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

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

> 
> > -----Original Message-----
> > From: Yigit, Ferruh <ferruh.yigit@intel.com>
> > Sent: Thursday, April 8, 2021 12:40 AM
> > To: Rong, Leyi <leyi.rong@intel.com>; Zhang, Qi Z <qi.z.zhang@intel.com>; Xing,
> > Beilei <beilei.xing@intel.com>
> > Cc: dev@dpdk.org
> > Subject: Re: [dpdk-dev] [PATCH] net/i40e: add Tx preparation for vector data
> > path
> >
> > On 3/31/2021 9:53 AM, Leyi Rong wrote:
> > > 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