Re: [dpdk-dev] [PATCH 2/5] net/tap: do not touch Tx offload flags

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

On Wed, Apr 07, 2021 at 05:15:39PM -0300, Flavio Leitner wrote:
> On Thu, Apr 01, 2021 at 11:52:40AM +0200, David Marchand wrote:
> > Tx offload flags are of the application responsibility.
> > Leave the mbuf alone and check for TSO where needed.
> > 
> > Signed-off-by: David Marchand <david.marchand@redhat.com>
> > ---
> 
> The patch looks good, but maybe a better approach would be
> to change the documentation to require the TCP_CKSUM flag
> when TCP_SEG is used, otherwise this flag adjusting needs
> to be replicated every time TCP_SEG is used.
> 
> The above could break existing applications, so perhaps doing
> something like below would be better and backwards compatible?
> Then we can remove those places tweaking the flags completely.

As a first step, I suggest to document that:
- applications must set TCP_CKSUM when setting TCP_SEG
- pmds must suppose that TCP_CKSUM is set when TCP_SEG is set

This is clearer that what we have today, and I think it does not break
anything. This will guide apps in the correct direction, facilitating
an eventual future PMD change.

> diff --git a/lib/librte_mbuf/rte_mbuf_core.h b/lib/librte_mbuf/rte_mbuf_core.h
> index c17dc95c5..6a0c2cdd9 100644
> --- a/lib/librte_mbuf/rte_mbuf_core.h
> +++ b/lib/librte_mbuf/rte_mbuf_core.h
> @@ -298,7 +298,7 @@ extern "C" {
>   *  - if it's IPv4, set the PKT_TX_IP_CKSUM flag
>   *  - fill the mbuf offload information: l2_len, l3_len, l4_len, tso_segsz
>   */
> -#define PKT_TX_TCP_SEG       (1ULL << 50)
> +#define PKT_TX_TCP_SEG       (1ULL << 50) | PKT_TX_TCP_CKSUM
>  
>  /** TX IEEE1588 packet to timestamp. */
>  #define PKT_TX_IEEE1588_TMST (1ULL << 51)

I'm afraid some applications or drivers use extended bit manipulations
to do the conversion from/to another domain (like hardware descriptors
or application-specific flags). They may expect this constant to be a
uniq flag.