Re: [dpdk-dev] Rx/Tx offloads checks behaviour in 18.05

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 5/11/2018 8:07 AM, Andrew Rybchenko wrote:
> Hi all,
> 
> I think that Rx/Tx offloads checks behaviour is inconsistent in next-net 
> as of today.
> 
> Consistency checks are removed from PMDs and substituted with error logs 
> in ethdev.

Yes.

> Basically application which is not switched to new offload API has no 
> way to find out if,
> for example, Rx scatter is supported. Rx scatter offload was introduced 
> in 17.11 to
> substitute corresponding flag in device Rx mode.
> 
> Not updated application could try to enable Rx scatter on device 
> configure and
> get failure if it is not supported.  Yes it is not fine-grained and it 
> could be numerous
> reasons behind the configure failure. With 18.05 configure will pass and 
> moreover
> hardware may be configured to do Rx scatter despite of no real support 
> in PMD.
> Consequences could be really different from simply dropping scattered 
> packet or
> delivery of truncated packets to spoiling of memory etc.
> 
> Similar could happen with multi-segment packet on Tx. Application configures
> Tx queue without NOMULTISEG flag, TxQ setup passes (with error log that
> multi-segment is not supported, but it is just an error log) and 
> application generates
> multi-segment packets which are simply truncated (if the first segment 
> length is used
> as packet length on transmit) or garbage is sent (if total packet length 
> is used, i.e.
> possible disclosure of security sensitive information since it could be 
> data from
> neighbour packet).

How common these error cases do you think?

> 
> I think we have the following options:
> 
> A. Rollback corresponding changes which remove checks from PMDs
>      (at least some PMDs will be not affected).
> 
> B. Fail configure if unsupported offload is requested (all PMDs must be 
> converted
>      in the release, so reporting of supported offloads must be correct) AND
>      add check that offloads requested on Tx queue level (derived from 
> txq_flags)
>      are supported at least somewhere (i.e. tx_offload_capa)

Issue is not PMDs, they should support new offload API. Concern is breaking
application which is out of our control.

With current approach some old application may request invalid offload and PMD
won't return an error to app, agreed this is a concern.
But adding error returns will break same applications, in a better more obvious
way, and has possibility to break more applications, ones really not concerned
about offload may be hit as well.

> 
> C. Describe the behaviour changes in release notes to try to make it at 
> least
>      create for DPDK users. I don't like the option at all.
> 
> Any other ideas?
> 
> I would vote for B since it is a step forward and even if it makes some apps
> to fail I think it is better than consequences of missing checks.
> I'll make a patch for option B and test it meanwhile.
> 
> Andrew
>