUM;
> >  		ret = gso_tcp4_segment(pkt, gso_size, ipid_delta,
> >  				direct_pool, indirect_pool,
> >  				pkts_out, nb_pkts_out);
> > --
> > 2.7.4