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

[Xin Li]

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