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

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

On Thu, Apr 08, 2021 at 08:21:58AM -0300, Flavio Leitner wrote:
> 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.

Yes. What I propose is just a first step: better document what is the
current expected behavior, before doing something else.

> > 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.

Honnestly no, I don't have any good example, just a (maybe unfounded) doubt.

I have in mind operations that are done with tables or vector
instructions inside the drivers, but this is mainly done for Rx, not Tx.
You can look at Tx functions like mlx5_set_cksum_table() or
nix_xmit_pkts_vector(), or Rx functions like desc_to_olflags_v() or
enic_noscatter_vec_recv_pkts() to see what kind of stuff I'm talking
about.