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Subject: Re: [PATCH] crypto: x86/crc32c-intel - Don't match some Zhaoxin CPUs
To:     Ard Biesheuvel <ardb@kernel.org>,
        Eric Biggers <ebiggers@kernel.org>
CC:     Herbert Xu <herbert@gondor.apana.org.au>,
        "David S. Miller" <davem@davemloft.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        X86 ML <x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
        Linux Crypto Mailing List <linux-crypto@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        <TimGuo-oc@zhaoxin.com>, <CooperYan@zhaoxin.com>,
        <QiyuanWang@zhaoxin.com>, <HerryYang@zhaoxin.com>,
        <CobeChen@zhaoxin.com>, <SilviaZhao@zhaoxin.com>
References: <1607686144-2604-1-git-send-email-TonyWWang-oc@zhaoxin.com>
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 <CAMj1kXH=u7+_DSEsUmFbtgGdPqCUs=AfTqX0mgL+DYvW2hAc8g@mail.gmail.com>
From:   Tony W Wang-oc <TonyWWang-oc@zhaoxin.com>
Message-ID: <7b826cf5-7371-77a2-b4c1-a6533787031e@zhaoxin.com>
Date:   Mon, 14 Dec 2020 10:29:01 +0800
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On 12/12/2020 18:54, Ard Biesheuvel wrote:
> On Sat, 12 Dec 2020 at 10:36, Ard Biesheuvel <ardb@kernel.org> wrote:
>>
>> On Fri, 11 Dec 2020 at 20:07, Eric Biggers <ebiggers@kernel.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@zhaoxin.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