Re: [dpdk-dev] ixgbe vector rx does not conform to rte_eth_rx_burst() API

["Zhao1, Wei" <wei.zhao1@intel.com>]

> -----Original Message-----
> From: Morten Brørup <mb@smartsharesystems.com>
> Sent: Monday, July 27, 2020 5:42 PM
> To: Zhao1, Wei <wei.zhao1@intel.com>; Guo, Jia <jia.guo@intel.com>; Xing,
> Beilei <beilei.xing@intel.com>; Zhang, Qi Z <qi.z.zhang@intel.com>; Wang,
> Xiao W <xiao.w.wang@intel.com>; Wu, Jingjing <jingjing.wu@intel.com>; Yang,
> Qiming <qiming.yang@intel.com>; Xu, Rosen <rosen.xu@intel.com>
> Cc: dev@dpdk.org; Thomas Monjalon <thomas@monjalon.net>; Yigit, Ferruh
> <ferruh.yigit@intel.com>; Andrew Rybchenko <arybchenko@solarflare.com>
> Subject: RE: [dpdk-dev] ixgbe vector rx does not conform to rte_eth_rx_burst()
> API
> 
> > From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Zhao1, Wei
> > Sent: Saturday, July 18, 2020 5:45 AM
> >
> > > From: dev <dev-bounces@dpdk.org> On Behalf Of Zhao1, Wei
> > > Sent: Saturday, July 18, 2020 11:32 AM
> > > To: Morten Brørup <mb@smartsharesystems.com>; Guo, Jia
> > > <jia.guo@intel.com>
> > >
> > > HI,
> > >
> > > > From: Morten Brørup <mb@smartsharesystems.com>
> > > > Sent: Thursday, July 16, 2020 5:09 PM
> > > >
> > > > > From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Zhao1, Wei
> > > > > Sent: Thursday, July 16, 2020 10:50 AM
> > > > >
> > > > > Hi,
> > > > >
> > > > > > From: Morten Brørup <mb@smartsharesystems.com>
> > > > > > Sent: Thursday, July 16, 2020 12:03 AM
> > > > > >
> > > > > > 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.
> 
> I looked at it, and it seems that most Intel Ethernet drivers suffer from this
> bug.
> 
> Some of them do not seem simple to fix, e.g. the fm40k behavior depends on
> the configured rx_free_thresh, which defaults to 32.

Yes

> 
> I think we can all agree that it is too risky for me to attempt to fix drivers for
> hardware which I do not have available for testing, so I have created a bug in
> Bugzilla, and am thus passing the baton to the Intel PMD developer team.
> 

Ok.

> >
> > >
> > > 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);
> > > }