-----Original Message-----
From: Yigit, Ferruh <redacted>
Sent: Tuesday, October 12, 2021 12:27 AM
To: Honnappa Nagarahalli <redacted>; Ruifeng Wang
[off-list ref]; dev@dpdk.org; Min Hu (Connor)
[off-list ref]; Yisen Zhuang [off-list ref]; Lijun
Ou [off-list ref]
Cc: Xing, Beilei <redacted>; Zhang, Qi Z <redacted>;
Richardson, Bruce [off-list ref]; jerinj@marvell.com;
hemant.agrawal@nxp.com; drc@linux.vnet.ibm.com; stable@dpdk.org; nd
[off-list ref]; 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:

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<snip>

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On 9/15/2021 9:33 AM, Ruifeng Wang wrote:

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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.

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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.

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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

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Fixes: 7b0cf70135d1 ("net/i40e: support ARM platform")
Cc: stable@dpdk.org

Signed-off-by: Ruifeng Wang <redacted>
Reviewed-by: Honnappa Nagarahalli <redacted>