Re: [dpdk-dev] [PATCH] net/i40e: add Tx preparation for vector data path

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

> > > > > Fill up dev->tx_pkt_prepare to i40e_pkt_prepare when on vector and
> > > > > simple data path selection, as the sanity check is needed ideally.
> > > > >
> > > > > Signed-off-by: Leyi Rong <leyi.rong@intel.com>
> > > > > ---
> > > > >   drivers/net/i40e/i40e_rxtx.c | 2 +-
> > > > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > > >
> > > > > diff --git a/drivers/net/i40e/i40e_rxtx.c
> > > > > b/drivers/net/i40e/i40e_rxtx.c index 61cb204be2..b3d7765e3b 100644
> > > > > --- a/drivers/net/i40e/i40e_rxtx.c
> > > > > +++ b/drivers/net/i40e/i40e_rxtx.c
> > > > > @@ -3412,7 +3412,7 @@ i40e_set_tx_function(struct rte_eth_dev *dev)
> > > > >   			PMD_INIT_LOG(DEBUG, "Simple tx finally be used.");
> > > > >   			dev->tx_pkt_burst = i40e_xmit_pkts_simple;
> > > > >   		}
> > > > > -		dev->tx_pkt_prepare = NULL;
> > > > > +		dev->tx_pkt_prepare = i40e_prep_pkts;
> > > > >   	} else {
> > > > >   		PMD_INIT_LOG(DEBUG, "Xmit tx finally be used.");
> > > > >   		dev->tx_pkt_burst = i40e_xmit_pkts;
> > > > >
> > > >
> > > > It seems prepare function is doing some sanity checks before handing
> > > > packets to the HW.
> > > > So with this change all Tx paths calls the same Tx prepare function,
> > > > if so why not set the function pointer outside of the if block,
> > > > instead of setting it in both legs of the if/else? This clarifies that Tx prepare
> > used always.
> > >
> > > Hi Ferruh,
> > >
> > > Yes, it make sense.
> > >
> > > Hi Konstantin,
> >
> > Hi Leyi,
> >
> > >
> > > Would that be something wrong if the prepare function goes for simple Tx
> > function although it does not support the offload feature yet?
> > >
> >
> > Current situation:
> > For simple TX path we set dev->tx_pkt_prepare = NULL.
> > That makes rte_eth_tx_prepare() a stub that does nothing and always returns:
> > "All packets are good".
> > That is unsafe off-course, and if upper layer will pass a packet that is not
> > supported, then it can lead to various bad things: bad cksum, corrupted packets,
> > TX hang, etc.
> > But at least it keeps simple TX path fast.
> > With that patch:
> > For simple TX path we set dev->tx_pkt_prepare = i40e_prep_pkts.
> > Now on TX path we invoke extra function that does a lot of checks, but it still
> > unsafe:
> > as i40e_prep_pkts() assumes that  full-featured TX function is in place (multi-segs
> > are allowed, etc.).
> > So our simple TX path became slower, but still is unsafe.
> > I think that if we want to introduce tx_prepare() for simple TX path, then the
> > proper way - create a new function for it (i40e_simple_prep_pkts() or so).
> > It will be aware that simple TX path is in place and more restrictions should be
> > met:
> > check that nb_segs==1 and no TX offloads (except FAST_FREE?) are enabled,
> > plus usual checks for min and max pkt_len.
> >
> > Konstantin
> >


Hi Leyi,
 
> Hi Konstantin,
> 
> Thanks for the explanation, I know the current full-featured prepare function will cost more CPU cycle, but not sure how to say is still
> unsafe?

Let say user will do:

mb = create_and_fill_multi_seg_pkt(...);
n =  rte_eth_tx_prepare(p, q, &mb, 1);
if (n == 1)
  n = rte_eth_tx_burst(p, q, &mb, 1);
else
  rte_pktmbuf_free(mb);

if dev->tx_pkt_prepare == i40e_prep_pkts and dev->tx_pkt_burs == i40e_xmit_pkts_simple,
then this code will TX the packet, even though it shouldn't in theory.

> Why I set the simple Tx prepare function to the current i40e_prep_pkts() is we may support more offload features like current full-featured
> Tx for vector path(which is included in simple Tx currently), if so, the current tx prepare function can be re-used.

AFAIK, for i40e current simple (and vector) TX path doesn't support all offloads that are supported by full-featured path
To be more specific: mulit-seg packets, TCP_CKSUM, TCP_SEG, etc.
Am I missing something obvious here?

Konstantin