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

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

> -----Original Message-----
> From: Ophir Munk [mailto:ophirmu@mellanox.com]
> Sent: Tuesday, April 24, 2018 10:44 AM
> To: Hu, Jiayu <jiayu.hu@intel.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 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.

But gso has no idea how this packet will be processed after it.
Caller can choose to calculate L3/L4 cksum in SW or might be going to use HW offloads.
In later case nothing stops the caller to update mbuf->ol_flags in a way he likes (TCP_CKSUM, IP_CKSUM, etc.).
Konstantin

> 
> > 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