linux-kernel - Re: [PATCH] crypto: x86/crc32c-intel

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Date:   Tue, 15 Dec 2020 10:15:29 +0800
From:   Tony W Wang-oc <TonyWWang-oc@...oxin.com>
To:     Eric Biggers <ebiggers@...nel.org>
CC:     <herbert@...dor.apana.org.au>, <davem@...emloft.net>,
        <tglx@...utronix.de>, <mingo@...hat.com>, <bp@...en8.de>,
        <x86@...nel.org>, <hpa@...or.com>, <linux-crypto@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <TimGuo-oc@...oxin.com>,
        <CooperYan@...oxin.com>, <QiyuanWang@...oxin.com>,
        <HerryYang@...oxin.com>, <CobeChen@...oxin.com>,
        <SilviaZhao@...oxin.com>
Subject: Re: [PATCH] crypto: x86/crc32c-intel - Don't match some Zhaoxin CPUs


On 15/12/2020 04:41, Eric Biggers wrote:
> On Mon, Dec 14, 2020 at 10:28:19AM +0800, Tony W Wang-oc wrote:
>> On 12/12/2020 01:43, Eric Biggers wrote:
>>> On Fri, Dec 11, 2020 at 07:29:04PM +0800, Tony W Wang-oc wrote:
>>>> The driver crc32c-intel match CPUs supporting X86_FEATURE_XMM4_2.
>>>> On platforms with Zhaoxin CPUs supporting this X86 feature, When
>>>> crc32c-intel and crc32c-generic are both registered, system will
>>>> use crc32c-intel because its .cra_priority is greater than
>>>> crc32c-generic. This case expect to use crc32c-generic driver for
>>>> some Zhaoxin CPUs to get performance gain, So remove these Zhaoxin
>>>> CPUs support from crc32c-intel.
>>>>
>>>> Signed-off-by: Tony W Wang-oc <TonyWWang-oc@...oxin.com>
>>>
>>> Does this mean that the performance of the crc32c instruction on those CPUs is
>>> actually slower than a regular C implementation?  That's very weird.
>>>
>>
>> From the lmbench3 Create and Delete file test on those chips, I think yes.
>>
> 
> Did you try measuring the performance of the hashing itself, and not some
> higher-level filesystem operations?
> 

Yes. Was testing on these Zhaoxin CPUs, the result is that with the same
input value the generic C implementation takes fewer time than the
crc32c instruction implementation.

Sincerely
Tony