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Date:   Thu, 7 Jan 2021 07:37:50 +0100
From:   Borislav Petkov <bp@alien8.de>
To:     Tony W Wang-oc <TonyWWang-oc@zhaoxin.com>
Cc:     herbert@gondor.apana.org.au, davem@davemloft.net,
        tglx@linutronix.de, mingo@redhat.com, x86@kernel.org,
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        linux-crypto@vger.kernel.org, linux-kernel@vger.kernel.org,
        TimGuo-oc@zhaoxin.com, CooperYan@zhaoxin.com,
        QiyuanWang@zhaoxin.com, HerryYang@zhaoxin.com,
        CobeChen@zhaoxin.com, SilviaZhao@zhaoxin.com
Subject: Re: [PATCH v1 1/3] x86/cpufeatures: Add low performance CRC32C
 instruction CPU feature
Message-ID: <20210107063750.GA14697@zn.tnic>
References: <1610000348-17316-1-git-send-email-TonyWWang-oc@zhaoxin.com>
 <1610000348-17316-2-git-send-email-TonyWWang-oc@zhaoxin.com>
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On Thu, Jan 07, 2021 at 02:19:06PM +0800, Tony W Wang-oc wrote:
> SSE4.2 on Zhaoxin CPUs are compatible with Intel. The presence of
> CRC32C instruction is enumerated by CPUID.01H:ECX.SSE4_2[bit 20] = 1.
> Some Zhaoxin CPUs declare support SSE4.2 instruction sets but their
> CRC32C instruction are working with low performance.
> 
> Add a synthetic CPU flag to indicates that the CRC32C instruction is
> not working as intended. This low performance CRC32C instruction flag
> is depend on X86_FEATURE_XMM4_2.
> 
> Signed-off-by: Tony W Wang-oc <TonyWWang-oc@zhaoxin.com>
> ---
>  arch/x86/include/asm/cpufeatures.h | 1 +
>  arch/x86/kernel/cpu/cpuid-deps.c   | 1 +
>  2 files changed, 2 insertions(+)
> 
> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index 84b8878..9e8151b 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -292,6 +292,7 @@
>  #define X86_FEATURE_FENCE_SWAPGS_KERNEL	(11*32+ 5) /* "" LFENCE in kernel entry SWAPGS path */
>  #define X86_FEATURE_SPLIT_LOCK_DETECT	(11*32+ 6) /* #AC for split lock */
>  #define X86_FEATURE_PER_THREAD_MBA	(11*32+ 7) /* "" Per-thread Memory Bandwidth Allocation */
> +#define X86_FEATURE_CRC32C		(11*32+ 8) /* "" Low performance CRC32C instruction */

Didn't hpa say to create a BUG flag for it - X86_BUG...? Low performance
insn sounds like a bug and not a feature to me.

And call it X86_BUG_CRC32C_SLOW or ..._UNUSABLE to denote what it means.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette