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

[Ferruh Yigit <ferruh.yigit@intel.com>]

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?

Is this performance drop valid for all Arm scalar datapath, or is it specific to
ThunderX2?

> 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>