Re: [PATCHv6 01/16] x86/cpu: Enumerate the LASS feature bits

[H. Peter Anvin]

On June 20, 2025 4:46:21 PM PDT, Xin Li <xin@xxxxxxxxx> wrote:
>On 6/20/2025 10:31 AM, Kirill A. Shutemov wrote:
>> On Fri, Jun 20, 2025 at 08:36:30AM -0700, Xin Li wrote:
>>> On 6/20/2025 6:53 AM, Kirill A. Shutemov wrote:
>>>> diff --git a/arch/x86/Kconfig.cpufeatures b/arch/x86/Kconfig.cpufeatures
>>>> index 250c10627ab3..9574c198fc08 100644
>>>> --- a/arch/x86/Kconfig.cpufeatures
>>>> +++ b/arch/x86/Kconfig.cpufeatures
>>>> @@ -124,6 +124,10 @@ config X86_DISABLED_FEATURE_PCID
>>>>    	def_bool y
>>>>    	depends on !X86_64
>>>> +config X86_DISABLED_FEATURE_LASS
>>>> +	def_bool y
>>>> +	depends on !X86_64
>>>> +
>>>>    config X86_DISABLED_FEATURE_PKU
>>>>    	def_bool y
>>>>    	depends on !X86_INTEL_MEMORY_PROTECTION_KEYS
>>> 
>>> You don't need to add X86_DISABLED_FEATURE_LASS, because the LASS code
>>> is NOT optional at build time, i.e., you now don't have CONFIG_X86_LASS.
>> 
>> Hmm. But it is optional. It depends on CONFIG_X86_64. I don't think we
>> want it to be advertised on 32-bit kernels.
>> 
>
>I kind of ignore 32-bit...
>
>But I simply hate adding a disabled feature that depends on !X86_64;
>x86_64 has a broad scope, and new CPU features are often intentionally
>not enabled for 32-bit.
>
>(X86_DISABLED_FEATURE_PCID is the only one before LASS)
>
>

More importantly, it is wrong. 

The 32-bit build can depend on this feature not existing, therefore it SHOULD be listed as a disabled feature.