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

[Flavio Leitner <fbl@sysclose.org>]

On Thu, Apr 08, 2021 at 09:41:59AM +0200, Olivier Matz wrote:
> 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

That's what I suggest above.

> - pmds must suppose that TCP_CKSUM is set when TCP_SEG is set

But that keeps the problem of implying the TCP_CKSUM flag in
various places.

> 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.

Interesting, do you have an example? Because each flag still has an
separate meaning.

-- 
fbl