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

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Message-ID: <D8783A84-119A-4981-9EB1-12C21BB34714@zytor.com>
Date: Fri, 20 Jun 2025 17:45:46 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Xin Li <xin@...or.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
CC: Andy Lutomirski <luto@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
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        Xiongwei Song <xiongwei.song@...driver.com>,
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        linux-kernel@...r.kernel.org, linux-efi@...r.kernel.org,
        linux-mm@...ck.org, Yian Chen <yian.chen@...el.com>
Subject: Re: [PATCHv6 01/16] x86/cpu: Enumerate the LASS feature bits

On June 20, 2025 4:46:21 PM PDT, Xin Li <xin@...or.com> 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.