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

[Gregory Etelson <getelson@nvidia.com>]

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 ?
 
> > 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.

> 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