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

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Date:   Sat, 12 Dec 2020 10:36:28 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     Tony W Wang-oc <TonyWWang-oc@...oxin.com>,
        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 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.

That means the crct10dif driver probably needs the same treatment.