[PATCH v2] app/testpmd: use Tx preparation in txonly engine

[Konstantin Ananyev]

> >
> >> On 1/11/2024 5:25 AM, Kaiwen Deng wrote:
> >>> Txonly forwarding engine does not call the Tx preparation API
> >>> before transmitting packets. This may cause some problems.
> >>>
> >>> TSO breaks when MSS spans more than 8 data fragments. Those
> >>> packets will be dropped by Tx preparation API, but it will cause
> >>> MDD event if txonly forwarding engine does not call the Tx preparation
> >>> API before transmitting packets.
> >>>
> >>
> >> txonly is used commonly, adding Tx prepare for a specific case may
> >> impact performance for users.
> >>
> >> What happens when driver throws MDD (Malicious Driver Detection) event,
> >> can't it be ignored? As you are already OK to drop the packet, can
> >> device be configured to drop these packages?
> >>
> >>
> >> Or as Jerin suggested adding a new forwarding engine is a solution, but
> >> that will create code duplication, I prefer to not have it if this can
> >> be handled in device level.
> >
> > Actually I am agree with the author of the patch - when TX offloads and/or multisegs are enabled,
> > user supposed to invoke eth_tx_prepare().
> > Not doing that seems like a bug to me.
> > If it still works for some cases, that's a lucky coincidence, but not the expected behavior.
> >
> 
> fair enough
> 
> > About performance - first we can check is that really a drop.
> > Also as I remember most drivers set it to non-NULL value, only when some TX offloads were
> > enabled by the user on that port, so hopefully for simple case (one segment, no tx offloads) it
> > should be negligible.
> >
> 
> +1 to measure the impact, that helps to decide
> 
> > Again, we can add manual check in testpmd tx-only code to decide do we need a TX prepare
> > to be called or not.
> >
> 
> What is the condition to call Tx prepare, is it only required when Tx
> offload are enabled?

Yes, as I remember, tx_prepare() need to be called only when TX offloads 
(cksum, tso, etc.) are requested.
It also does some extra sanity checks - min pkt_len, max number of segments, etc.,
so there is no harm to call it even when no tx offloads are enabled, but then 
user needs to be prepared for some extra overhead. 
In theory user can do all these things manually I his own code, but as requirements vary between HW models,
I suppose it will be a real pain for the user.

> 
> 
> @Kaiwen can you please give some details on the problematic case, "MSS
> spans more than 8 data fragments", and how to produce it?