Re: [PATCH v1 1/3] x86/cpufeatures: Add low performance CRC32C instruction CPU feature

[Borislav Petkov]

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@xxxxxxxxxxx>
> ---
>  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