[dpdk-dev] ixgbe vector rx does not conform torte_eth_rx_burst() API

[Zhao1, Wei]

> -----Original Message-----
> From: dev <dev-bounces at dpdk.org> On Behalf Of Zhao1, Wei
> Sent: Saturday, July 18, 2020 11:32 AM
> To: Morten Brørup <mb at smartsharesystems.com>; Guo, Jia
> <jia.guo at intel.com>
> Cc: dev at dpdk.org; Thomas Monjalon <thomas at monjalon.net>; Yigit, Ferruh
> <ferruh.yigit at intel.com>; Andrew Rybchenko <arybchenko at solarflare.com>
> Subject: Re: [dpdk-dev] ixgbe vector rx does not conform torte_eth_rx_burst()
> API
> 
> HI,
> 
> > -----Original Message-----
> > From: Morten Brørup <mb at smartsharesystems.com>
> > Sent: Thursday, July 16, 2020 5:09 PM
> > To: Zhao1, Wei <wei.zhao1 at intel.com>; Guo, Jia <jia.guo at intel.com>
> > Cc: dev at dpdk.org; Thomas Monjalon <thomas at monjalon.net>; Yigit, Ferruh
> > <ferruh.yigit at intel.com>; Andrew Rybchenko <arybchenko at solarflare.com>
> > Subject: RE: [dpdk-dev] ixgbe vector rx does not conform
> > torte_eth_rx_burst() API
> >
> > > From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Zhao1, Wei
> > > Sent: Thursday, July 16, 2020 10:50 AM
> > >
> > > Hi,
> > >
> > > > -----Original Message-----
> > > > From: Morten Brørup <mb at smartsharesystems.com>
> > > > Sent: Thursday, July 16, 2020 12:03 AM
> > > > To: Zhao1, Wei <wei.zhao1 at intel.com>; Guo, Jia <jia.guo at intel.com>
> > > > Cc: dev at dpdk.org; Thomas Monjalon <thomas at monjalon.net>; Yigit,
> > > Ferruh
> > > > <ferruh.yigit at intel.com>; Andrew Rybchenko
> > > <arybchenko at solarflare.com>
> > > > Subject: ixgbe vector rx does not conform to rte_eth_rx_burst()
> > > > API
> > > >
> > > > Wei, Jeff,
> > > >
> > > > For the ixgbe driver using vector functions, i.e.
> > > ixgbe_recv_pkts_vec(), calling
> > > > rte_eth_rx_burst() with nb_pkts > RTE_IXGBE_MAX_RX_BURST only
> > > > returns RTE_IXGBE_MAX_RX_BURST packets. E.g. calling
> > > > rte_eth_rx_burst() with
> > > > nb_pkts=64 only returns 32 packets.
> > > >
> > > >
> > > > The API description of rte_eth_rx_burst() says:
> > > >
> > > > <quote>
> > > > The rte_eth_rx_burst() function returns the number of packets
> > > actually
> > > > retrieved, which is the number of rte_mbuf data structures
> > > effectively supplied
> > > > into the rx_pkts array. A return value equal to nb_pkts indicates
> > > that the RX
> > > > queue contained at least rx_pkts packets, and this is likely to
> > > signify that other
> > > > received packets remain in the input queue. Applications
> > > > implementing
> > > a
> > > > "retrieve as much received packets as possible" policy can check
> > > > this
> > > specific
> > > > case and keep invoking the rte_eth_rx_burst() function until a
> > > > value
> > > less than
> > > > nb_pkts is returned.
> > > > </quote>
> > > >
> > > > The driver implementation does not conform to the documented
> > > > behavior
> > > for
> > > > "retrieve as much received packets as possible" applications.
> > >
> > > It seems not an issue, this function has comment bellow, it is
> > > design work in that way.
> > >
> > >
> > > /*
> > >  * vPMD receive routine, only accept(nb_pkts >=
> > > RTE_IXGBE_DESCS_PER_LOOP)
> > >  *
> > >  * Notice:
> > >  * - nb_pkts < RTE_IXGBE_DESCS_PER_LOOP, just return no packet
> > >  * - nb_pkts > RTE_IXGBE_MAX_RX_BURST, only scan
> > RTE_IXGBE_MAX_RX_BURST
> > >  *   numbers of DD bit
> > >  * - floor align nb_pkts to a RTE_IXGBE_DESC_PER_LOOP power-of-two
> > > */
> > >
> >
> > I noticed this already. And yes, ixgbe_recv_pkts_vec() does what its
> > comments says.
> >
> > However, when ixgbe_recv_pkts_vec() is used as the driver's
> > implementation of the rte_eth_rx_burst() function call, the
> > rte_eth_rx_burst() function does not do what is expected of the
> rte_eth_rx_burst() function.
> >
> > The implementation must conform to the API that it implements.
> >
> > If you don't want to update the ixgbe_recv_pkts_vec() function, I
> > propose that you add a wrapper function that calls
> > ixgbe_recv_pkts_vec() repeatedly, and use the wrapper function as the
> > implementation of the rte_eth_rx_burst() function.

A code review will be do for that change, it is need because that is a important change.

> 
> Get your point, I know what you need, but is there any risk for
> ixgbe_recv_pkts_vec? I am not sure.
> Maybe you can have a try first, if it work well, you can submit a patch.
> What you need is this:
> 
> uint16_t
> i40e_recv_scattered_pkts_vec_avx2(void *rx_queue, struct rte_mbuf
> **rx_pkts,
> 			     uint16_t nb_pkts)
> {
> 	uint16_t retval = 0;
> 	while (nb_pkts > RTE_I40E_VPMD_RX_BURST) {
> 		uint16_t burst = i40e_recv_scattered_burst_vec_avx2(rx_queue,
> 				rx_pkts + retval, RTE_I40E_VPMD_RX_BURST);
> 		retval += burst;
> 		nb_pkts -= burst;
> 		if (burst < RTE_I40E_VPMD_RX_BURST)
> 			return retval;
> 	}
> 	return retval + i40e_recv_scattered_burst_vec_avx2(rx_queue,
> 				rx_pkts + retval, nb_pkts);
> }