linux-kernel - Re: [PATCH v10 01/15] x86/cpu: Enumerate the LASS feature bits

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Message-ID: <5d95d421-1413-46de-a578-c2a0e44e3aa1@intel.com>
Date: Tue, 7 Oct 2025 11:28:36 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
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Subject: Re: [PATCH v10 01/15] x86/cpu: Enumerate the LASS feature bits

On 10/7/25 11:19, Edgecombe, Rick P wrote:
>>  	{ X86_FEATURE_SPEC_CTRL_SSBD,		X86_FEATURE_SPEC_CTRL },
>> +	{ X86_FEATURE_LASS,			X86_FEATURE_SMAP      },
> Aha! So SMAP is required for LASS. This makes the stac/clac patch make more
> sense. Please those comments less seriously. Although I think a comment is still
> not unwarranted.

STAC/CLAC are also architected to:

	#UD - If CPUID.(EAX=07H, ECX=0H):EBX.SMAP[bit 20] = 0.

So, even though LASS _technically_ doesn't require SMAP, it would be a
real pain without SMAP and STAC/CLAC. Thus, this series relies on SMAP
being present.

Actually, it might be worth breaking this dependency hunk out into its
own patch, just so there's a nice clean place to discuss this.