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

Xin Li
2025-06-24T04:58:37Z

On 6/23/2025 7:04 PM, H. Peter Anvin wrote:
> On June 23, 2025 10:40:59 AM PDT, Xin Li <xin@zytor.com> wrote:
>> On 6/20/2025 5:50 PM, H. Peter Anvin wrote:
>>> On 2025-06-20 17:45, H. Peter Anvin wrote:
>>>>>
>>>>> 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.
>>>>
>>>
>>> Ok, that was word salad. What I meant was that the original patch is correct, and we SHOULD have this as a disabled feature.
>>
>> Agreed!
>>
>>> The reason is that it reduces the need to explicitly test for 32/64 bits for features that don't exist on 32 bits. When they are flagged as disabled, they get filtered out *at compile time*.
>>
>> It's better to make it depend on X86_32 directly rather than !X86_64:
>>
>> config X86_DISABLED_FEATURE_LASS
>> def_bool y
>> depends on X86_32
>>
>>
>> But the disabled feature list due to lack of 32-bit enabling will keep
>> growing until we remove 32-bit kernel code.
>>
>> Wondering should we bother enforcing cpuid_deps[] on 32-bit?
>>
>> IOW, turn off the feature when its dependency isn’t satisfied on 32b-it;
>> don’t just throw a warning and hope for the best.
>>
>> Thanks!
>> Xin
>>
>
> We should have the dependencies enforced; in fact, preferably we would enforce them at build time as well.
>
>

Yeah, sounds something we can do later :)