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

[Tony W Wang-oc]

On 07/01/2021 14:37, Borislav Petkov wrote:
> 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.
> 

This issue will be enhanced by hardware and patch submit will be pending.

Sincerely
Tonyw