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Date:   Mon, 14 Dec 2020 10:29:01 +0800
From:   Tony W Wang-oc <TonyWWang-oc@...oxin.com>
To:     Ard Biesheuvel <ardb@...nel.org>,
        Eric Biggers <ebiggers@...nel.org>
CC:     Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        Linux Kernel Mailing List <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 12/12/2020 18:54, Ard Biesheuvel wrote:
> On Sat, 12 Dec 2020 at 10:36, Ard Biesheuvel <ardb@...nel.org> wrote:
>>
>> On Fri, 11 Dec 2020 at 20:07, Eric Biggers <ebiggers@...nel.org> 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.
>>>
>>
>> This driver does not use CRC instructions, but carryless
>> multiplication and aggregation. So I suppose the pclmulqdq instruction
>> triggers some pathological performance limitation here.
>>
> 
> Just noticed it uses both crc instructions and pclmulqdq instructions.
> Sorry for the noise.
> 
>> That means the crct10dif driver probably needs the same treatment.
> 
> Tony, can you confirm that the problem is in the CRC instructions and
> not in the PCLMULQDQ code path that supersedes it when available?

CRC instructions.

sincerely
Tony

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