Re: [dpdk-dev] [PATCH v2] app/testpmd: fix TX checksum calculation for tunnel

[Olivier Matz <olivier.matz@6wind.com>]

On Thu, Jul 29, 2021 at 10:31:45AM +0000, Gregory Etelson wrote:
> Hello Olivier,
> 
> [:snip:]
> > >
> > > Correct. Inner checksum is offloaded and outer computed in software.
> > 
> > I think this approach is not sane: the value of the outer checksum depends on
> > the inner checksum, so it has to be calculated after. There is a comment in the
> > code about this:
> > 
> >         /* Then process outer headers if any. Note that the software
> >          * checksum will be wrong if one of the inner checksums is
> >          * processed in hardware. */
> >         if (info.is_tunnel == 1) {
> >                 tx_ol_flags |= process_outer_cksums(outer_l3_hdr, &info,
> >                                 tx_offloads,
> >                                 !!(tx_ol_flags & PKT_TX_TCP_SEG));
> >         }
> 
> I agree. That computation order led to error in my case.
> What if the engine could analyze port TX offload options and select 
> processing order according to existing configuration ?

I really think hardware inner checksum + software outer checksum is
broken by design, because outer checksum depends on inner checksum.

> > > Consider this example:
> > > Tunneled packet arrived at port A and being forwarded through port B.
> > > The packet arrived at port A with correct inner checksums - L3 and L4.
> > > Port B TX offloads inner L3 only.
> > >
> > > process_inner_cksums() sets "ipv4_hdr->hdr_checksum = 0;"
> > unconditionally.
> > > Inner L3 checksum value will be restored by port B TX checksum
> > > offload, but when
> > > process_outer_cksums() runs software calculation on outer L4 it will use 0
> > and produce wrong result.
> > >
> > > Therefore, the patch zeros inner checksum values only before actual
> > software calculations.
> > 
> > I better understand your use case, thanks.
> > 
> > However, with your patch, if the inner L4 checksum is wrong when it arrives
> > on port A, I think it will result in a packet with a wrong outer
> > L4 checksum and a correct inner L4 checksum. Is it what you expect?
> > 
> 
> Wrong checksum reflects ongoing issue that must be investigated and fixed.
> I don't expect forwarding engine to fix wrong checksum because it can hide
> a real problem.

Yes and no :)

We should keep in mind that this engine is a demo / test program for
checksum API. A real-world application should detect and drop a packet
with a wrong checksum.

> > I don't argue against the patch itself. What you suggest better matches the
> > offload API than what we have today. Can you please send another version
> > that better explains the use-case?
> > 
> 
> I posted v3 with updated comment.
> 
> > One more suggestion, maybe for later. Currently, the csumonly engine can be
> > configured to do the checksum in sw or in hw. Maybe we could add a "dont-
> > touch" option, to keep the value in the packet. Would it help for your use-
> > case?
> > 
> 
> That's a very good idea.
> Packets can arrive with pre-calculated correct checksums. Keeping these values
> can save processing time.
> 
> [:snip:]
> 
> Regards,
> Gregory