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: sta...@dpdk.org
>
> Signed-off-by: Ruifeng Wang <ruifeng.w...@arm.com>
> Reviewed-by: Honnappa Nagarahalli <honnappa.nagaraha...@arm.com>
