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

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Message-ID: <aa91aadb-758e-42db-86ab-451384e466ed@zytor.com>
Date: Fri, 20 Jun 2025 16:46:21 -0700
From: Xin Li <xin@...or.com>
To: "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>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
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        Xin Li <xin3.li@...el.com>, "Mike Rapoport (IBM)" <rppt@...nel.org>,
        Brijesh Singh <brijesh.singh@....com>,
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 <rick.p.edgecombe@...el.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 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)