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

H. Peter Anvin
2025-06-24T02:04:59Z

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.