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

["Zhang, Qi Z" <qi.z.zhang@intel.com>]

> -----Original Message-----
> From: Yigit, Ferruh <ferruh.yigit@intel.com>
> Sent: Tuesday, October 12, 2021 12:27 AM
> To: Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>; Ruifeng Wang
> <Ruifeng.Wang@arm.com>; dev@dpdk.org; Min Hu (Connor)
> <humin29@huawei.com>; Yisen Zhuang <yisen.zhuang@huawei.com>; Lijun
> Ou <oulijun@huawei.com>
> Cc: Xing, Beilei <beilei.xing@intel.com>; Zhang, Qi Z <qi.z.zhang@intel.com>;
> Richardson, Bruce <bruce.richardson@intel.com>; jerinj@marvell.com;
> hemant.agrawal@nxp.com; drc@linux.vnet.ibm.com; stable@dpdk.org; nd
> <nd@arm.com>; humin29@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.

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

> 
> >
> >>
> >>> Fixes: 7b0cf70135d1 ("net/i40e: support ARM platform")
> >>> Cc: stable@dpdk.org
> >>>
> >>> Signed-off-by: Ruifeng Wang <ruifeng.wang@arm.com>
> >>> Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
> >