[dpdk-stable] [PATCH v2 2/2] net/i40e: fix risk in Rx descriptor read in scalar path

[Ruifeng Wang]

Hi Ferruh,

> > > -----Original Message-----
> > > From: Yigit, Ferruh <ferruh.yigit at intel.com>
> > > Sent: Tuesday, October 12, 2021 12:27 AM
> > > To: Honnappa Nagarahalli <Honnappa.Nagarahalli at arm.com>; Ruifeng
> > Wang
> > > <Ruifeng.Wang at arm.com>; dev at dpdk.org; Min Hu (Connor)
> > > <humin29 at huawei.com>; Yisen Zhuang <yisen.zhuang at huawei.com>;
> > Lijun Ou
> > > <oulijun at huawei.com>
> > > Cc: Xing, Beilei <beilei.xing at intel.com>; Zhang, Qi Z
> > > <qi.z.zhang at intel.com>; Richardson, Bruce
> > > <bruce.richardson at intel.com>; jerinj at marvell.com;
> > > hemant.agrawal at nxp.com; drc at linux.vnet.ibm.com; stable at dpdk.org;
> nd
> > > <nd at arm.com>; humin29 at huawei.com
> > > Subject: Re: [dpdk-stable] [PATCH v2 2/2] net/i40e: fix risk in Rx
> > > descriptor read in scalar path
> > >
> > > On 9/29/2021 4:29 PM, Honnappa Nagarahalli wrote:
> > > > <snip>
> > > >>
> > > >> On 9/15/2021 9:33 AM, Ruifeng Wang wrote:
> > > >>> Rx descriptor is 16B/32B in size. If the DD bit is set, it
> > > >>> indicates that the rest of the descriptor words have valid values.
> > > >>> Hence, the word containing DD bit must be read first before
> > > >>> reading the rest of the descriptor words.
> > > >>>
> > > >>> Since the entire descriptor is not read atomically, on relaxed
> > > >>> memory ordered systems like Aarch64, read of the word containing
> > > >>> DD field could be reordered after read of other words.
> > > >>>
> > > >>> Read barrier is inserted between read of the word with DD field
> > > >>> and read of other words. The barrier ensures that the fetched
> > > >>> data is correct.
> > > >>>
> > > >>> Testpmd single core test showed no performance drop on x86 or
> > N1SDP.
> > > >>> On ThunderX2, 22% performance regression was observed.
> > > >>>
> > > >>
> > > >> Is 22% performance drop value correct? That is a big drop, is it
> > acceptable?
> > > > Agree, it is a big drop. Fixing it will require using the barrier less
> frequently.
> > > For ex: read 4 descriptors (4 words containing the DD bits) before
> > > using the barrier.
> > > >
> > > >>
> > > >> Is this performance drop valid for all Arm scalar datapath, or is
> > > >> it specific to ThunderX2?
> > > > This is specific to ThunderX2. N1 CPU does not see any impact. A72
> > > > is not
> > > tested. Considering that the ThunderXx line of CPUs are not in
> > > further development, and it is scalar path, I would not suggest to
> > > make further changes to the code.
> > > >
> > > > It would be good to test this on Kunpeng servers and get some
> feedback.
> > >
> > > Hi Connor, Yisen, Lijun,
> > >
> > > Can you please check this patch? I don't know if you are using i40e
> > > nic on your platform but if you do can you please test it?
> > >
> > > Overall this patch cause a big performance drop on Arm for i40e, I
> > > just want to be sure this is not impacting any user negatively.
I cannot speak for vendors. But my test on a Huawei aarch64 server showed no performance drop.
NIC in use is XXV710.
Just FYI.

Thanks,
Ruifeng
> >
> > Folks:
> > 	This patch has been dropped from dpdk-next-net-intel, as still
> > waiting for your confirm.
> > 	Btw Patch 1/2 was still in dpdk-next-net-intel.
> > Thanks
> > Qi
> >
> Hi Qi, Ferruh,
> 
> Do you have any suggestion on how to progress this patch?
> It is fixing possible violation of hardware access from architecture point of
> view.
> Negative performance impact may happen because barriers are added.
> I don't think we received objections until now.
> 
> Thanks,
> Ruifeng
> > >
> > > >
> > > >>
> > > >>> Fixes: 7b0cf70135d1 ("net/i40e: support ARM platform")
> > > >>> Cc: stable at dpdk.org
> > > >>>
> > > >>> Signed-off-by: Ruifeng Wang <ruifeng.wang at arm.com>
> > > >>> Reviewed-by: Honnappa Nagarahalli
> <honnappa.nagarahalli at arm.com>
> > > >